## ﻿وب تریلر فیلم

- ﻿وب فور ایرونی
 Dictionary ofComputer andInternet TermsTenth EditionDouglas A. Downing, Ph.D.School of Business and EconomicsSeattle Pacific UniversityMichael A. Covington, Ph.D.Artificial Intelligence CenterThe University of GeorgiaMelody Mauldin CovingtonCovington InnovationsAthens, GeorgiaCatherine Anne CovingtonCovington InnovationsAthens, GeorgiaWith the assistance ofSharon Covington ABOUT THE AUTHORSDouglas Downing teaches economics and quantitative methods at the School of Business and Economics at Seattle Pacific University. He is the author of several books in both Barron’s Easy Way and Business Review series. He is also the author of Java Programming the Easy Way and Dictionary of Mathematics Terms, published by Barron’s Educational Series, Inc. He holds the Ph.D. degree in economics from Yale University.Michael Covington is Associate Director of the Artificial Intelligence Institute at the University of Georgia. He is the author of several books and over 250 magazine articles. He holds the Ph.D. degree in linguistics from Yale University.Melody Mauldin Covington is a graphic designer living in Athens, Georgia. She is the author of Dictionary of Desktop Publishing (published by Barron’s).Catherine Anne Covington is a student at the Lamar Dodd School of Art (University of Georgia).Sharon Covington is a student at Emory University.© Copyright 2009, 2006, 2003, 2000, 1998, 1996, 1995, 1992, 1989, and 1986 by Barron’s Educational Series, Inc.All rights reserved.No part of this book may be reproduced or distributed in any form or by any means without the written permission of the copyright owner.All inquiries should be addressed to:Barron’s Educational Series, Inc. 250 Wireless Boulevard Hauppauge, NY 11788 www.barronseduc.comISBN-13: 978-0-7641-4105-8ISBN-10: 0-7641-4105-8Library of Congress Catalog Card No. 2008044365Library of Congress Cataloging-in-Publication DataDowning, Douglas.Dictionary of computer and Internet terms / Douglas A. Downing, Michael A. Covington, Melody Mauldin Covington. — 10th ed.p.  cm.ISBN 978-0-7641-4105-81. Computers—Dictionaries. 2. Internet—Dictionaries. I. Covington, Michael A., 1957– II. Covington, Melody Mauldin. III. Title.QA76.15.D667 2009	004.03—dc22	2008044365PRINTED IN CHINA 9 8 7 6 5 4 3 2 1 CONTENTSAbout the Authors	iiTo the Reader	ivDictionary of Computer and Internet Terms	1Numbers	1Greek Letters	5A	7B	38C	71D	124E	159F	185G	211H	223I	242J	264K	272L	276M	296N	322O	336P	349Q	389R	392S	421T	468U	498V	510W	521X	538Y	543Z	545Visual Dictionary of Characters and Symbols	547Country Codes for Top-Level Domains	552 TO THE READERComputers are no longer just for specialists. Today, computing is not just a profession and a hobby; it is also a tool used in virtually all human activities.That’s why we’ve compiled this book of background knowledge. Its pur-pose is to tell you the things other people think you already know.We design this book to have a convenient size so it can be easily carried around. In compiling a book this size, we have had to be selective. The quickest way to identify a word that you can’t find in a book is probably to do a web search (see SEARCH ENGINE). Also, some terms are almost always abbreviated, and in that case you should look for the abbreviation rather than the full term.Much has changed since the first edition of this book was published more than twenty years ago. New terms are being invented every day. We regularly update the book, and this edition contains new entries on a vari-ety of topics including Windows Vista and Mac OS X. We’ve also cut out material that was showing its age.Terms are marked slang or humorous if they are seldom used in serious writing. They are marked as jargon if, in our estimation, they are somewhat pretentious new names for old concepts and are not likely to endure. We provide occasional Usage notes to explain grammar, spelling, and proper use of words, such as the exact difference between disc and disk.Throughout, we use SMALL CAPITALS to mark important words that are defined elsewhere in this book. By following cross-references, you can quickly find many entries that pertain to whatever interests you. Here are some entries you may wish to start with to learn about particular topics:•	Internet culture: CHAT ROOM•	right and wrong: COMPUTER ETHICS•	safe computing: COMPUTER SECURITY•	solving exceptionally difficult problems: ARTIFICIAL INTELLIGENCE•	productively using computers in business and daily life: APPLICATIONPROGRAM•	listening to music: DIGITAL MUSIC•	taking pictures: DIGITAL CAMERA•	creating web pages: HTML•	writing computer programs: PROGRAMMING LANGUAGE•	software that controls a computer: OPERATING SYSTEM•	how a computer works: COMPUTER ARCHITECTURE•	networking and the Internet: INTERNET•	connecting computers wirelessly: WIRELESS COMMUNICATION•	electronic components: TRANSISTOR iv v	TO THE READERBe sure to notice the visual dictionary of symbols at the end of the book. If you don’t know what ∑ or ≈ or • is called, don’t worry; you can look it up there.All four of us want to thank The University of Georgia and Seattle Pacific University for access to facilities and for accommodating us as we worked on the project. We also want to thank Robert Downing for help with 1960s data processing terminology; Sharon Covington for help with current Internet culture; and Brantley Coile of Coraid, Inc., for permission to adapt material from Coraid’s glossary of networking and data storage.Many of the words used in this book are registered trademarks. We have made no attempt to determine or report their legal status. For further infor-mation about any product name, consult the manufacturer’s literature.  1	10base-2NUMBERS1-2-3 see LOTUS 1-2-3.3Com a leading producer of networking hardware, mainly focusing on res-idential and small to medium businesses. In recent years the company has sharpened its focus in this area by acquiring U.S. Robotics but sell-ing off Palm (see PALM). Their web address is www.3com.com.3D see THREE-DIMENSIONAL GRAPHICS.4, 8, 16 . . . 64 (etc.) describing a CD or DVD drive, able to transfer data at 4, 8, 16 (etc.) times the speed of normal audio or video. For example, a 16 CD-R drive can record a full CD, equivalent to about an hour of audio, in about four minutes.5.1 a format of SURROUND SOUND with five speakers that transmit the full audio spectrum and one that transmits only bass. The five full-range speakers are positioned as front left, center, and right, and rear left and right. The bass speaker, or SUBWOOFER, is usually placed in front. See Fig. 253, p. 464. Compare 6.1, 7.1.6.1 a format of SURROUND SOUND with six full-range speakers in the left front, center front, right front, left, right, and rear center positions, plus a SUBWOOFER for additional bass. Compare 5.1.7-layer model see DATA COMMUNICATION.7.1 a format of SURROUND SOUND with seven full-range speakers in the left front, center front, right front, left, right, left rear, and right rear posi-tions, plus a SUBWOOFER for additional bass. Compare 5.1.8.3 filename a filename consisting of up to 8 letters or digits, a dot (period), and up to three more letters or digits, as in DOS and Windows 3.10/100 (describing a network adapter) capable of operating at 10 or 100 megabits per second. See 10BASE-T; 100BASE-T.10/100/1000 (describing a network adapter) capable of operating at 10, 100, and 1000 megabits per second. See 10BASE-T; 100BASE-T; 1000BASE-T.10base-2 thinwire Ethernet; a type of Ethernet connection using thin coax-ial cable with BNC T-connectors, a bus topology, and a maximum data rate of 10 megabits per second. Cable segments can range from 2 feet (0.6 m) to 607 feet (185 m) in length. See ETHERNET; THINWIRE.Usage note: In this and similar terms, 10 stands for the data rate in megabits per second; base means baseband (not modulated on a higher-frequency carrier); and 2 is the approximate maximum cable length in hundreds of meters. The hyphen is often left out. 10base-5	210base-5 thickwire Ethernet; a type of Ethernet connection using thick coaxial cable with special cable-piercing taps, a bus topology, and a maximum data rate of 10 megabits per second. Cable segments can range from 8.2 feet (2.5 m) to 1640 feet (500 m) in length. See ETHER-NET; THICKWIRE.10base-F fiber-optic Ethernet; a type of Ethernet connection using fiber-optic cable and a maximum data rate of 10 megabits per second. Cables can be as long as 1.2 miles (2 km). See ETHERNET; FIBER OPTICS.10base-T twisted-pair Ethernet using Category 3 or Category 5 cable and RJ-45 modular connectors, a star topology with hubs, and a maximum data rate of 10 megabits per second. Each cable can be up to 328 feet (100 m) long. However, because they are unshielded, these cables are somewhat subject to electrical noise if placed close to motors or fluo-rescent lights. See ETHERNET; CROSSOVER CABLE; CATEGORY 5 CABLE.16-bit program a program that runs on Intel microprocessors using only the features of the 8088 or 80286, with 16-bit internal registers. Most DOS applications and many earlier Windows applications are 16-bit pro-grams. Contrast 32-BIT PROGRAM.24-bit graphics graphical images that use 24 bits to represent each color, so that each color is made by mixing red, green, and blue, each of which is measured on a scale of 0 to 255, and a total of 16,777,216 colors is available. Often called “millions of colors.”24  7 (or 24/7, 24-7) available 24 hours a day, 7 days a week.32-bit program a program that uses the 32-bit internal registers and large memory capacity of the Intel 386, 486, Pentium, or other compatible micro-processor; generally faster than a 16-bit program doing the same computa-tion on the same CPU. Contrast 16-BIT PROGRAM. See also WIN32S.32-bit Windows Microsoft Windows 95, NT, and their successors for the Pentium and related processors, as distinct from Windows 1.0–3.1 (apart from 32-bit add-ons) or Windows CE. See WINDOWS.35-mm equivalent the focal length of lens, on a 35-mm film camera, that would cover the same field of view as a particular digital camera and lens. See CROP FACTOR; FOCAL LENGTH; ZOOM.47 USC 227 the 1991 U.S. law that banned “junk faxing” (unsolicited advertising by fax). See JUNK FAX.100base-F fast fiber-optic Ethernet, like 10base-F but with a maximum data rate of 100 megabits per second.100base-T fast twisted-pair Ethernet using Category 5 cable and RJ-45 modular connectors; like 10base-T but with a maximum data rate of 100 megabits per second. Many network cards and hubs are compatible with 3	486both 10base-T and 100base-T transmission. Thus, you can convert a 10base-T network to 100base-T component-by-component and switch to the higher speed when all the components have been modernized.386 the first Intel microprocessor with 32-bit internal registers and good support for multitasking and extended memory; able to run Windows 95, but too slow for most present-day software. See MICROPROCESSOR.403 FORBIDDEN HTTP error message indicating that the HTTP server is not permitted to read a file. This usually means that the owner of the web page has not set the correct permissions on the file. See PERMISSION.404 NOT FOUND HTTP error message indicating that a web address is invalid. See DEAD LINK.419 scam, 4-1-9 scam a form of fraud conducted through e-mail, usually from Nigeria, where it violates section 4-1-9 of the criminal code, hence the name.The perpetrator sends out mass e-mail claiming to be a bank officer or government official who needs help sneaking some money out of the country and wants to use someone else’s bank account. In return, the vic-tim will get thousands or millions of dollars.What actually happens is that the victim’s bank account is emptied, or the victim’s information is used for further fraud. Some victims have even been lured into traveling overseas without proper visas so that they could be trapped and blackmailed.The 419 scam is so common that many active Internet users receive more than one solicitation per day. Newer versions of the scam no longer mention Nigeria, and many of them claim to offer lottery winnings, inheritances, or business deals.See also SPAM; COMPUTER ETHICS.486 an Intel microprocessor similar to the 386 but faster; predecessor of the Pentium. See MICROPROCESSORTABLE 1IEEE 802.11 STANDARDS FOR WIRELESS NETWORKINGSpecification	Popular name	Frequency	Speed	Compatible with				802.11a	Wireless-A	5 GHz	54 Mbps	Wireless-A802.11b	Wireless-B	2.4 GHz	11 Mbps	Wireless-B802.11g	Wireless-G	2.4 GHz	54 Mbps	Wireless-B, -G802.11n	Wireless-N	2.4 GHz	100 Mbps	Wireless-B, -G, -N				 802.11	4802.11 (more fully, IEEE 802.11) a set of specifications for wireless net-working that give performance similar to 10base-T or 100base-T and implement Wi-Fi product compatibility standards (Table 1).Note that the three 2.4-GHz specifications are downward compatible; that is, a Wireless-B computer will work in a Wireless-G or Wireless-N network. Of course, in that case, communication takes place at the lower speed of Wireless-B.802.16 see WIMAX.1394, 1394a, 1394b see FIREWIRE.2000 see YEAR 2000 PROBLEM.2600 a number used as an identifying code by groups of people who exchange detailed information about how to break into computers, tam-per with telephone systems, duplicate credit cards, and the like, whether for the purpose of preventing or encouraging these acts. There is a mag-azine (2600: The Hacker Quarterly), a newsgroup (alt.2600), and a variety of loosely organized local “2600” groups. See HACKER (definition 3); CRACKER; PHREAK.The number 2600 is from the 2600-Hz control tone formerly used in telephone systems. The Atari 2600 video game machine is completely unrelated.8088 the Intel microprocessor used in the original IBM PC (1981). It has 16-bit registers and an 8-bit external bus. See MICROPROCESSOR.68000 the series of Motorola microprocessors originally used in the Apple Macintosh. See MICROPROCESSOR.80286 the Intel microprocessor used in the IBM PC AT (1984). It is faster than the 8088 and supports extended memory but does not have 32-bit registers or the built-in ability to emulate multiple 8088s; for that reason, multitasking operating systems did not become common until the 386 was introduced. See MICROPROCESSOR.80386, 80486 unofficial names for the Intel 386 and 486 microprocessors. See 386, 486, and references there. 5	πGREEK LETTERSα (alpha) the opacity of a layer in a graphical image. See ALPHA.γ (gamma) a measure of the contrast of photographic film or the nonlin-earity of an electronically obtained image. See GAMMA.μ (mu) abbreviation for micro- (one-millionth). See METRIC PREFIXES.μC abbreviation for microcontroller.μP abbreviation for microprocessor.π (pi) the ratio of the circumference of a circle to its diameter, approxi-mately 3.14159. See PI. THE GREEK ALPHABET	6 The Greek AlphabetA	α	alphaB	β	betaΓ	γ	gammaδdeltaΕεepsilon (  in some typefaces)Ζ	ζ	zetaΗ	η	etatheta (  in some typefaces)Ι	ι	iotaΚ	κ	kappaΛ	λ	lambdaΜ	μ	muΝ	ν	nuΞ	ξ	xiΟ	ο	omicronΠπpi (   in some typefaces)Ρρrho (  in some typefaces)Σσsigma (  at ends of words)Τ	τ	tau	υ	upsilonΦφphi (  in some typefaces)Χ	χ	chiΨ	ψ	psiΩ	ω	omega 7	absAA1.	abbreviation used in HTML to indicate an anchor, a link to another location. For an example, see HTML.2.	(on a digital camera) aperture-priority autoexposure, the mode in which the user sets the lens opening (f-ratio) and the camera chooses the exposure time; same as Av. Contrast P, TV, S.A4 the standard size of typing paper everywhere except the United States, 210  297 mm, about 81⁄4  113⁄4 inches. American typing paper is 81⁄2  11 inches.A4 is part of an ISO standard for paper sizes (chosen so that A0 paper (840  1189 mm) has an area of 1 square meter and each size can be cut in half to make the next smaller one. Thus, the area of a sheet of A4 paper is 1⁄16 m2). For table, see PAPER SIZES (ISO).FIGURE 1. A4 paper is longer and narrower than letter sizeAAC Advanced Audio Coding, an audio compression format newer and more efficient than MP3, used internally by iTunes and Nintendo Wii. See www.mpeg.org/MPEG/aac.html.ABC Atanasoff Berry Computer, a machine developed in 1939 at Iowa State University by John Atanasoff and Clifford Berry for solving equa-tion systems. Although it did not allow for stored programs, it was an important predecessor of the ENIAC and other digital computers.abort to cancel an action or command.Abort, Retry, Fail? an error message displayed by DOS and similar oper-ating systems when a disk is unreadable or some other input or output operation is physically impossible. An earlier version said, “Abort, Retry, Ignore?”abs the function that calculates absolute value in many programming lan-guages and on scientific calculators. It converts negative numbers to positive while leaving positive numbers unchanged. For example, abs(37) = 37; abs(–37) = 37; abs(–2.5) = 2.5; abs(0) = 0. ABSOLUTE URL.COMPUTER ARCHITEC-8absolute addressabsolute address1. a fixed location in the computer’s memory. SeeTURE; OFFSET.2. in a spreadsheet program, a cell address that refers to a fixed location that will not change when a formula is copied to another location. In Excel, absolute addresses are indicated by placing a dollar sign before the column and row indicator. For example, if the formula 2*$D$7 is entered into a cell, then $D$7 is an absolute address. If this formula is copied to another cell, the address $D$7 will not change. Contrast RELA-TIVE ADDRESS.3. Seeabsolute URL a URL that contains the full address, identifying the machine, directory, and file. For example, if a web page contains the link:<a href=”http://www.census.gov/2010census/about_2010_census/”>it will find about _2010_census in the directory 2010census at the com-puter labeled www.census.gov. Contrast RELATIVE URL.abstract1.	a summary of a document or file. For example, in Java programming, a JAR FILE contains class files together with an encrypted abstract (sum-mary) calculated with a kind of hash function. If one of the class files is tampered with, the hash function calculated from the downloaded files will not match the hash function in the abstract, so the verifier will not allow the class to load. See also MANIFEST.2.	not tied to a specific pre-existing example. For example, an abstract data type is one that does not correspond exactly to anything in the archi-tecture of the computer; instead, it is declared by the programmer to suit the purposes of the program.In object-oriented programming, a class is declared abstract if there will not be any data or methods specific to that class; instead, it is to be used as a superclass for other classes that will have specific data. An abstract class cannot be instantiated, but other classes can extend it.accelerator a device that makes an operation run faster. For example, a graphics accelerator is a card that contains built-in circuits for perform-ing graphics operations, allowing the system to render graphics more quickly than would be the case if the microprocessor bore the entire load.accents marks added to letters (as in é è ê ë) to indicate differences of pro-nunciation; said to have been introduced by Aristophanes of Byzantium c. 200 B.C. to preserve the pitch accent of ancient Greek, which was dying out. The only major languages that do not require accents are English and Latin.Most computer software treats a letter with an accent as a single char-acter. More sophisticated systems represent the accent and the letter sep-arately, so that any accent can be put on any letter. See ANSI. 9	accessibility acceptable-use policy a policy established by the owner of a computer sys-tem, or by an Internet service provider, concerning acceptable use of the computer and network facilities. Acceptable-use policies should gener-ally include the following points:1.	Users are accountable for what they do. Deliberate snooping, harassment, or interference with other users will not be tolerated, nor will any deliberate unauthorized activity.2.	The computer shall be used only for its intended purposes. For example, you generally can’t use your employer’s computer to run another business on the side; nor can you run private money-making schemes on a computer owned by a state university. Employees are accountable for how they use their time at work.3.	Passwords must be kept secret. See PASSWORD.4.	The service provider has the right to suspend accounts that are being misused. People accused of misconduct have the right to a fair hearing.5.	Users must abide by the acceptable-use policies of newsgroups and other electronic discussion forums, which are mostly paid for by other people. On the Internet you are always someone’s guest.6.	Chain letters and mass e-mailing are expensive, unwelcome, and generally not permitted. The correct way to reach a wide audience is to use an appropriate newsgroup.7.	Cyberspace is not above the law. Practices that are illegal in the real world, such as forgery, gambling, obscenity, and threatening or inciting violence, are still illegal when you do them on the computer.8.	Losing an account is not necessarily the only penalty for miscon-duct. The service provider cannot shield users from criminal or civil liability when they break laws or deliberately harm others. Really destructive computer abusers generally have several accounts and must be stopped by other means.See also COMPUTER ETHICS; COMPUTER LAW.Access a powerful, highly programmable RELATIONAL DATABASE marketed by Microsoft as part of the Office suite of products.access control list in Windows, the list of which users or groups are allowed to use a file, directory, or device. See CACLS.access provider see INTERNET SERVICE PROVIDER.access time the amount of time needed by a memory device to transfer data to the CPU.accessibility the measure of how fully a computer product can be used by people of varying abilities. For example, a blind computer user visiting a web page may use speech synthesis software to read the page aloud. A web site where images all have alternate text descriptions is more acces-sible than a web site without such tags. See also WAI. account	10account authorization to use a computer or any kind of computer service, even if free of charge. An account consists of an identifying name and other records necessary to keep track of a user. Sometimes an account belongs to another computer or a computer program rather than a human being.accounting system software that reads in data for transactions and gener-ates income statements, balance sheets, and related financial reports. See also QUICKEN.accumulator the register where a computer stores the result of an arith-metic operation. For example, in 8086 assembly language, the instruc-tion ADD AX,10 means “Add 10 to the number in the accumulator, and leave the result there.” Some computers can use more than one register as an accumulator. See COMPUTER ARCHITECTURE; ASSEMBLY LANGUAGE.ACL see ACCESS CONTROL LIST.ACM (Association for Computing Machinery) a worldwide association of computer professionals headquartered in the United States. Their web address is www.acm.org.ACPI (Advanced Configuration and Power Interface) a set of standard hardware/software interactions that give the operating system the ability to direct power management of hardware devices. For example, a com-puter with ACPI can turn itself off under software control as the last step in shutting down the operating system.acquire to obtain a file (for editing) from a scanner or a camera. Similar to IMPORT, except that the image is not coming from a file.Acrobat software from ADOBE SYSTEMS, INC., for creating and reading PDF (Portable Document Format) files. Acrobat Reader is a browser plug-in available free from Adobe’s web site (www.adobe.com) that enables users to view and print PDF files that they receive from others. The full version of Acrobat is a powerful multi-use utility designed to facilitate annotation and distribution of digital documents. With Acrobat, com-ments and highlights can be added to documents. It’s possible to perform minor text edits, although large changes to page layout are not possible. Forms can be made interactive. Multiple .pdf documents can be com-bined or pages may be extracted into separate files. Acrobat also includes the ability to add a secure digital signature to .pdf documents.See PDF.acronym a word formed from the initial parts of other words. For example, BASIC stands for Beginner’s All-Purpose Symbolic Instruction Code.See also TLA.activate1. to choose a window in which you want to type. This is done by mov-ing the mouse pointer into the window and clicking one button. In some 11	ActiveXoperating systems you must click on the window’s title bar. See WINDOW. 2. to start a piece of software by double-clicking on its name or icon.See CLICK; ICON.3. to make a software product usable by informing the manufacturer that it has been installed and obtaining an activation code. This can be done on line or by making a telephone call. See REGISTRATION (definition 1).active color the color currently selected (in a painting or drawing pro-gram). Whatever tool is being used will paint or draw in the active color.Active Desktop in Windows, the ability to use a WEB PAGE as the desktop, i.e., the screen itself, not just as one of the programs running on it. This makes it easy to display a web page that is constantly updated, such as weather or stock price information, without having to start and run aBROWSER. See also DESKTOP; WORLD WIDE WEB.active matrix a type of liquid crystal display (LCD) that produces higher contrast than earlier passive-matrix displays by incorporating transistors into the LCD matrix.active window the window currently in use, the one in which the user is typing, drawing, or making menu choices (see Figure 2). There can only be one active window at a time. See WINDOW; ACTIVATE.FIGURE 2. Active windowActiveX a marketing name used by Microsoft for many types of software components implemented in the COM (Component Object Model) architecture (see COM).An ActiveX control is a small piece of software designed to be used as part of a larger one. Some ActiveX controls are simply object libraries or subroutine libraries used by programmers—a more sophisticated kind of DLL. Others work more like Java applets (see APPLET).At one time it was common to include ActiveX controls in web pages, as programs to be executed on the client computer, but because of secu-rity risks, many web browsers no longer accept them. actor	12actor in computer animation, any object that moves in a specified manner along a path, whether or not it represents a human being. Even a bounc-ing ball is an actor.actual parameter the value actually passed to a function or procedure in a programming language. For example, if you compute ABS(X) and the value of X is –2.5, then –2.5 is the actual parameter of ABS. See FORMAL PARAMETER; PARAMETER.A/D converter see ANALOG-TO-DIGITAL CONVERTER.Ada a programming language developed in the late 1970s for the U.S. Department of Defense. It is named for Augusta Ada Byron, Countess of Lovelace, who worked with Babbage’s mechanical calculator in the nineteenth century.Ada subprograms can be compiled separately and linked together before execution. In the sample program, the with and use statements specify that this program uses a library of precompiled subroutines called I_O_PACKAGE.Much of the original motivation for designing Ada was the need for a better language for real-time programming, that is, programming com-puters to control automatic or semiautomatic equipment. Toward this end, Ada allows the programmer to create multiple tasks that run con-currently (see TIMESHARING) to pass signals from one task to another and to introduce controlled time delays.with I_O_PACKAGE; procedure FACTORIAL isuse I_O_PACKAGE;--This program reads a number and --computes its factorial.NUM, FACT, COUNT: INTEGER; beginGET(NUM);FACT := 1;for COUNT in 2..NUM loop FACT := FACT * COUNT;end loop;PUT(”The factorial of ”); PUT(NUM); PUT(” is ”); PUT(FACT);end;FIGURE 3. Ada programadaptive technology technology that helps people work around physical lim-itations. Computer-related examples include magnified screen displays, speech recognition devices, and keyboards with latching shift and control keys for people who can press only one key at a time. See ACCESSIBILITY. 13	ADSLADC see ANALOG-TO-DIGITAL CONVERTER.add-in a package providing additional features to a program such as a spreadsheet; for example, the Solver add-in for Microsoft Excel.add noise a paint program filter that adds a speckled texture to a picture. This is done by adding noise (random variation) to the pixel values.Compare WHITE NOISE.FIGURE 4. Add Noise filter adds texture to imageaddress1.	a number or bit pattern that uniquely identifies a location in a com-puter memory. Every location has a distinct address.2.	a letter and number identifying the column and row of a cell in a spreadsheet. See RELATIVE ADDRESS; ABSOLUTE ADDRESS.3.	a set of numbers identifying a machine on the Internet. See IPADDRESS.4.	an electronic mail address. See ELECTRONIC MAIL; INTERNET.5.	a URL identifying a web page. See URL.address book a facility in an e-mail program, chat program, or web browser for storing addresses of individuals or web sites. Addresses may be added to the address book automatically when the user replies to e-mail.ADF (automatic document feeder) a device for feeding documents into a SCANNER automatically, sheet by sheet.admin abbreviation for ADMINISTRATOR.Administrator the account name used by the system administrator under Windows NT and its successors. Compare ROOT.Adobe Systems, Inc. (San Jose, California) the software company that developed the PDF format for distributing documents on the web and the PostScript command language for output devices. Other products include Photoshop, Illustrator, GoLive, PageMaker, Premiere for digital video production, and a library of type styles. See PDF and POSTSCRIPT. Web address: www.adobe.comADSL (Asymmetric Digital Subscriber Line) a widely used way of pro-viding a high-speed Internet connection through ordinary telephone advance fee fraud	14lines; called asymmetric because the upstream and downstream data rates are different. Full-rate ADSL provides data rates of up to 8 Mbps downstream and 1.5 Mbps upstream. The variety of ADSL commonly provided to homes is called G.lite and provides data rates up to 1.5 Mbps downstream and 0.5 Mbps upstream. See DSL and cross-references there.advance fee fraud scams promising you a large amount of money provided you pay some kind of fee in advance. Needless to say, do not send money to anyone that sends such an e-mail to you. See 419 SCAM.adventure game a game in which the player navigates through an interac-tive story, solving puzzles and exploring areas. There may be multiple endings to the game, and the player’s actions determine which ending is reached. Adventure games test reasoning skills instead of reflexes.The first adventure games, such as Zork, had no graphics, only tex-tual descriptions:“You are in a maze of twisting passages, all alike...”Other notable examples are many games produced by Sierra, such as the King’s Quest series, and the Myst series from Cyan Worlds.adware software whose main purpose is to display advertisements on the user’s computer. Sometimes adware refers to legitimate software sent out as samples or sales presentations, but more often, the term denotes software installed without the user’s full knowledge and consent. Adware of the latter type can make advertisements pop up on the screen even when the web browser is blocking pop-up ads. See POP-UP AD.Software tools for detecting and removing malicious adware can be found at www.safer-networking.org and www.lavasoftusa.com. See alsoMALWARE; SPYWARE..aero a suffix indicating that a web or e-mail address belongs to some part of the aeronautical industry (in any country). Contrast .COM. See alsoTLD; ICANN.Aero the new user interface of Windows Vista, including many subtle changes from earlier versions, the most noticeable of which is that win-dows can have transparent borders. Compared to Windows 2000 and XP, Aero shows considerable influence from MAC OS X. Graphics features such as transparency can be turned off to improve performance on slow CPUs.AES (Advanced Encryption Standard) data encryption standard adopted by the U.S. government in 2002, using key sizes of 128 to 256 bits. SeeENCRYPTION.AF (audio frequency) a frequency within the range of human hearing, 20 to 20,000 hertz. Contrast RF.AFAIK online abbreviation for “as far as I know.”AFAIR online abbreviation for “as far as I remember.” 15	AirPortaffiliate program a promotional program whereby a business makes pay-ments or provides free services to others who refer customers to them. For example, many web sites participate in the affiliate program of AMA-ZON.COM to sell books.afk online abbreviation for “away from keyboard.”agent a piece of software that performs a service for someone, usually silently and automatically. For example, an agent might run on a CLIENT computer to keep the SERVER informed of its needs.aggregation point a point where signals from different wireless network nodes are collected and then connected by wire to the rest of the Internet.AGP (Accelerated Graphics Port) a fast bus connection that allows the graphics adapter to communicate with the CPU at a higher speed than the conventional ISA, EISA, or PCI bus. AGP was introduced with Intel’s Pentium II processor.AI see ARTIFICIAL INTELLIGENCE.AIM AOL Instant Messenger; an application that allows computer users to correspond with friends while online. A competing service is Microsoft’s Windows LIVE MESSENGER. A key feature of IM programs is their ability to display your status, whether “Online” or “Away,” to your regular con-tacts. IM programs also allow you to designate “Friends” and provide the ability to block communication with unwanted persons. IM mes-sages are typically brief and heavily abbreviated. Icons are sometimes used to express emotions.Messaging programs are also becoming popular with businesses, especially when members of a tight-knit work group are traveling.airbrush a tool available in some paint programs that simulates the effect of spraying paint; the edges are soft and the colors are translucent. The softness of the edge, size of the spray pattern, and the degree of opaque-ness can be controlled (see Figure 5).In bitmap-editing programs that do not offer an airbrush tool, there is usually a SPRAY CAN, which is basically a coarser version of the airbrush.FIGURE 5. AirbrushAirPort trade name used by Apple for various wireless networking adapters for the MACINTOSH. Ajax	16Ajax (Asynchronous JavaScript and XML) a method for providing dynamic content on web pages, often used with PHP server-side scripting.ALGOL (Algorithmic Language) a pair of programming languages that had a strong impact on programming language design. The first, ALGOL 60 (developed by an international committee around 1960), was an immediate ancestor of Pascal and introduced many Pascal-like fea-tures that have been adopted by numerous other languages, including:•	recursion;•	begin and end keywords to allow grouping statements into blocks;•	the “block if” statement, of the form:if condition then beginstatements endelse beginstatements end•	the symbol := for arithmetic assignment;•	semicolons between statements, leaving the programmer free to arrange statements in any convenient layout rather than putting one statement on each line.The key idea is that whereas a BASIC or FORTRAN program is a list of numbered instructions, an ALGOL program is a set of blocks of state-ments embedded within larger blocks. Thus, hierarchical design is eas-ier to establish and follow.But the ALGOL 60 standard did not specify statements for input and output, since these were considered machine-specific, and as a result, although much admired for its design, ALGOL 60 was not widely used in practice.ALGOL 68 (released in 1968) is a much more abstract language with a reputation for being powerful but hard to learn. It introduced wide-spread use of pointer variables (called refs) and variant types (called unions). An important principle of ALGOL 68 is orthogonality, which means that all meaningful combinations of features are allowed. (In geometry, two things are orthogonal if they meet at right angles.)Discontent with the complexity of ALGOL 68 led Niklaus Wirth to design first ALGOL W and then Pascal (see PASCAL), which almost com-pletely replaced ALGOL in practical use. See also RECURSION; STRUC-TURED PROGRAMMING; POINTER; VARIANT.algorithm a sequence of instructions that tells how to solve a particular problem. An algorithm must be specified exactly, so that there can be no doubt about what to do next, and it must have a finite number of steps. A computer program is an algorithm written in a language that a com-puter can understand, but the same algorithm can be written in several ANTIALIASING; SAMPLING RATE.17	allocation unitdifferent languages. An algorithm can also be a set of instructions for a person to follow.A set of instructions is not an algorithm if it does not have a definite stopping place, or if the instructions are too vague to be followed clearly. The stopping place may be at variable points in the general procedure, but something in the procedure must determine precisely where the stop-ping place is for a particular case.There are well-understood algorithms for many common computa-tions (for example, see SELECTION SORT). However, some problems are so complicated that there is no known algorithm to solve them, and in other cases, the only known algorithm takes impossibly large amounts of time.See HEURISTIC; LIMITS OF COMPUTER POWER.algorithmically unsolvable problem see LIMITS OF COMPUTER POWER.alias1.	(Macintosh) a copy of a file icon that provides an alternate way of starting an application program or opening a file, folder, or disk. You can place the alias anywhere that’s convenient—the desktop, the Apple menu, or a special folder. The title of an alias icon is in italics and dis-plays a small arrow in the lower left corner of the icon. In Windows, an alias is called a SHORTCUT.2.	(UNIX) an alternative way of typing a command. The alias command creates aliases. For example, if you execute the commandalias dir ls -althen from then on, dir will mean ls -al (the complete file listing command).FIGURE 6. Aliasingaliasing the appearance of false stairsteps or bands in an image, or false fre-quencies in digitized sound, due to interaction of the original signal withthe sampling rate. Seealign to make things line up, either horizontally or vertically. Most draw-ing programs and page layout programs have specific commands to help you align objects and text. See Figure 7.All Your Base Are Belong To Us a phrase from a poorly translated Japanese video game (Zero Wing, 1989) that achieved brief but wide-spread popularity as a catchphrase.allocation unit the units of disk space that can be allocated to a file. For example, if a disk drive uses 4096-byte allocation units, the space occu-pied by every file will be a multiple of 4096 bytes, regardless of how small the file is. Also called CLUSTER. See FAT32. AlltheWeb.com	18FIGURE 7. AlignAlltheWeb.com a search engine that in March 2004 became part of Yahoo!, and now uses the Yahoo! database.alpha (α) a measure of the opacity, or visibility, of an object in a graphical image. A transparent object has an alpha of 0 and is invisible; most objects have an alpha of 1 and completely cover the objects behind them.alpha channel (in paint programs) a CHANNEL that defines a selection. Instead of specifying a color of ink to print, the alpha channel marks part of the image for special treatment. An image may have multiple alpha channels. See SELECTION TOOLS.alpha testing the first stage of testing of a new software product, carried out by the manufacturer’s own staff. Contrast BETA TESTING; GAMMA TESTING.alphabet soup (slang) unrecognizable abbreviations.alphanumeric characters letters and digits (but not punctuation marks, mathematical symbols, or control codes).On large IBM computers, the characters @, #, and $count as alpha-betic, and hence as alphanumeric. They are called national characters because they print differently on computers designed for use in different countries.alt1. prefix identifying “alternative” Usenet newsgroups, those that have not been voted on by the members, such as alt.folklore.urban. SeeNEWSGROUP.2. a key on a computer keyboard that is used to give an alternate mean-ing to other keys. It is used like the Shift key; that is, you hold it down while pressing the other key. For example, to type Alt-P, type P while holding down Alt. See ASCII; ANSI; MODIFIER KEY. 19 AmigaAltair a pioneering microcomputer marketed to hobbyists in 1975, signifi-cant because the version of BASIC for this machine was the first Microsoft product.AltaVista a popular World Wide Web SEARCH ENGINE originally developed by Digital Equipment Corporation. Their web address is www.altavista.com.aluminum chemical element (atomic number 13) added to silicon to create a P-type SEMICONDUCTOR.Amazon.com the first prominent E-TAIL merchant. Established as a book-store in 1995 in Seattle, Washington, Amazon has since expanded to sell a wide variety of products around the world. Web address: www.amazon.comambient lighting (in three-dimensional computer graphics) the overall lighting of a scene. Ambient lighting in a computer scene appears to have no specific source.AMD (Advanced Micro Devices) a manufacturer of digital integrated cir-cuits, including the Athlon and Duron, which are compatible substitutes for Intel’s Pentium. AMD is headquartered in Sunnyvale, California, and can be reached on the Web at www.amd.com. See also MICROPROCESSOR;PENTIUM.AMD64 see X64.America Online (AOL) an online information service based in Dulles, Virginia. America Online offers its subscribers e-mail, conferencing, software, computing support, interactive magazines and newspapers, online classes, and Internet access. In 2000, AOL merged with Time Warner. Web address: www.aol.com. See INTERNET; ONLINE (usage note).AMI (American Megatrends, Inc.) the leading supplier of the BIOS soft-ware built into PC motherboards (see BIOS). AMI also makes diagnostic software, RAID disk array controllers, and other products. The company is headquartered in Norcross, Georgia (a suburb of Atlanta), and can be reached on the Web at www.ami.com.Amiga a computer marketed by Commodore Business Machines in the 1980s to mid-90s. Similar in size and cost to the IBM PC, it had a quite different architecture and was ahead of its time in many ways, offering multitasking, windowing, an advanced graphics system, and MIDI music. Like the Macintosh, it used Motorola 68000-series microprocessors.Despite being admired by knowledgeable programmers, the Amiga never achieved the popularity of the PC or Macintosh. Although Commodore went bankrupt in 1994, a new company named Amiga continued development on the Amiga platform (web address: www.amiga.com). ampere, amp 20ampere, amp a unit for measuring electric current. A current of 1 ampere means that 6.25  1018 electrons are flowing by a point each second. A group of 6.25  1018 electrons has a charge of 1 coulomb, so 1 ampere = 1 coulomb per second.ampersand the character &, which stands for the word and. For an illus-tration of ampersands in various typefaces, see Figure 8.FIGURE 8. Ampersandsamplified speaker a speaker that includes its own amplifier to produce louder sound and stronger bass. See SOUND CARD; MULTIMEDIA.anacronym1. a reinterpreted abbreviation or acronym. For example, DVD origi-nally stood for digital video disc but is now said to stand for digital ver-satile disc. Compare BACKRONYM.2. an apparent abbreviation or acronym that does not actually stand for a series of words. For example, POSIX is apparently a blend of the words portable and UNIX but has no exact official interpretation.analog representing data in a form other than binary bits. The image picked up by a conventional film camera or scanner and the sound picked up by a microphone are examples of analog data that must be digitized (con-verted into the computer’s internal representation) in order to be stored in a computer. See also ANALOG COMPUTER; ANALOG-TO-DIGITAL CON-VERTER. Contrast DIGITAL.analog computer a computer that represents information in a form that can vary smoothly between certain limits rather than having discrete values. (Contrast DIGITAL COMPUTER.) A slide rule is an example of an analog computer because it represents numbers as distances along a scale.All modern, programmable computers are digital. Analog computer circuits are used in certain kinds of automatic machinery, such as auto-motive cruise controls and guided missiles. Also, a fundamental analog computer circuit called an operational amplifier is used extensively in audio, radio, and TV equipment.analog television transmission of television signals to the viewer as analog (non-digitized) signals in a format such as NTSC. See NTSC. ContrastDIGITAL TELEVISION.analog-to-digital converter (ADC) a device that changes data from analog to digital form. For example, a sound card uses an analog-to-digital con-verter to convert audio waveforms into digital representations. Laptop computers use analog-to-digital converters to measure the voltages of their batteries. See ANALOG COMPUTER; SOUND CARD; CODEC. BINARY ADDI-21 animationanalytic processing See OLAP.Analytical Engine see COMPUTERS, HISTORY OF.anchor a marked position in an HTML document (web page), to which the user can jump from elsewhere. For example, the HTML command<A NAME=”Elephants”>All about Elephants</A>marks its position as an anchor named “Elephants,” and if it resides in file http://www.vet.uga.edu/animals.html, then the full address of the anchor ishttp://www.vet.uga.edu/animals.html#ElephantsSee HTML; WORLD WIDE WEB.AND gate a logic gate that produces an output of 1 only when all of its inputs are 1, thus:Inputs Output0 0 00 1 01 0 01 1 1The symbol for an AND gate is shown in Figure 9.AND gates are used in computer arithmetic. In addition, AND gates with more than two inputs are used to recognize signals coming in simul-taneously on several wires, such as memory addresses. SeeTION; DECODER; LOGIC CIRCUITS.FIGURE 9. AND gate (logic symbol)angle brackets the characters < > or, more properly, < >, used in mathe-matics to enclose ordered pairs and the like. (Strictly speaking, < and > are the less-than and greater-than signs, respectively.) Contrast SQUARE BRACKETS; CURLY BRACKETS; PARENTHESES.animated gif see GIF89A.animation the simulation of motion by showing a series of still images redrawn many times per second. (See Figure 10.) Computers are now the primary way to create animation for theatrical releases, television pro-grams, and commercials. Animation studios such as Pixar and Dreamworks Animation utilize custom software written by their own programmers.On a more accessible level, animations produced with Macromedia Flash are very popular on the World Wide Web. (See FLASH.) Small icons anonymous FTP 22and graphics can also be animated by saving a series of images as an ANI-MATED GIF file. See GIF89A.FIGURE 10. Animationanonymous FTP see FTP.anonymous variable in Prolog, a variable (written _) that does not retain a value. If several anonymous variables occur in the same fact or rule, they are not the same variable. In pattern matching, anonymous variables match anything. See PROLOG.ANSI (American National Standards Institute) the main industrial stan-dardization organization in the United States. There are official ANSI standards in almost all industries, and many of them have to do with computers. In computer programming, ANSI most often refers to one of the following:1. ANSI standard versions of C, FORTRAN, COBOL, or other pro-gramming languages. Typically, a particular manufacturer’s ver-sion of a language will include all of the features defined in the ANSI standard, plus additional features devised by the manufac-turer. To be easily transportable from one computer to another, a program should not use any features that are not in the ANSI stan-dard. The programmer can then produce executable versions of it for different types of computers by compiling the same program with different compilers.2 ANSI standard escape sequences for controlling the screen of a computer terminal or microcomputer. An escape sequence is a series of character codes which, when sent to the screen, causes the screen to do something other than simply display the characters to which the codes correspond. The ANSI escape sequences all begin with the ASCII Escape character (code 27).3 The ANSI extended character set used in Microsoft Windows, and shown in Table 2. It includes all the ASCII characters plus many others. See ASCII; WINDOWS (MICROSOFT); IBM PC; UNICODE.To type any ANSI character in Microsoft Windows, hold down the Alt key while typing 0 followed by the character code number on the numeric keypad at the right-hand side of the keyboard. For example, to type é, hold down Alt and type 0233. You may prefer to use the Character Map utility to select characters and copy them to the Clipboard, and then paste them into your application. 23 antialiasingTABLE 2ANSI CHARACTER SET USED IN MICROSOFT WINDOWSantialiasing1. a technique for eliminating the stairstep appearance of slanted and curved lines on computer displays by partly illuminating some of the pixels adjacent to the line. See Figure 11.2. a technique for eliminating spurious tones in digitized sound by fil-tering out all frequencies above, or too close to, the sampling rate. See ALIASING. antivirus software 24FIGURE 11. Antialiasingantivirus software software that protects a computer from viruses (secretly destructive software modifications), either by blocking the modifica-tions that a virus tries to make, or by detecting a virus as soon as possi-ble after it enters the machine. See also VIRUS.AoE (ATA over Ethernet) a method of communicating with a disk drive on a server using the same protocol as if it were an internally mounted ATA drive, but sending the data back and forth as Ethernet packets. For more information, see www.coraid.com. Compare ISCSI.AOL see AMERICA ONLINE.Apache a web (HTTP) server program used by many web sites on a vari-ety of computers. It is an example of open-source software, where the source code is published and a variety of people make contributions. The first version was released in 1995. Within a year, it became the leading web server software. Apache is available from the Apache Software Foundation at www.apache.org.API (Application Program Interface) the set of services that an operating system makes available to programs that run under it. For example, the Windows API consists of a large number of procedures and data areas that can be used by programs running under Windows. With modern operating systems, it is important for programs to use the operating sys-tem API, as far as possible, rather than manipulating hardware directly, because direct manipulation of hardware can interfere with other pro-grams that are running concurrently.APL a programming language invented by Ken Iverson in the early 1960s and still used for some kinds of mathematical work. APL stands for A Programming Language, the title of Iverson’s 1962 book.APL has its own character set, so that most operations are represented by special characters rather than keywords. Additionally, in APL, arrays rather than single numbers are considered the basic data type; a single number is merely a one-element array. Here is an APL program that reads a series of numbers into an array and computes their average:AVG[1] X ← ■[2] K ← ρX[3] (+/X) ÷ K[4] 25 applicationHere X ← ■ means “read something from the keyboard into X,” and X becomes an array if the user types a series of numbers rather than just one. Then ρX is the number of elements in X, and (+/X) is the operation of addition distributed over X, i.e., the sum of all the elements. The last line of the program is an expression that defines the result.app (slang) APPLICATION PROGRAM.append to put something at the end of something else, for example, to append information at the end of a file or append additional files at the end of a tape.Apple an influential manufacturer of personal computers and entertainment equipment. The company, located in Cupertino, California, was founded by Steve Jobs and Steve Wozniak, who began work in a garage. (Web address: www.apple.com.)The Apple II, introduced in 1977, was one of the earliest popular microcomputers. It was based on the 8-bit MOS technology 6502 micro-processor. The Apple II was widely used in educational institutions, and the first microcomputer spreadsheet program (VisiCalc) ran on the Apple II.In 1984 Apple introduced the Macintosh, which was the first widely used computer with a graphical user interface (GUI). The Macintosh became widely used for desktop publishing and artistic applications, and it became one of the two main standards for microcomputers. Apple pro-duces both the hardware and the operating system software for the Macintosh (which is very different from the situation with Windows-based computers, the other main standard). See MACINTOSH. Apple also produces the QUICKTIME software for playing audio and video on micro-computers, now used on Windows computers as well as the Macintosh.Recent big sellers include iPod digital music player and theiPhone (which includes a portable phone, web browser, and music and video player). In 2003 Apple introduced the iTunes music store, allowing users to legally download songs for 99 cents each. See DIGITAL MUSIC; IWORK.Apple menu (Macintosh) the menu at the far left of the top menu bar that holds log out options, System Preferences, Recent Items list, and other controls.applet1. (obsolete usage) a small application program that is inexpensive and designed to do a small, specific job. Most operating systems come with several applets, such as a calculator, a calendar, and a note editor.2. an application program that is downloaded automatically through a World Wide Web browser and executed on the recipient’s machine. Applets are normally written in Java. See WORLD WIDE WEB; BROWSER; JAVA. application see APPLICATION PROGRAM. application framework 26application framework a set of predefined procedures or classes that saves the programmer much of the work of writing a program with a sophisti-cated user interface. Using an application framework, the programmer need not write code to handle menu choices, mouse movements, etc., because that work has already been done.An important early application framework was MacApp for the Macintosh. Newer programming languages such as Delphi, Visual Basic, and Java have application frameworks built in.application program a computer program that performs useful work not related to the computer itself. Examples include WORD PROCESSORs, PRE-SENTATION GRAPHICS programs, DESKTOP PUBLISHING programs, SPREADSHEETs, DATABASE MANAGEMENT programs, CAD and CAM systems, and ACCOUNTING SYSTEMs. Contrast UTILITY; OPERATING SYSTEM.applications programmer a person who writes programs that use the com-puter as a tool to solve particular problems, rather than just to manage the computer itself. Contrast SYSTEMS PROGRAMMER.arc1. part of a circle.2. a data compression program for the IBM PC formerly produced by System Enhancement Associates in the mid-1980s, a precursor of ZIP.See ZIP FILE.arccos, arc cosine the inverse of the trigonometric cosine function. If x = cos y, then y = arccos x. Many computer languages provide the arc tan-gent function but not the arc cosine function. You can work around this by using the relation:arccos x = arctan 1 − x2 x where x is positive and angles are expressed in radians. See alsoTRIGONOMETRIC FUNCTIONS.archival storage storage for data that must be kept for a long time but will seldom be used, such as backup copies of working programs. Microcomputers often use CD-R, DVD-R, or external hard disks for archival storage. Magnetic tape is a traditional form of archival storage, but the tapes should be copied every 2 or 3 years if their contents are still needed, since tapes can become demagnetized with age. See also GRAND-FATHER TAPE.archive1. a filing system for information designed to be kept for a long time.See ARCHIVAL STORAGE.2. a file containing the compressed contents of other files. The original files can be reconstructed from it. See ARC; ZIP FILE; TAR FILE; DATA COMPRESSION. INTERNET; WIDE AREA NET-27 arrange3. in Windows, a file attribute that says whether or not the file has been backed up by copying to another disk or tape. The attrib command can be used to examine or change archive bits. The archive bit makes it pos-sible to back up only the files that have not already been backed up. SeeATTRIBUTES.arcsin, arc sine the inverse of the trigonometric sine function. If x = sin y, then y = arcsin x. Many computer languages provide the arc tangent function but not the arc sine function. You can work around this by using the relation: arcsin x = arctan x1 − x2 in which all angles are expressed in radians. See TRIGONOMETRICFUNCTIONS.arctan, arc tangent the inverse of the trigonometric tangent function. If x = tan y, then y = arctan x. In BASIC, the arc tangent function is calledATN. See TRIGONOMETRIC FUNCTIONS.arguments (actual parameters) values passed to a function or procedure by the calling program. See ACTUAL PARAMETER.ARPANET a computer network originally developed for the U.S. Defense Advanced Research Projects Agency (ARPA, now known as DARPA) to link research institutions. ARPANET introduced the TCP/IP protocolsand eventually developed into the Internet. SeeWORK; TCP/IP; PROTOCOL.arrange1. to place the icons on the screen in neat rows and columns, retrieving any that have been moved off the edge of the screen (Figure 12).In Windows, “Arrange Icons” is on the menu that pops up when you right-click on an empty area of the desktop; it is also on the “View” menu of individual windows.If you want the computer to keep the icons arranged automatically, turn on the “Auto arrange” feature; a check mark shows that it is selected. See also CASCADE; TILE. FIGURE 12. “Arrange Icons” array 282. to place an item in relation to other items. In drawing programs, there is usually an Arrange menu that contains commands (ALIGN; SEND TO BACK; TO BACK; etc.) relating to the placement of selected objects. Objects are layered as if they were opaque pieces of paper.array a collection of data items that are given a single name and distin-guished by numbers (subscripts). For example, in C and related lan-guages, the declarationint x[5];creates an array of five elements that can be referred to as x[0], x[1], x[2], x[3], and x[4]. (In some languages, such as BASIC, the elements are numbered from 1 rather than from 0.)FIGURE 13. Array (one-dimensional)You can store numbers in these elements with statements such asx[0] = 10; x[1] = 43; x[2] = 8; x[3] = 91; x[4] = -5;just as if each element were a separate variable. You can also perform input and output on array elements just as if they were ordinary variables.Arrays are useful because they let you use arithmetic to decide which element to use at any particular moment. For example, you can find the total of the numbers in the five-element array x by executing the state-ments:total = 0;for (i=0; i<5; i++){total = total + x[i];}Here total starts out as 0 and then gets each element of x added to it. Arrays can have more than one dimension. For example, the declara-tion int y[3][5] creates a 3  5 array whose elements are:y[0][0] y[0][1] y[0][2] y[0][3] y[0][4] y[1][0] y[1][1] y[1][2] y[1][3] y[1][4] y[2][0] y[2][1] y[2][2] y[2][3] y[2][4] 29 artificial intelligenceMultidimensional arrays are useful for storing tables of data, such as three test grades for each of five students. See also DATA STRUCTURES; SORT.FIGURE 14. Array (two-dimensional, 3  5)arrow keys keys that move the cursor up, down, left, or right. The effect of these keys depends on the software being used. In a GUI environment, the arrow keys are basically an alternative to a mouse. Some drawing environments let you NUDGE the selected object with the arrow keys, giv-ing you greater precision. Touch typists sometimes prefer the arrow keys to a mouse because it allows them to keep their hands on the keyboard.See KEYBOARD; MOUSE; NUDGE.artifact any unwanted part of a signal or image that results from the way it was recorded or processed. For instance, a low-level hiss is a familiar artifact of tape recording.If you greatly increase the brightness or contrast of a JPEG image, you may see unusual stripes or blocks of color. They are artifacts of JPEG compression, in parts of the picture where the JPEG algorithm chose to sacrifice low-contrast detail in order to make the file smaller.See JPEG.artificial intelligence (AI) the use of computers to simulate human think-ing. Artificial intelligence is concerned with building computer pro-grams that can solve problems creatively, rather than just working through the steps of a solution designed by the programmer.One of the main problems of AI is how to represent knowledge in the computer in a form that can be used rather than merely reproduced. In fact, some workers define AI as the construction of computer programs that utilize a knowledge base. A computer that gives the call number of a library book is not displaying artificial intelligence; it is merely echo-ing back what was put into it. Artificial intelligence would come into play if the computer used its knowledge base to make generalizations about the library’s holdings or construct bibliographies on selected sub-jects. See EXPERT SYSTEM.Computer vision and robotics are important areas of AI. Although it is easy to take the image from a TV camera and store it in a computer’s memory, it is hard to devise ways to make the computer recognize the objects it “sees.” Likewise, there are many unsolved problems associ-ated with getting robots to move about in three-dimensional space—to ASC 30walk, for instance, and to find and grasp objects—even though human beings do these things naturally.AI also includes natural language processing—getting computers to understand speech, or at least typewritten input, in a language such as English. In the late 1950s it was expected that computers would soon be programmed to accept natural-language input, translate Russian into English, and the like. But human languages have proved to be more complex than was expected, and progress has been slow. See NATURAL LANGUAGE PROCESSING.Do computers really think? Artificial intelligence theorist Alan Turing proposed a criterion that has since become known as the Turing test: A computer is manifesting human-like intelligence if a person communi-cating with it by teletype cannot distinguish it from a human being. Critics have pointed out that it makes little sense to build a machine whose purpose is to deceive its makers. Increasing numbers of AI work-ers are taking the position that computers are not artificial minds, but merely tools to assist the human mind, and that this is true no matter how closely they can be made to imitate human behavior. See also COGNITIVE ENGINEERING; COGNITIVE PROSTHESIS; COGNITIVE SCIENCE; COMPLEXITY THEORY; ELIZA; LIMITS OF COMPUTER POWER; NEURAL NETWORK; TURING TEST.ASC the function, in BASIC, that finds the ASCII code number associated with a given character. (See ASCII.) For example, ASC(”A”) is 65 because the ASCII code of the character A is 65 (expressed in decimal).ascender the part of a printed character that rises above the body of the let-ter. For instance, the letter d has an ascender and the letter o does not.See DESCENDER; TYPEFACE; X-HEIGHT.FIGURE 15. AscendersASCII (American Standard Code for Information Interchange) a standard code for representing characters as numbers that is used on most micro-computers, computer terminals, and printers. In addition to printable characters, the ASCII code includes control characters to indicate car-riage return, backspace, etc. Table 3 (page 31) shows all the ASCII char-acter codes as decimal numbers. For two popular extensions of ASCII, see IBM PC (character set chart) and ANSI character set (Table 2, page 23). For alternatives to ASCII, see EBCDIC and UNICODE.ASCII file a text file on a machine that uses the ASCII character set. (See TEXT FILE.) In Windows, each line ends with a carriage return and line feed (code 13 and 10). 31 ASCII graphicsTABLE 3ASCII CHARACTER CODES (DECIMAL)FIGURE 16. ASCII graphicsASCII graphics an amusing technique of drawing pictures using only the standard keyboard characters (Figure 16). By making use of the intrin-sic shapes of the characters, or of their relative densities, the artist can render surprisingly realistic graphics. These pictures are best when designed and displayed in a fixed-pitch font. (See FIXED-PITCH TYPE.) ASSEMBLY LANGUAGE; MACHINE LANGUAGE;ASIC 32Because ASCII graphics uses only standard characters, these pictures can easily be transmitted by e-mail or in newsgroup postings and are popular in a SIGNATURE FILE.ASIC (Application Specific Integrated Circuit) an integrated circuit (sili-con chip) specifically made for a particular complete piece of electronic equipment. For example, the video controller in a typical PC is an ASIC designed specifically for use on that particular make and model of video card, whereas the memory chips are standard ICs also used in other types of computers.Ask.com a search engine originally called “Ask Jeeves” that began in 1996 as a natural-language search engine able to understand English questions such as “what is a platypus?” In February 2005 it was reformed as a con-ventional search engine at Ask.com.ASP1. (Active Server Pages) on Windows web servers, a system for gener-ating web pages partly or completely by computation, not by launching a separate program as in CGI, but by running scripts interpretively within the web pages as they are delivered to the client. Thus, a web page can be written partly in VBscript or another programming language in order to do computation as it is being served.The current version of ASP, using the .NET Framework, is called ASP.NET. See IIS. Contrast CGI (definition 1); PHP.2. (Application Service Provider) a network service provider that also provides application software, such as networked database programs.Compare INTERNET SERVICE PROVIDER, CONTENT PROVIDER.aspect ratio the ratio of height to width.ASPI (Advanced SCSI Programming Interface) a standard way for appli-cation programs to access SCSI hardware. See SCSI.assembler a computer program that translates assembly language intomachine language. SeeCOMPILER.assembly in Microsoft’s .NET Framework, a set of files containing soft-ware components that work together as a single program. All the com-ponents are explicitly and uniquely identified so that accidental substitutions cannot occur. (Assembly in this sense has nothing to do with assembly language.) See .NET FRAMEWORK. Contrast DLL HELL.assembly language a computer language in which each statement corre-sponds to one of the binary instructions recognized by the CPU. Assembly-language programs are translated into machine code by an assembler.Assembly languages are more cumbersome to use than regular (or high-level) programming languages, but they are much easier to use than 33 assignment statementpure machine languages, which require that all instructions be written in binary code.Complete computer programs are seldom written in assembly lan-guage. Instead, assembly language is used for short procedures that must run as fast as possible or must do special things to the computer hardware. For example, Figure 17 shows a short routine that takes a number, checks whether it is in the range 97 to 122 inclusive, and subtracts 32 if so, oth-erwise leaving the number unchanged. (That particular subtraction hap-pens to convert all lowercase ASCII codes to their uppercase equivalents.)This particular assembly language is for the Intel 8086 family of proces-sors (which includes all PC-compatible computers); assembly languages for other processors are different. Everything after the semicolon in each line is a comment, ignored by the computer. Two lines (PROC and ENDP) are pseudo instructions; they tell the assembler how the program is organized. All the other lines translate directly into binary codes for the CPU.Many of the most common operations in computer programming are hard to implement in assembly language. For example, there are no assembly language statements to open a file, print a number, or compute a square root. For these functions the programmer must write compli-cated routines from scratch, use services provided by the operating sys-tem, or call routines in a previously written library. Example of IBM PC assembly language Accepts a number in register AX; subtracts 32 if it is in the range 97-122; otherwise leaves it unchanged.SUB32 PROC ; procedure begins here CMP AX,97 ; compare AX to 97 JL DONE ; if less, jump to DONE CMP AX,122 ; compare AX to 122 JG DONE ; if greater, jump to DONE SUB AX,32 ; subtract 32 from AXDONE: RET ; return to main programSUB32 ENDP ; procedure ends hereFIGURE 17. Assembly languageassignment statement a statement in a computer language that calculates the value of an expression and stores that value in a variable. For exam-ple, this is an assignment statement in C:y = sqrt(a*x*x + b*x + c);This statement calculates the value of ax 2  bx  c and then gives that value to the variable y. Another example isi = i + 1;which makes i take on a value 1 greater than its previous value. HAYES COMPATIBILITY.associate 34associate to tell a computer that a particular file should always be processed by a particular program, so that the next time the user opens the file, that program will automatically start.Under Windows, associations are largely based on filename exten-sions. (See EXTENSION, definition 2.) To change the association of a file extension, open any directory (folder); then select View, Folder Options, File Types. Associations can also be changed by editing the REGISTRY.asterisk the star-shaped character *. An asterisk is usually used to mark a footnote.In Windows, UNIX, and other operating systems, an asterisk is used as a wild card character, to match any characters occurring in a particu-lar place in a filename. For example, if you type dir *.exe at a command prompt, you will get a list of all files whose names end in exe. See WILD CARD.FIGURE 18. Asterisksasynchronous not synchronized. For example, most computer terminals use asynchronous data transmission, in which the terminal or the com-puter is free to transmit any number of characters at any time. The bits constituting each character are transmitted at a fixed rate, but the pauses between characters can be of any duration.asynchronous discussion a discussion where the participants do not all have to be present at the same time. For example, in a newsgroup, participants can read comments about a particular topic that have been previously posted by other participants and then add to them. SeeNEWSGROUP.AT (Advanced Technology) the class of IBM PCs originally introduced in 1984 using the 80286 microprocessor and a 16-bit bus. See IBM PC.Contrast XT.AT command set the set of commands used to control Hayes modems;they all begin with the letters AT. Seeat sign the symbol @, which stands for “at” and was originally used in price quotes (e.g., 4 items @$3 each) and is now used in e-mail addresses (jones@somewhere.com). Some colorful nicknames for @ include “elephant’s ear” and “cinnamon bun.”In Windows .bat files, commands beginning with @ are not displayed on the screen. FIGURE 19. At sign ARC TANGENT; TRIGONOMETRIC FUNCTIONS.35	AUPATA (AT Attachment, where AT means PC AT) the interface used by IDE hard disks, essentially a buffered connection to the system bus. See also IDE.ATA over Ethernet see AOE.ATAPI (AT Attachment Packet Interface) the interface used by CD-ROM drives and other devices other than hard disks that are connected to an IDE hard disk controller.Athlon a high-speed Pentium-compatible microprocessor made by AMD.ATM1.	online abbreviation for “at the moment” (i.e., now).2.	in banking, abbreviation for automatic teller machine.3.	in networking, abbreviation for Asynchronous Transfer Mode, a set of high-speed data transmission protocols. See PROTOCOL. ContrastIPX/SPX; NETBEUI; TCP/IP.ATN the function, in BASIC, that computes the trigonometric arc tangent.Seeattachment a file transmitted as part of a piece of electronic mail. See ELEC-TRONIC MAIL.atto- metric prefix meaning ÷1,000,000,000,000,000,000. Atto- is derived from the Danish word for “eighteen” (because it signifies 10–18). SeeMETRIC PREFIXES.attributes the properties of files in Windows and similar operating sys-tems. Files can be marked as hidden, system, read-only, and archive (which means that they have changed since last backed up to tape). The attrib command displays and changes file attributes.auction a sale in which buyers make bids (offers) and the highest offer is accepted. For information about online auctions see EBAY. See alsoDUTCH AUCTION; RESERVE PRICE; BID.audio sound, represented by means of electronic signals. See also SOUND CARD; FM SYNTHESIS; WAVETABLE SYNTHESIS; MIDI. Contrast VIDEO.FIGURE 20. Audio (sound) waveform displayed graphicallyaudit trail a record kept by a computer program that shows how data was entered into the computer. These records are essential for ensuring the reliability of financial data processing systems.AUP see ACCEPTABLE-USE POLICY. autodimensioning	36autodimensioning a CAD feature that keeps imported graphics correctly scaled as the drawing or diagram is completed.AUTOEXEC.BAT in DOS and early versions of Windows, a file that con-tains commands to be executed as the computer boots up. It is stored in the root directory of the boot disk, which is normally drive C. See BAT FILE.In Windows NT, 2000, XP, and their successors, the function of AUTOEXEC.BAT has been taken over by the Registry. However, if an AUTOEXEC.BAT file exists, Windows normally reads it and executes the SET and PATH commands in it. Whether this happens is controlled by the reg-istry key ParseAutoexec. A separate file, AUTOEXEC.NT, is executed at the beginning of every DOS-mode program. See also CONFIG.SYS.autojoin a feature of drawing programs that automatically joins endpoints that are within a certain distance of each other, so that you can draw a closed curve without having to come back to the exact pixel where the curve started. (See Figure 21.) If you are having trouble getting curves to close so you can fill them, try increasing the autojoin setting. See your software manual for details.FIGURE 21. Autojoinautomagically (slang, comical) automatically, as if by magic.AutoPlay the operation that happens automatically when a CD or DVD is inserted into the drive, unless it is disabled. Audio and video discs are played automatically; Windows software discs behave as specified in their AUTORUN.INF files.autorun.inf the file on a CD-ROM or DVD-ROM that tells Windows how to AUTOPLAY it. The file specifies the name of a program to execute and can also specify an icon to identify the disc.autotrace (DRAW PROGRAM) a command that instructs the computer to fit a curve to the outline of a bitmap. When the bitmap is traced (converted into VECTOR GRAPHICS), it is more easily manipulated in the drawing program. FIGURE 22. Autotracing a bitmap 37	AX.25AV1.	abbreviation for audio-visual; pertaining to the recording and repro-duction of sounds and pictures.2.	(for “aperture value,” on a digital camera) aperture-priority autoex-posure. See A. Contrast P, TV, S.avatar an image representing the user of a visual chat or virtual reality pro-gram. Avatars range from simple images to complex, personalized three-dimensional models. In Hinduism, an avatar is an incarnation or materialization of a god.AVI file (Audio-Video Interleave file) a file containing video and audio (i.e., moving pictures with sound) in any of several formats, identified by a filename ending in .avi. Compare QUICKTIME.AWK a programming language for scanning text files and processing lines or strings that match particular patterns, now largely superseded by Perl. AWK was developed by A. V. Aho, P. J. Weinberger, and B. W. Kernighan, and the name “AWK” is an acronym composed of their ini-tials. Compare PERL; PYTHON; REXX.AWT (Abstract Window Toolkit) a set of tools provided in Java for using a graphical user interface. Crucially, AWT is not tied to Windows, Mac OS, or any other operating system; programs that use AWT are portable.AX.25 a standard format used in amateur radio for transmitting data in packets. It is an adaptation of the ITU-T (formerly CCITT) X.25 stan-dard. See PACKET RADIO; X.25. B-spline	38BB-spline a smooth curve that approximately connects two points. B-splines can be joined together to make a smooth curve passing close to any num-ber of points. For an illustration see SPLINE.Each segment of a B-spline curve is influenced by four points—the two that it lies between, plus one more in each direction. This makes computation of a B-spline much quicker than computation of a cubic spline, because every part of a cubic spline is influenced by all the points to be joined.To plot a B-spline defined by four points (x1,y1), (x2,y2), (x3,y3), and (x4,y4), let t range from 0 to 1 and compute values of x and y for each t as follows:a = –t3/6 + t2/2 – t/2+1/6 b = t3/2 – t2 + 2/3c = –t3/2 + t2/2+ t/2 + 1/6 d = t3/6x = ax1 + bx2 + cx3 + dx4 y = ay1 + by2 + cy3 + dy4That gives you a curve that lies approximately between (x2,y2) and (x3,y3). You can then advance by one point (letting the old x4 become the new x3, and so on) to plot the next segment. See also BÉZIER SPLINE; CUBIC SPLINE.Babbage, Charles (1791–1871) inventor of a number of computing machines, including the “Analytical Engine,” which introduced concepts that were later used in electronic computers. Babbage was the first to envision a machine controlled by a program stored in its memory.back browser command that returns you to the most recently viewed web page.back door an alternate way of entering a computer system. For example, the original programmer of the system may have programmed a secret way of logging onto the system not requiring the normal password entrance. Viruses often create backdoors.back end the part of a computer system not directly interacting with the user. For example, a database system running on a mainframe computer is the back end of a system, whereas the microcomputers used by those accessing the system are the front end of the system. See THREE-TIERARCHITECTURE.back one drawing program command that sends the selected object down one level. See also ARRANGE; DRAW PROGRAM.back one; send backward comparable commands that send the selected object down one layer. See also ARRANGE; BACK ONE; DRAW PROGRAM;FORWARD ONE. 39	backtrackingbackbone the main communication path in a WIDE-AREA NETWORK; the set of cables or connections that carries most of the traffic. Other data paths branch off from the backbone.background the field or color against which objects are drawn or displayed on the screen.background execution the continued execution of a program while it is not visible on the screen (or does not occupy much of the screen) and the user is free to run other programs at the same time. Background execu-tion is possible only in multitasking operating systems. See MULTITASK-ING; OS/2; UNIX; WINDOWS (MICROSOFT).backlit illuminated from behind (as in the liquid crystal displays on laptop computers and some calculators).backronym an acronym that is made up by choosing the abbreviation first and then (possibly much later) finding words to fit it. The name of the programming language BASIC (said to stand for Beginner’s All-purpose Symbolic Instruction Code) appears to be a backronym.Compare ACRONYM, ANACRONYM.backslash the character \ as opposed to the forward slash /.backtracking a method of solving problems by trying various combina-tions of moves until a successful combination is found.Backtracking works as follows: First, choose a possible move and make it. Then, proceed from there by choosing a possible move, and so on, until a solution is found. If you reach a point where you have not found a solution but no more moves are possible, back up to the most recent untried alternative. That is, undo one or more moves until you get back to an alternative you did not take. Follow that different choice to see if the solution lies in that direction. If not, keep backing up until you find what you’re looking for (you may have to try all possible moves).Backtracking is built into the programming language Prolog, which uses it extensively (see PROLOG). In this article we will discuss how to implement backtracking in a conventional language.For example, consider this problem: Find three whole numbers x[l], x[2], and x[3], each between 1 and 5, such that their sum is equal to their product. While you might try to solve this problem mathematically, it is easier just to try all the combinations. One way to get all the com-binations is to set up three nested loops as follows:FOR x[1]:=1 to 5 DO FOR x[2]:=1 to 5 DOFOR x[3]:=1 to 5 DOIF x[1]+x[2]+x[3] = x[1]*x[2]*x[3] THEN write (x[1],x[2],x[3],’ is a solution’); This is possible because you know in advance that the solution will take three moves (choosing one number in each move), so you can set up three loops. backtracking	40 Backtracking occurs in this example because if the innermost loop doesn’t find a solution, control returns to the middle loop, which incre-ments x[2] and starts the innermost loop over again. Similarly, the mid-dle loop backtracks to the outer loop if it fails to find a solution. The loop counters keep track of untried alternatives, but it’s important to under-stand that any method of generating alternatives one by one would work just as well.With some problems, you don’t know how many moves a solution will take. In such a case you must use recursion to nest the loops at run time. Figure 23 shows a program that solves the problem we’ve just dis-cussed but uses recursion rather than nested loops.class backtrack {/*	Java program that demonstrates backtracking	*//*	This program displays all triples x[1], x[2], x[3],		for values of the x’s from 1 to 5, and displays ”**”		next to a triple if x[1]+x[2]+x[3]=x[1]*x[2]*x[3]	*/static int x[]={0,0,0}; static void choose(int n){if (n<3){for (int i=1; i<=5; i++){x[n]=i; choose(n+1);}}else{System.out.print(” (”+x[0]+”,”+x[1]+”,”+x[2]+”)”); if ((x[0]+x[1]+x[2])==(x[0]*x[1]*x[2])){System.out.print(”**”);}else{System.out.print(” ”);}if (x[2]==5){System.out.println(” ”);}}}public static void main(String args[]){choose(0);}}FIGURE 23. Backtracking In Prolog, where backtracking is built in, all you have to do is spec-ify possible values for the integers and specify the condition they must meet. You can solve our example problem by typing the query: ARCHIVAL STORAGE; HARD DISK MANAGEMENT; GRANDFATHER TAPE;41	Backus normal form ?- member(X1,[1,2,3,4,5]), member(X2,[1,2,3,4,5]), member(X3,[1,2,3,4,5]), Sum is X1+X2+X3, Product is X1*X2*X3, Sum == Product, write([X1,X2,X3]).Each clause in the query specifies a condition that the answer must meet. Whenever the computer encounters a clause whose condition cannot be satisfied, it backs up to an untried alternative in a previous clause and tries again.backup copy a copy of working programs and related files that can be used to restore lost or damaged programs and files. You should have a full backup copy of your hard disk on CD-R discs, DVD-R discs, or an external hard disk stored separately from the computer. It’s a good idea to also make daily backups of work in progress. Store your backup copies in a secure place, preferably a fireproof box. Then, in case of any hardware or software problems, you will be able to restore your files.See alsoCD-R.Backus-Naur form (BNF) a notation used to describe the syntax of lan-guages. BNF was devised by J. Backus and Peter Naur and introduced in the first official description of ALGOL 60 (see ALGOL); it is some-times referred to as Backus normal form.Each BNF statement describes some syntactic unit by giving one or more alternative expansions of it, separated by the symbol |. For example, the following is a BNF description of the assignment statement in BASIC (we assume that <line number>, <variable name>, and <expression> have already been defined, and that <empty> stands for the absence of any writ-ten symbol):<let keyword> ::= LET | <empty><assignment statement> ::=<line number> <let keyword> <variable name> = <expression>The first statement defines <let keyword> as standing for either the word let or no symbol, in order to indicate that the word let is optional. The second statement defines <assignment statement> as consisting of a line number, an optional let keyword, a variable name, an equals sign, and an expression.Many languages contain syntactic rules that cannot be expressed in BNF; for instance, BNF provides no way to say that an integer cannot exceed 32,767. Still, BNF descriptions are handy because they are con-cise and definitive and because parsers can be generated directly from them. See PARSING.Backus normal form see BACKUS-NAUR FORM. backward compatible	42backward compatible able to substitute for an older computer or operat-ing system. For example, Windows Vista is mostly backward compati-ble with DOS; that is, it can run most DOS software.bacn (slang; pronounced “bacon”) pesky e-mail that arrives because you subscribed to it or gave permission for it to be sent to you; something resembling SPAM but slightly more desirable. See SPAM.BAK chat-room abbreviation for “back at keyboard” and ready to resume the conversation. Contrast AFK.balloon popup, balloon prompt a message that pops up on the Windows taskbar in the shape of a balloon or cartoon speech bubble (Fig. 24).FIGURE 24. Balloon popup (balloon prompt)banding the appearance of strips of colors in an image due to the inherent difficulty of representing gradations of tones with a limited color palette. See Figure 25. Increasing the number of bands gives a smoother color transition.FIGURE 25. Banding (right)bandwidth the rate at which a communication system can transmit data; more technically, the range of frequencies that an electronic system can transmit. High bandwidth allows fast transmission or the transmission of many signals at once. On a monitor screen, high bandwidth provides a sharp image. On a computer network, the bandwidth of a connection is limited by the slowest link in the chain connecting two computers.bang (slang) the character !, better known as the exclamation mark. 43	bar graphbank switching the ability to use more than one set of memory chips at dif-ferent times, while giving them the same addresses. This makes it possi-ble to equip a computer with more memory than it was originally designed for—simply store some data in one set of chips, then switch over and use the other set of chips.Bank switching was used to give the original IBM PC more than 640K of memory (called expanded memory). Bank switching is also used on advanced color graphics cards.banner1.	an area of a web page reserved for an advertisement. See BANNER AD.2.	a sign made by piecing together pages of computer printout.3.	an extra page with an identifying name in large letters, printed to identify a printout.banner ad an advertisement placed on a web page by a third party, who provides free services or payment in return. Many useful web sites, such as www.weather.com, are supported by banner ads. Some sites provide free web space to individuals in return for being allowed to place banner ads on the web pages.A “banner” is an area of a web page reserved for such an ad. ComparePOP-UP AD, POP-UNDER AD.It is unwise to allow a third party to place on your web page ads that you cannot control. Generally, the ad resides on the advertiser’s site, and the advertiser can change its content at any time. Web users have been embarrassed to find that formerly innocuous banner ads have changed into obnoxious material or even ads for their direct competitors.bar code a pattern of wide and narrow bars printed on paper or a similar material. A computer reads the bar code by scanning it with a laser beam or with a wand that contains a light source and a photocell. The most familiar bar code is the Universal Product Code (Figure 26), used with cash registers in supermarkets, but bar codes have been utilized to encode many kinds of data, including complete computer programs. Circular bar codes are sometimes used on boxes or pieces of luggage that may be scanned from many different directions.FIGURE 26. Bar codebar graph a type of chart that displays information by representing quan-tities as rectangular bars of different heights. Sometimes symbols are stacked or stretched to the appropriate heights to lend some visual inter-est to the chart (see PICTOGRAPH, definition 2). bare metal	44Bar graphs are usually preferred for representing and contrasting data collected over a period of time.FIGURE 27. Bar graphbare metal (slang) the computer hardware itself. “Programming to the bare metal” means controlling the hardware directly rather than relying on operating system services. See API.barebone system, barebones system (slang) a partly assembled computer consisting of just the “bare bones” of the system, usually a case, power supply, and motherboard with no memory or disk drives.base1.	the middle layer of a bipolar transistor. See TRANSISTOR.2.	a number raised to an exponent; for example, in y = ax, a is the base.3.	the number of digits used in a number system. For example, decimal numbers use base 10 and binary numbers use base 2. See BINARY NUM-BER; DECIMAL NUMBER; HEXADECIMAL NUMBER; OCTAL.4. in BASIC, the starting point for numbering the elements of an array. For example, with base 1, an array declared as X(2) will have two ele-ments X(1) and X(2). With base 0, it would also have an element X(0).base-10 (describing a number) written with the conventional digits 0 to 9 in the usual way, as opposed to binary, octal, or hexadecimal.base-16 see HEXADECIMAL NUMBER.base-2 encoded using just the digits 0 and 1; BINARY NUMBER.base-8 see OCTAL.Base64 a system, used in MIME and other contexts, for encoding any kind of data in printable, nonblank ASCII characters so that they can be sent through e-mail systems that only handle text.To do the encoding, every group of three 8-bit bytes is treated as a 24-bit sequence, which is split into four 6-bit fields, each of which is encoded as a base-64 number using A-Z, a-z, 0-9, +, and –, in that order as the 64 digits, and = as a padding character in unused positions at the end.Base64 files contain lines of equal length (usually 64 characters) con-sisting of seemingly random letters and digits with no spaces or punctu-ation. The UNIX uuencode and uudecode commmands convert to and from this format. 45	BAT file (batch file)baseband the range of frequencies needed to convey a signal itself, with-out a higher-frequency carrier. For example, the video signal from a TV camera is a baseband signal. When modulated onto a radio-frequency carrier so that it can share a cable with many other TV signals, it becomes a broadband signal. Likewise, baseband Ethernet carries one packet at a time; broadband Ethernet carries many different packets, or data packets plus other types of signals, on different high-frequency car-riers. See BROADBAND.BASIC (Beginner’s All-purpose Symbolic Instruction Code) a computer language designed by John Kemeny and Thomas Kurtz in 1964 and pop-ularized by Microsoft in the 1970s.BASIC is good for programming simple calculations quickly, and you do not have to learn much of the language in order to begin using it. Because no declarations are required, programs can be quite short.Figure 28 shows a simple BASIC program and the results of running it. In  the  1970s  and  1980s,  numerous  software  vendors,  especially Microsoft, added features to BASIC to support STRUCTURED PROGRAM-MING and a wide variety of DATA STRUCTURES. Today, BASIC is one of the most complex programming languages in wide use, incorporating fea-tures from Pascal, FORTRAN, and C. Line numbers are no longer nec-essary. However, BASIC is still easy for beginners to use, and the newest BASIC compilers still accept programs written in Kemeny and Kurtz’soriginal language.Usage note: Since it is an acronym, BASIC is usually written in all capital letters. Newer Microsoft publications, however, write Basic by analogy to Pascal. We spell it that way when referring to newer Microsoft products. See VISUAL BASIC.10 REM Temperature conversion program20 PRINT ”Temperature in Fahrenheit”;30 INPUT F40 LET C=(F-32)*5/950 PRINT F;” F equals ”;60 PRINT C;” C”70 ENDTemperature in Fahrenheit? 98 98 F equals 36.66667 CFIGURE 28. BASIC program and its outputBAT file (batch file) in Windows, a file whose name ends in .BAT and that contains a list of commands. For example, if you store the commandsdir a: dir b: dir c:on a file called THREEDIR.BAT, then you can type threedir and the three dir commands in the file will all be executed in succession. See alsoAUTOEXEC.BAT. batch processing	46batch processing the noninteractive use of computers. In batch processing, the user gives the computer a “batch” of information, referred to as a job—for example, a program and its input data—and waits for it to be processed as a whole. Batch processing contrasts with interactive pro-cessing, in which the user communicates with the computer by means of a terminal while the program is running. The crucial difference is that with batch processing the user must put all of the data into the computer before seeing any of the results, while with interactive processing the user can decide how to handle each item on the basis of the results obtained with earlier items.baud a unit that measures the speed with which information is transferred. The baud rate is the maximum number of state transitions per second: for instance, a system whose shortest pulses are 1/300 second is operating at 300 baud.On an RS-232 serial link, the baud rate is equal to the data rate in bits per second (bps). With other kinds of communication, the data rate may be considerably faster than the baud rate. For instance, a 2400-baud modem takes 2400-baud (2400-bps) serial data and encodes it into an audio signal whose true baud rate is 600, near the maximum rate at which a telephone line can transmit pulses. Each pulse carries more than one bit of information. At the receiving end, another modem transforms the signal back into 2400-baud serial data.Modems connected to telephone lines are still used extensively, but a large amount of Internet traffic is moving to faster BROADBAND services.bay a place provided for a disk or tape drive in a computer enclosure.Bayer matrix (or Bayer filter mosaic) the pattern of alternating red, green, and blue filters placed in front of the pixels of a color CMOS or CCD image sensor (Fig. 29), invented by Bryce Bayer (pronounced BY-er) of Kodak.The idea behind the Bayer matrix is that adjacent points in an image are likely to be the same color, even if they are not the same brightness. Thus, each pixel determines its own brightness, but its color is deter-mined by comparing it to two adjacent pixels with differently colored fil-ters. Green gets twice as many filters as the other two primary colors because it is in the middle of the visible spectrum.The filters do not block complementary colors completely; they only make each pixel prefer a specific color. The process of decoding colors (DE-BAYERIZATION) is done by the CPU in the digital camera.Bayes’ theorem the formulaPr (B|A)Pr (A)Pr(A|B) =Pr (B|A)Pr (A) +  Pr (B|A ′ )Pr (A′)where A and B are two events; Pr(A|B) is the conditional probability that event A happens given that event B happens; Pr(B|A) is the probability 47	Bayesian spam filterof B given A, and A′ is the event A complement (the event that A does not happen). The vertical line is read as “given that.”For example, if you have a jar with 2 big red marbles, 4 big blue mar-bles, 6 small red marbles, and 8 small blue marbles, and you randomly choose one marble, the probability that it will be a big marble given that it is a blue marble will be:Pr (blue | big )Pr (big)Pr(big| blue) =Pr (blue | big )Pr (big ) + Pr (blue | small )Pr (small)								4		6									=						6			20										4			6				8			14					6		20			14			20																										4									4																							4		1	=					20						20							4		8				12	12    3	20		20				20						In words: “There is a one-third probability that a marble will be big, given that it is blue”; or another way of saying it: “one-third of the blue marbles are big.”The theorem is named after Rev. Thomas Bayes, English theologian and mathematician (1702–1761).See BAYESIAN SPAM FILTER.FIGURE 29. Bayer matrix of red, green, and blue filtersBayesian spam filter a spam filter that uses BAYES’ RULE to adjust its esti-mate of the probability that a message is spam. Some words have a high probability of being contained in spam, and other words have a high probability of being contained in legitimate e-mail. These probabilities can be different for different users, so a Bayesian spam filter can adjust bbl	48probabilities for different users as users train the system by indicating whether a particular message is spam or not. In general, the Bayesian approach to statistics views probabilities as being conditional on avail-able information, and provides a way to revise them as more information is obtained. However, spammers can try to subvert the filter by includ-ing extraneous text in the message that the filter is likely to think is legit-imate, or misspelling words that have a high probability of being contained in spam.bbl chat-room abbreviation for “[I’ll] be back later.”BBS (bulletin board system) an online message board, especially those that were run on non-networked computers with dial-up modems in the 1980s. See MESSAGE BOARD.Bcc business abbreviation for “blind copies” (compare CC). In e-mail head-ers, Bcc: precedes additional addresses to which copies of the message should be sent. Unlike the Cc: header, the Bcc: header is not sent out with the message, so the copies are “blind” (i.e., recipients of the mes-sage do not know that copies are being distributed).Using Bcc:, you can send a message to a large number of people with-out giving them each other’s e-mail addresses, thus protecting their privacy.BCD see BINARY-CODED DECIMAL.BD see BLU-RAY DISC.bells and whistles elaborate features added to a computer program. The phrase bells and whistles usually connotes that the features are unneces-sary and confusing; it originally referred to sound effects devices on the-ater organs in the silent movie era.benchmark a computer program used to test the performance of a computer or a piece of software. For example, the speed with which computers do arithmetic is often measured by running a prime-number-finding algo-rithm called the Sieve of Eratosthenes.Benchmark results are always somewhat untrustworthy because no single program tests all aspects of a computer’s operation. A particular benchmark may exaggerate a difference between two machines that is unimportant in practice, or it may conceal an important difference. See also MIPS.Beowulf a type of cluster computing system using machines running LINUX, named for a hero of early English literature. See www.beowulf.org.Berners-Lee, Tim developer of the hypertext concepts that became the World Wide Web, while he was at the European physics laboratory CERN in the early 1990s.best practices highly recommended procedures for maintaining computer security or performing other management tasks. 49	big-endianbeta testing the second stage of testing a new software product that is almost ready for market. Beta testing is carried out by volunteers in a wide variety of settings like those in which the finished product will be used. Contrast ALPHA TESTING; GAMMA TESTING.Bézier spline a curve that connects two points smoothly and is further defined by two more points that it does not pass through. Most draw pro-grams represent curves as Bézier splines. For an illustration, see SPLINE. (Bézier is pronounced, roughly, “bay-zee-ay.”)A Bézier spline can be thought of as a gradual transition from one line to another. Call the four control points that define the curve P1, P2, P3, and P4. Then the curve starts out heading from P1 toward P2. But it curves around so that by the time it gets to P4 it is approaching from the direction of P3.To plot a Bézier spline, let (x1,y1), (x2,y2), (x3,y3), and (x4,y4) by the coordinates of P1 . . . P4 respectively, and let t range from 0 to 1. Thencompute x and y from t as follows:a = –t3 + 3t2 – 3t + 1 b = 3t3 – 6t2 + 3tc = –3t3 + 3t23x = ax1 + bx2 + cx3 + t3x4 y = ay1 + by2 + cy3 + t y4Notice that as t goes from 0 to 1, a drops from 1 to 0, and t3 rises from 0 to 1. This computation was first described by Pierre Bézier in 1970.FIGURE 30. Bézier splineBHO (Browser Help Object) a software component that can be added to Internet Explorer to add new functions. However, BHOs have also been used as SPYWARE.bid an offer to buy something for a particular price. See AUCTION; EBAY;RESERVE PRICE.bidirectional able to transmit data in two directions. See PARALLEL PORT.big-endian a system of memory addressing in which numbers that occupy more than one byte in memory are stored “big end first,” with the upper-most 8 bits at the lowest address. bilingual	50For example, the 16-digit binary number 1010111010110110 occu-pies two 8-bit bytes in memory. On a big-endian computer such as the Macintosh, the upper byte, 10101110, is stored at the first address and the lower byte, 10110110, is stored at the next higher address. On a lit-tle-endian machine, the order is reversed. Contrast LITTLE-ENDIAN.The terms big-endian and little-endian are from Gulliver’s Travels; they originally referred to the parties in a dispute over which end of a boiled egg should be broken first.bilingual1.	using more than one language.2.	(describing FIREWIRE cables) having a Firewire 1394b 9-pin connec-tor on one end and a Firewire 1394a 4- or 6-pin connector on the other.binary addition the process of calculating a sum of numbers expressed in base-2 form. It is one of the basic arithmetic operations performed by computers, and understanding it is the key to understanding how machines do arithmetic.If each of the numbers to be added has only one digit, addition is simple:0 + 0 = 000 + 1 = 011 + 0 = 011 + 1 = 10A circuit that implements this function is called a half adder and can be made out of standard logic gates as shown in Figure 31. Notice that there are two digits of output; the higher digit is called the carry bit because it is carried to the next column when adding multi-digit numbers.To add numbers with more than one digit, we proceed one digit at a time, starting at the right, just as with pencil-and-paper arithmetic. Each step of the addition can have three inputs: one digit from each of the num-bers to be added, plus a digit carried from the previous column to the right. Accordingly, for each column except the rightmost, we need a cir-cuit called a full adder, which takes three one-digit inputs. Its output still has only two digits because the largest value that can be obtained is 1 + 1 + 1 = 11. A full adder can be built out of two half adders (Figure 32).To add two 16-bit binary numbers, a computer needs 15 full adders and one half adder, with the carry output of each adder connected to an input of the adder to its left. All of this circuitry is part of the CPU; when the CPU receives an add instruction, it sends the contents of two registers to the inputs of the set of adders, and then stores the output in a register.binary-coded decimal (BCD) a way of representing numbers by means of codes for the decimal digits. For example, consider the number 65. In binary, 65 is 01000001, and that is how most computers represent it. But some computer programs might represent it as the code for 6 followed by the code for 5 (i.e., 0110 0101). 51	binary multiplicationThe advantage of BCD shows up when a number has a fractional part, such as 0.1. There is no way to convert 0.1 into binary exactly; it would1require an infinite number of digits, just like converting  3  into decimal(see ROUNDING ERROR). But in BCD, 0.1 is represented by the code for 1 immediately after the point, and no accuracy is lost.BCD arithmetic is considerably slower and takes more memory than binary arithmetic. It is used primarily in financial work and other situa-tions where rounding errors are intolerable. Pocket calculators use BCD.FIGURE 31. Binary addition: half adderFIGURE 32. Binary addition: full adderbinary file a file containing bits or bytes that do not necessarily represent printable text. The term binary file usually denotes any file that is not a text file, such as executable machine language code. Crucially, special software is required to print a binary file or view it on the screen.Contrast TEXT FILE.binary multiplication a basic operation in computer arithmetic. For single-digit numbers, the binary multiplication table is very simple and is the same as the Boolean AND operation (see AND GATE):0  0 = 00  1 = 01  0 = 01  1 = 1For numbers with more than one digit, the computer does something very similar to what we do to decimal numbers in pencil-and-paper arithmetic. To find 13  21 in decimal, we proceed like this: binary number	5221  136321273First, find 3  21. Then, find 10  21, and add these two results together to get the final product. Note that the product has more digits than either of the two original numbers.You can follow the same procedure to multiply two binary numbers:10101  011011010100000101011010100000100010001Notice that each of the partial products is either zero or a copy of 10101 shifted leftward some number of digits. The partial products that are zero can, of course, be skipped. Accordingly, in order to multiply in binary, the computer simply starts with 0 in the accumulator and works through the second number to be multiplied (01101 in the example), checking whether each digit of it is 1 or 0. Where it finds a 0, it does nothing; where it finds a 1, it adds to the accumulator a copy of the first number, shifted leftward the appropriate number of places.binary number a number expressed in binary (base-2) notation, a system that uses only two digits, 0 and 1. Binary numbers are well suited for use by computers, since many electrical devices have two distinct states: on and off. Writing numbers in binary requires more digits than writing numbers in decimal, so binary numbers are cumbersome for people to use. Each digit of a binary number represents a power of 2. The right-most digit is the 1’s digit, the next digit leftward is the 2’s digit, then the 4’s digit, and so on:Decimal	Binary20	= 1	121	= 2	1022	= 4	10023	= 8	100024	= 16	10000Table 4 shows examples of numbers written in binary and decimal form.See also DECIMAL NUMBER; HEXADECIMAL NUMBER; OCTAL. 53				binary subtraction			TABLE 4				DECIMAL-BINARY EQUIVALENTS								Decimal	Binary	Decimal	Binary					0	0	11	1011	1	1	12	1100	2	10	13	1101	3	11	14	1110	4	100	15	1111	5	101	16	10000	6	110	17	10001	7	111	18	10010	8	1000	19	10011	9	1001	20	10100	10	1010	21	10101						binary search a method for locating a particular item from a list of items in alphabetical or numerical order. Suppose you need to find the location of a particular word in a list of alphabetized words. To execute a binary search, look first at the word that is at the exact middle of the list. If the word you’re looking for comes before the midpoint word, you know that it must be in the first half of the list (if it is in the list at all). Otherwise, it must be in the second half. Once you have determined which half of the list to search, use the same method to determine which quarter, then which eighth, and so on. At most, a binary search will take about N steps if the list contains about 2N items.binary subtraction a basic operation in computer arithmetic. The easiest way to subtract two binary numbers is to make one of the numbers neg-ative and then add them. Circuits for doing binary addition are readily constructed with logic gates (see BINARY ADDITION). The negative coun-terpart of a binary number is called its 2-complement.Suppose that we have a number x, represented as a binary number with k digits. The 2-complement of x (written as x) isx = 2k – xThen, to find the difference a – x we can computea – x = a + x – 2kThis is easier than it looks, for two reasons. First, subtracting 2k is triv-ial, because 2k is a binary number of the form 1000, 100000, and so on, with k +1 digits. So all we have to do is discard the leftmost digit to get our k-digit answer.Second, finding the 2-complement of x is easy: just invert all the dig-its of x (changing 0’s to 1’s and 1’s to 0’s) and then add 1. See INVERTER. binary subtraction	54 Suppose we want to compute 5 – 2 using 4-digit binary representa-tions. That is, we want to compute:0101 – 0010First, change the second number to its complement, change the minus to a plus, and subtract 2k:0101 + 0010 – 10000To actually compute the complement, invert the digits of 0010 and add 1, so the whole computation becomes:0101 + (1101 + 1) – 10000Evaluate this expression by performing the two additions0101 + 1101 + 1 = 10011and then throwing away the leftmost digit, giving 0011 (= 3), which is the answer.This method for handling subtraction suggests a way to represent neg-ative numbers. Suppose we want to represent –3. Positive 3 is binary 011. Negative 3 can be represented by the 2-complement of 3, which is the binary representation of 5: 101. However, we need an extra bit to indicate that 101 indicates –3 instead of 5. The bit indicating the sign will be included as the first digit of the number, with 1 indicating nega-tive and 0 indicating positive.The range of numbers that can be represented is different than before. Without the sign bit, 4 binary digits can hold numbers from 0 to 15; with the sign bit, the numbers range from –8 to 7. The table shows how.Positive	Numbers		Negative	NumbersDecimal	Binary		Decimal	Binary				0	0 0 0 0			1	0 0 0 1		–1	1 1 1 12	0 0 1 0		–2	1 1 1 03	0 0 1 1		–3	1 1 0 14	0 1 0 0		–4	1 1 0 05	0 1 0 1		–5	1 0 1 16	0 1 1 0		–6	1 0 1 07	0 1 1 1		–7	1 0 0 1			–8	1 0 0 0				On real computers it is typical to use 16 bits (2 bytes) to store integer values. Since one of these bits is the sign bit, this means that the largest positive integer that can be represented is 215 – 1 = 32,767, and the most negative number that can be represented is –(215)= –32,768. Some pro-gramming languages also provide an “unsigned integer” data type that ranges from 0 to 65,535. 55	bitbind to associate symbols with data, or to associate one piece of data with another, in several different ways, among them:1.	to give a variable a value; to INITIALIZE it.2.	to associate a network protocol with a particular Ethernet port or the like. See PROTOCOL.3.	to map an XML document onto a set of variables or objects in Java or another programming language.4.	to put together the pages of a book.binding see BIND (all definitions).biometrics measurable physical characteristics of the human body, used to identify an individual for security purposes. They include fingerprints, the distinctive appearance of faces and eyes, and the distinctive sound quality of one’s voice. There are computer input devices to read these characteristics.BIOS (Basic Input Output System) a set of procedures stored on a ROM chip inside PC-compatible computers. These routines handle all input-output functions, including screen graphics, so that programs do not have to manipulate the hardware directly. This is important because if the hardware is changed (e.g., by installing a newer kind of video adapter), the BIOS can be changed to match it, and there is no need to change the application programs.The BIOS is not re-entrant and is therefore not easily usable by mul-titasking programs. Windows programs do not call the BIOS; instead, they use procedures provided by the operating system.BIOS enumerator the BIOS routine that tells a PLUG AND PLAY system what hardware is installed.bipolar transistor a semiconductor device formed by sandwiching a thin layer of P- or N-type semiconductor between two layers of the opposite type of semiconductor. (See TRANSISTOR.) The other general type of tran-sistor is the field-effect transistor (FET).bis Latin for “a second time,” used to denote revised CCITT and ITU-T standards. See CCITT; ITU-T.BIST (built-in self test) a feature included in newer integrated circuits and other electronic equipment. An electronic device that has BIST can test itself thoroughly whenever it is turned on. See INTEGRATED CIRCUIT.bit a shorthand term for binary digit. There are only two possible binary digits: 0 and 1. (See BINARY NUMBER.) Bits are represented in computers by two-state devices, such as flip-flops. A computer memory is a collec-tion of devices that can store bits.A byte is the number of bits (usually 8) that stand for one character. Memory is usually measured in units of kilobytes or megabytes. SeeMEMORY. bit bucket	56One important measure of the capability of a microprocessor is the number of bits that each internal register can contain. For example, the classic Z80 microprocessor had 8-bit registers. The Intel 8088, used in the original IBM PC, had 16-bit registers but only an 8-bit bus, leading to some confusion as to whether it should really have been called a 16-bit processor. Newer microprocessors have 32 or 64 bits per register. In general, a processor with a greater number of bits per instruction can process data more quickly (although there are other factors to consider that also determine a computer’s speed). See also MICROPROCESSOR.The number of colors that can be displayed is sometimes given by listing the number of bits used to represent a color. For example, a 24-bit color system uses 8 bits for red, 8 for green, and 8 for blue, so it can display 28 = 256 different levels of each of the three primary colors, or 224 = 16,777,216 different mixtures of colors. See COLOR.The term bit is also used to indicate the quality of digitized sound, as in 8 bit or 16 bit. See SAMPLING RATE.bit bucket (slang) a place where data is lost. For example, under UNIX, the filename /dev/null can be used as a bit bucket; anything written to it will be ignored, but the program will think it is successfully writing to a file.bit depth in graphics, the number of bits that are used to record the inten-sity and color of each pixel. For example, 1-bit graphics can distinguish only black and white; 8-bit graphics can distinguish 256 shades of gray or 256 colors; and 24-bit graphics can distinguish more than 16 million colors. Sometimes bit depth denotes the number of levels of each color; for example, an image in which each pixel has 8 bits each for red, green, and blue might be called either a 24-bit image or an 8-bit RGB image.bitblt (bit-block transfer, pronounced “bitblit”) the rapid copying of a block of memory or a portion of an image from one place to another.Compare BLIT.bitlocker a security feature of Vista that encrypts data on a hard drive, using an encryption key contained in a separate microchip in the com-puter, or provided on a flash drive.bitmap a graphical image represented as an array of brightness values. For example, if 0 represents white and 1 represents black, then000000000111111001000010010000100111111000000000is a bitmap of a black rectangle on a white background. Each point for which there is a value is called a PIXEL. See also DIGITAL IMAGE PROCESSING. 57	blacklistBitmaps can be imported into other application programs such as word processors and page layout programs, but you will not be able to edit bitmaps in those environments. You must use a PAINT PROGRAM to change bitmaps. Contrast VECTOR GRAPHICS. See also DRAW PROGRAM;PAINT PROGRAM.bitmap graphics a method of displaying pictures on a computer. The pic-ture is treated as a large array of pixels (see PIXEL), each of which is stored in a specific memory location. The picture is drawn by specifying the color of each pixel. Contrast VECTOR GRAPHICS. See also BITMAP; DRAW PROGRAM; PAINT PROGRAM.bitness (slang) the property of using a specific number of bits. For exam-ple, a single-precision integer and a double-precision integer differ in bitness.BITNET a wide-area network linking university computer centers all over the world. It originated in the northeastern United States in the early 1980s and was later combined with the Internet. Its most common use was to transmit electronic mail among scholars who were working together.BitTorrent a peer-to-peer file sharing system that reduces dependency on the original host (or the SEED) by having everyone who downloads the file also offer it for anonymous upload to others. The more people who download the file (and therefore host the pieces they already have), the faster the file is downloaded. This format is especially useful for large files such as rich media (movies, music, etc.). See www.bittorrent.com..biz a suffix indicating that a web or e-mail address belongs to a business (in any country). Contrast .COM. See also ICANN; TLD.black hat someone who attempts to break into computers maliciously; a villain (like the characters in old Western movies who wore black hats).Contrast WHITE HAT.BlackBerry a wireless device produced by Research In Motion, Inc., which is a combination cellular telephone, PDA, and web browser. Web address: www.blackberry.com. See also PDA.Blackberry thumb an informal name for painful repetitive stress injuries caused by excessive typing on small keyboards like the ones on Blackberries or cellular phones. See CARPAL TUNNEL SYNDROME.Blackcomb Microsoft’s code name for the version of Windows that will succeed Windows Vista. Blackcomb is better known as Windows 7.blacklist a list of senders or sites from which messages will not be accepted. Synonyms: IGNORE LIST; KILL FILE. blend	58blend1.	a drawing program command that computes the intermediate shapes between two selected objects. You would use the blend command to make the smooth highlights on a rendering of a three-dimensional object.In many ways, the blend command is like the morphing special effects we see on television commercials. You could make the letter C turn into a cat, for example. However, blend has practical applications as well as the playful ones. You can use it to create equally spaced objects, such as lines for a business form. Align two identical objects, and then set the intermediate blend steps to the desired number.2.	a paint program filter that smooths colors and removes texture over a selected area.3.	A piece of digital art where several images have been combined seam-lessly into a visually interesting whole. Figures and objects are often lay-ered so that it takes several seconds to identify what you are seeing.FIGURE 33. Blend (in a draw program)blind copies see BCC.blit block image transfer, the rapid copying of a portion of an image (or, sometimes, any type of memory contents) from one place to another. Compare BITBLT.blittable capable of being copied rapidly by BLIT.bloatware (slang) bloated software; inefficient, slow software that requires unreasonable amounts of disk space, memory, or CPU speed. Too many added features can make bloatware difficult to use (see CREEP-ING FEATURISM) and prone to crashes. Many critics claim that much mod-ern software is designed to sell computers larger and faster than are actually needed to do the computations efficiently.block move the operation of moving a section of a file from one place to another within the file. See EDITOR.block protect to mark a block of text so that it will not be split across pages when printed out. This is useful to prevent a table or formula from being broken up.blog a “web log”; a type of personal column posted on the Internet. Most blogs consist of small, plentiful entries. Some blogs are like an individ- 59	Bluetoothual’s diary while others have a focused topic, such as recipes or political news.Blogger a web site (www.blogger.com) providing one of the most popular and oldest web log services. Anyone can maintain a BLOG there and update it from any computer with an Internet connection. Blogger has been owned by GOOGLE since 2003. Compare LIVEJOURNAL; WORDPRESS;XANGA.blogosphere The world of BLOGs; the very loosely-knit community of blog writers and their audiences. The blogosphere provides important forums for political discussion and news reporting separate from the established news media.Blu-Ray disc an optical disc similar to a DVD and the same size, but read and written with a blue or violet laser, whose shorter wavelength makes a higher data density possible. Blu-Ray discs can hold 25 GB (single layer) or 50 GB (double layer). Contrast HD DVD.Blue Screen of Death (slang) (sometimes written BSOD) in Windows, a seri-ous error message displayed in white type on a blue screen, without any use of windows or graphics (see Figure 34). It usually means that the entire operating system has become inoperative. The memory addresses and file-names it displays are sometimes explained on www.microsoft.com, but they are usually meaningful to only the authors of Windows.After experiencing a “Blue Screen of Death,” one should always restart the computer in order to load a fresh copy of theFIGURE 34. Blue Screen of Deathoperating system into memory. Windows Vista usually reboots after a serious error of this type, often bypassing the blue screen.Bluetooth a standard for wireless networking of relatively slow devices in the same room; for details see www.bluetooth.org. The name alludes to a medieval Danish king. Contrast 802.11. blur	60blur a paint program filter that throws the image slightly out of focus. Blur can be repeated until the desired effect is achieved. See also MOTION BLUR.FIGURE 35. Blur filter.bmp the filename extension for files in Microsoft Windows that contain bitmap representations of images. See BITMAP.BNC connector a push-and-twist connector (see Figure 36) used to join coaxial cables in thinwire Ethernet networks and in some types of video equipment. See 10BASE-2; COAXIAL CABLE; ETHERNET. Contrast RCA PLUG.FIGURE 36. BNC connectorsboard1.	a printed circuit board for a computer, the MOTHERBOARD, or an add-on board, sometimes also called a card. Most computers contain expan-sion slots where you can add additional boards to enhance the capabilities of the machine.2.	a bulletin board system (BBS) or similar discussion forum. See BBS.boat anchor (slang) obsolete, useless machine.BODY tag used in HTML to indicate the main part of the material for a web page, as opposed to the HEAD. For an example see HTML.BOF (birds of a feather) an informal meeting of a group of computer pro-fessionals with an interest in common, held as part of a larger convention. FULL-TEXT61	Boolean variablebogus (slang. obsolescent) fake, incorrect, or useless. (In computer slang, this word covers a much wider range of meanings than in ordinary English; it can be applied to almost anything that is defective in any way.)bold a type style that appears heavier and darker than normal type. The entry terms in this dictionary are set in bold type. See WEIGHT.bomb (slang) to fail spectacularly (either computer programs or human performers); to CRASH. When a program bombs on a Macintosh, an alert box containing a picture of a bomb appears—the computer must be restarted and all changes made since the last “Save” will be lost.Contrast FREEZE UP; HANG.bookmark1.	a remembered position in a file that is being edited. Some editors let the user set bookmarks in order to return quickly to specific points in the file.2.	a remembered address on the WORLD WIDE WEB. Web browsers nor-mally let the user record the addresses of web pages in order to go directly to them in the future without having to type the address.3.	a placeholder that allows one to return to a specific point in a multi-media presentation.Boole, George (1815–1864) the mathematician who discovered that logi-cal reasoning can be represented in terms of mathematical formulas (BOOLEAN ALGEBRA). Boole’s work is the basis of modern digital com-puting.Boolean algebra the study of operations carried out on variables that can have only two values: 1 (true) and 0 (false). Boolean algebra was devel-oped by George Boole in the 1850s; it was useful originally in applica-tions of the theory of logic and has become tremendously important in that area since the development of the computer.Boolean query a query formed by joining simpler queries with and, or, and not. For example: “Find all books with author ‘Downing’ and subject‘Computers’  and  not  published  before  1987.”  See  alsoSEARCH; SEARCH ENGINE.Boolean variable a variable in a computer program that can have one of two possible values: true or false. See BOOLEAN ALGEBRA.Boolean variables are useful when the results of a comparison must be saved for some time after the comparison is done. Also, they can be operated on repeatedly to change their values. For example, the follow-ing Java program segment reads numbers in an n-element array called a (which has already been defined) and reports whether a number over 100 was encountered; the Boolean variable is used somewhat as an integer would be used to keep a running total. boot	62boolean b=false;for (int i=0; i<=n-1; i++){b = (b | (a[i]>100)); /* vertical line means OR */}System.out.println(”Was there a number over 100?”); if (b){System.out.println(”There was a number over 100.”);}FIGURE 37. Boolean variable in Javaboot to start up a computer. The term boot (earlier bootstrap) derives from the idea that the computer has to “pull itself up by the bootstraps,” that is, load into memory a small program that enables it to load larger programs.The operation of booting a computer that has been completely shut down is known as a dead start, cold start, or cold boot. A warm start or warm boot is a restarting operation in which some of the needed instruc-tions are already in memory.boot disk a disk, diskette, or CD that can be used to BOOT (start up) a computer.boot image see IMAGE (definition 2).Borland International (briefly renamed Inprise Corporation in the late 1990s) a manufacturer of microcomputer software, founded by Philippe Kahn and headquartered in Scotts Valley, California. Its first products were Turbo Pascal, an extremely popular Pascal compiler released in 1984 (see TURBO PASCAL), and Sidekick, a set of IBM PC utilities that are always resident in RAM and can be called up at any time, even in the middle of another task. Later products include compilers for C and C++, the spreadsheet program Quattro, the database program Paradox, and, most recently, Delphi, Kylix, and Java development tools. Web address: www.borland.com.boron chemical element (atomic number 5) added to silicon to create a P-type SEMICONDUCTOR.bot (slang) see ROBOT (definition 2).bottleneck the part of a computer system that slows down its performance, such as a slow disk drive, slow modem, or overloaded network. Finding and remedying bottlenecks is much more worthwhile than simply speed-ing up parts of the computer that are already fast.The Von Neumann bottleneck is a limit on computer speed resulting from the fact that the program and the data reside in the same memory. Thus, at any moment the CPU can be receiving a program instruction or 63	breadcrumb menua piece of data to work on, but not both. Newer computers overcome the Von Neumann bottleneck by using pipelines and caches. See CACHE; PIPELINE; VON NEUMANN ARCHITECTURE.bounce1.	to return a piece of E-MAIL to its sender because of problems deliver-ing it.2.	to transfer a piece of incoming e-mail to another recipient without indicating who forwarded it.3.	(slang) to turn a piece of equipment off and on again (to POWER-CYCLEit).bounding box an invisible box surrounding a graphical object and deter-mining its size. See Figure 38.FIGURE 38. Bounding boxbox1.	(slang) a computer, especially a small one. For example, a Linux box is a computer that runs Linux.2.	(jargon) a set of presumed limits. See THINKING OUTSIDE THE BOX.boxing (in Microsoft .NET Framework) the automatic conversion of sim-ple data types, such as numbers and STRUCTs, into OBJECTs (definition 1) so that they can be processed by object-oriented routines. See OBJECT-ORIENTED PROGRAMMING.Bps (with capital B, Bps) bytes per second.bps (with lowercase b, bps) bits per second. See also BAUD.BR HTML tag that indicates a line break. For an example see HTML.braces the characters { and }, sometimes called CURLY BRACKETS.brackets the characters [ and ], also called SQUARE BRACKETS.brb chat-room abbreviation for “[I’ll] be right back.”breadcrumb menu a menu on a WEB PAGE that indicates its place in the hierarchical organization of a web site, such as:Home > Products > Cameras > DSLR cameras This means that you are on the “DSLR cameras” page, which you probably reached from “Cameras,” which you probably reached from breakpoint	64“Products” and then from “Home.” By clicking on any of these, you can go back to them.breakpoint a place in a program where normal execution is interrupted and can be resumed after manual intervention, typically as an aid in debugging.bridge a device that links two or more segments of a network. Unlike a hub, a bridge does not pass along all data packets that it receives. Instead, a bridge examines each packet and passes it along the path to its destination. In this way, local traffic can be prevented from flooding a larger network. Compare HUB; ROUTER; SWITCH (definition 2).briefcase a feature of Windows allowing you to synchronize files that you work on using different computers, making sure that the version of the file on your main computer will include the most recent changes you made on another computer. See VERSION PROBLEM.brightness1.	a paint program filter that has the same effect as the brightness con-trol on a TV or monitor; it lightens or darkens the entire area that it’s applied to. Brightness may be combined with the contrast filter since the two attributes affect each other.2.	a software control normally available with scanners, used to adjust the overall brightness of the image.3.	the total amount of light emitted or reflected by a colored object.FIGURE 39. Brightness—light, normal, and darkbring forward; forward one comparable commands that send the selected object down one layer. See also ARRANGE; BRING TO FRONT; TO FRONT;DRAW PROGRAM; SEND BACKWARD; BACK ONE; SEND TO BACK; TO BACK.bring to front; to front comparable commands that send the selected object to the top layer. See also ARRANGE; BRING FORWARD; FORWARD ONE;DRAW PROGRAM; SEND BACKWARD; BACK ONE; SEND TO BACK; TO BACK. FIGURE 40. Bring to front 65	browserbrittle working correctly but easily disrupted by slight changes in condi-tions; the opposite of ROBUST.broadband covering a wide range of frequencies; permitting fast data transfer. In this sense, ADSL lines, T1 lines, and all kinds of Internet connections that are appreciably faster than a modem are often described as broadband.In a narrower sense, broadband denotes systems of modulating many signals onto different high-frequency carriers so that they can share the same cable. Cable television is a simple example; many video signals are delivered at once, on different frequencies. Broadband Ethernet allows many networks, or a network and other types of signals, to coexist on the same cable by using different high-frequency carriers. Contrast BASEBAND.broadcast flag a code embedded in a DIGITAL TELEVISION broadcast that is intended to prevent copying or recording by the recipient. In the United States, the FCC issued a rule requiring television receivers (including video recorders and computers) to obey the broadcast flag. This rule was overturned by the U.S. Court of Appeals in 2005 (American Library Association, et. al. v. Federal Communications Commission). The court ruled that the FCC did not have the authority to regulate the use of elec-tronic devices when they were not receiving a broadcast signal (such as when they were playing back a recording). However, Congress could require the use of the broadcast flag by legislation. The issue is a matter of ongoing dispute.broken hyperlink a link in a web page that points to a document that is no longer at that address. See also DEAD LINK.broken pipe a communication failure between two programs that are run-ning concurrently. Typically, a broken pipe occurs when a network con-nection is lost or one of the programs terminates while the other is still trying to communicate with it. See PIPE (definition 1).brownout an extended period of insufficient power-line voltage. It can damage computer equipment. See POWER LINE PROTECTION.browse1.	to explore the contents of the World Wide Web or, more generally, the Internet. See BROWSER.2.	to explore the contents of a disk drive or a computer network.browse master in Windows, the computer on the local area network that tells the other computers what shared resources are available. The browse master is chosen automatically from the computers that are on the network at a particular time.browser a computer program that enables the user to read HYPERTEXT in files or on the WORLD WIDE WEB (Figure 41). Popular World Wide Web browsers include Firefox and Microsoft Internet Explorer. See WORLD WIDE WEB; HTML; VRML; FIREFOX; INTERNET EXPLORER; OPERA. BSD	66FIGURE 41. Browser displaying a web pageBSD a version of UNIX that was developed at the University of California at Berkeley (UCB). (See UNIX.) BSD UNIX introduced the vi full-screen editor and a number of other enhancements. SunOS (Solaris) and System V are combinations of BSD UNIX with the original AT&T UNIX.BSOD see BLUE SCREEN OF DEATH.BTW online abbreviation for “by the way.”bubble sort an algorithm for arranging items in order, as follows: First, examine the first two items in the list. If they are in order, leave them alone; if not, interchange them. Do the same with the second and third items, then with the third and fourth items, until you have reached the last two. At this point you are guaranteed that the item that should come last in the list has indeed “bubbled” up to that position. Now repeat the whole process for the first n – 1 items in the list, then for n – 2, and so on.Bubble sort is not a particularly fast sorting algorithm on conven-tional computers, but it works well on parallel computers that can work on different parts of the list simultaneously.Figure 42 shows a Java program that performs a bubble sort. For clar-ity, this version of the algorithm does not use element 0 of the array.buddy list a set of online friends, with contact information. Most instant messaging programs have buddy lists, which not only keep track of your chatting buddies but show whether or not they are presently logged in. 67	bufferclass bubblesort{/* This Java program performs a bubble sort */ /* Array to be sorted and number of items in it.Element a[0], which contains 0 here, is ignored. */ static int a[] = {0,29,18,7,56,64,33,128,70,78,81,12,5}; static int n=12;public static void main(String args[]){/* Perform the bubble sort */for (int i=1; i<=n; i++){for (int j=1; j<=(n-i); j++){if (a[j]>a[j+1]){int t=a[j]; a[j]=a[j+1]; a[j+1]=t;}}}/* Display the results */for (int i=1; i<=n; i++){System.out.println(a[i]);}}}FIGURE 42. Bubble sortbuffer1.	areas in memory that hold data being sent to a printer or received from a serial port. The idea here is that the printer is much slower than the computer, and it is helpful if the computer can prepare the data all at once, and then dole it out slowly from the buffer as needed. Similarly, a serial port needs a buffer because data may come in when the computer is not ready to receive it.2.	an area in memory that holds a file that is being edited. Some editors allow you to edit more than one file at once, and each file occupies its own buffer.3.	an area in memory that holds data being sent to, or received from, a disk. Some operating systems allow you to adjust the size or number of disk buffers to fit the speed of your disk drive. See also UNDERRUN.4.	an area in memory that holds signals from keys that have been pressed but have not yet been accepted by the computer.5.	an electronic device whose output is the same as its input (e.g., an amplifier for driving long cables). bug	68bug an error in a computer program. The term is somewhat misleading because it suggests that errors have a life of their own, which they do not. Bugs fall into at least three classes: syntax errors, where the rules of the programming language were not followed; semantic errors, where the programmer misunderstood the meaning of something in the pro-gramming language; and logic errors, where the programmer specified some detail of the computation incorrectly. Nowadays there is beginning to be a serious problem with a fourth class, which can be called infra-structure errors, where the programmer fell victim to something wrong with the operating system or a programming tool.build1.	(verb) to put together a piece of software from its components by compiling, linking, and doing whatever else is necessary to make a working, deliverable version. See COMPILER; LINKER.2.	(noun) the result of building a piece of software on a particular occa-sion. Some software developers keep track of build numbers, which change much more rapidly than version numbers.built fraction a fraction that is composed by setting the numerator and denominator as regular numerals separated by a forward slash (1/2, 1/4).Contrast CASE FRACTION; PIECE FRACTION.FIGURE 43. Built fraction (center) vs. case and piece fractionsbullet a character such as • used to mark items in a list.bulletin board a message board; an online service where people can post messages. See MESSAGE BOARD.bundled software software that is sold in combination with hardware. For example, software for processing speech and music is often bundled with sound cards.burn (slang) to record information on a CD or DVD disc or an EPROM.bus the main communication avenue in a computer. For a diagram, seeCOMPUTER ARCHITECTURE.The bus consists of a set of parallel wires or lines to which the CPU, the memory, and all input-output devices are connected. The bus con-tains one line for each bit needed to give the address of a device or a location in memory, plus one line for each bit of data to be transmitted 69	button barin a single step, and additional lines that indicate what operation is being performed.Most personal computers today use a 32-bit bus, but on PC-compati-bles, the bus can also work in 8-bit and 16-bit mode.Here is how an 8-bit bus works: If the CPU wants to place the value 00011001 into memory location 10100000, it places 00011001 on the data lines, places 10100000 on the address lines, and places 1 (rather than the usual 0) on the “write memory” line. The memory unit is responsible for recognizing the “write memory” request, decoding the address, and storing the data in the right location.The bus can transmit data in either direction between any two com-ponents of the system. If the computer did not have a bus, it would need separate wires for all possible connections between components. See also EISA; ISA; PCI; PCMCIA; SERIAL BUS.button a small circle or rectangular bar within a windowed DIALOG BOX that represents a choice to be made. One of the buttons in a group is normally highlighted, either by having a black circle inside of it or having a heavy black border. This represents the DEFAULT choice. You can choose any one of the buttons by clicking on it with the mouse.There are two kinds of buttons: OPTION BUTTONS (sometimes called radio buttons) and command buttons. Option buttons represent mutually exclusive settings; that is, you can choose only one. They are usually small and round but are sometimes diamond-shaped. If you change your mind and click on another option button, your original choice will be grayed (dimmed). See CLICK; DIALOG BOX; DIMMED; HIGHLIGHT.Command buttons cause something to happen immediately when you click on them. They are usually rectangular and larger than option but-tons. The most familiar examples are the OK and Cancel buttons that are in almost every dialog box. If the command button brings up another dialog box, you will see an ELLIPSIS (. . . ) after its label.FIGURE 44. Buttons: option buttons (left) and command buttons (right)button bar a row of small icons usually arranged across the top of the workspace on the screen. Each icon represents a commonly used com-mand; many programs allow you to customize your button bar to suit your taste. bwahahahaha	70FIGURE 45. Button barsbwahahahaha typewritten representation of an evil laugh.Byron, Augusta Ada see ADA.byte the amount of memory space needed to store one character, which is normally 8 bits. A computer with 8-bit bytes can distinguish 28 = 256 dif-ferent characters. See ASCII for the code that most computers use to rep-resent characters.The size of a computer’s memory is measured in kilobytes (= 210 = 1024 bytes) or megabytes (= 220 =1,048,576 bytes).bytecode the concise instructions produced by compiling a Java program. This bytecode is the same for all platforms; it is executed by a Java vir-tual machine. See JAVA; JVM. See also .NET FRAMEWORK. 71	CCC a programming language developed at Bell Laboratories in the 1970s, based on the two earlier languages B (1970) and BCPL (1967). A C com-piler is provided as a part of the UNIX operating system (see UNIX), and C was used to write most of UNIX itself. In addition, C is popular as an alternative to assembly language for writing highly efficient microcom-puter programs. There is a widespread (and often mistaken) belief that programs written in C are more efficient than programs written in any other language.C is a general-purpose language like Pascal and ALGOL, but, unlike other general-purpose languages, it gives the programmer complete access to the machine’s internal (bit-by-bit) representation of all types of data. This makes it convenient to perform tasks that would ordinarily require assembly language, and to perform computations in the most efficient way of which the machine is capable./* CHKSUM.C *//* Sample program in C —M. Covington 1991 */ /* Based on a program by McLowery Elrod *//* Reads a character string from the keyboard */ /* and computes a checksum for it. */#include <stdio.h>#define N 256main(){int i, n; char str[N];puts(”Type a character string:”); gets(str);printf(”The checksum is %d\n”,chksum(str));}chksum(s,n) char* s; int n;{unsigned c; c = 0;while (n— >0) c = c + *s++; c = c % 256;return(c);} FIGURE 46. C program C++	72In C, things that are easy for the CPU are easy for the programmer, and vice versa. For example, character string handling is somewhat clumsy because the CPU can only do it through explicit procedure calls. But inte-ger arithmetic is very simple to program because it is simple for the CPU to execute. Most programmers who use C find themselves writing efficient programs simply because the language pushes them to do so.Figure 46 shows a program written in C. This language encourages structured programming; the three loop constructs are while, do, and for. The comment delimiters are /* */. A semicolon comes at the end of every statement (unlike Pascal, where semicolons only come between statements).C allows operations to be mixed with expressions in a unique way. The expression i++ means “retrieve the value of i and then add 1 to it.” So if i equals 2, the statement j = (i++)*3 will make j equal 6 (i.e., 2  3) and will make i equal 3 (by adding 1 to it after its value is retrieved; but notice that its old value was used in the multiplication).Another noteworthy feature of C is the #define statement. The sam-ple program contains the line#define N 256which tells the compiler that wherever N occurs as a symbol in the pro-gram, it should be understood as the number 256. See also C++.C++ an object-oriented programming language developed by Bjarne Stroustrup at Bell Laboratories in the mid-1980s as a successor to C. In C and C++, the expression C++ means “add 1 to C.”Figure 47 shows a program in C++. Comments are introduced by // and object types are declared as class. The part of an object that is accessible to the outside world is declared as public. For an explanation of what this program does, see OBJECT-ORIENTED PROGRAMMING, where the equivalent Java code is explained.In C++, input-output devices are known as streams. The statementcout << ”The answer is ” << i;sends “The answer is” and the value of i to the standard output stream. This provides a convenient way to print any kind of data for which a print method is defined.C++ lets the programmer overload operators (give additional mean-ings to them). For example, + normally stands for integer and floating-point addition. In C++, you can define it to do other things to other kinds of data, such as summing matrices or concatenating strings.C++  is  the  basis  of  the  Java  and  C#  programming  languages.See JAVA; C#. 73	CA //	SAMPLE.CPP//	Sample C++ program —M. Covington 1991//	Uses Turbo C++ graphics procedures#include <graphics.h>class pnttype { public:int x, y;void draw() { putpixel(x,y,WHITE); }};class cirtype: public pnttype { public:int radius;void draw() { circle(x,y,radius); }};main(){int driver, mode; driver = VGA; mode = VGAHI;initgraph(&driver,&mode,”d:\tp\bgi”);pnttype a,b; cirtype c;a.x = 100; a.y = 150; a.draw();c.x = 200; c.y = 250;c.radius = 40; c.draw();closegraph;}FIGURE 47. C++ programC# (pronounced “C sharp”) a programming language developed by Anders Hejlsberg (the developer of Turbo Pascal and Delphi) for Windows pro-gramming under Microsoft’s .NET Framework.C# is similar in appearance and intent to Java, but it is more tightly tied to the object-oriented operating-system interface of the .NET Framework. In some ways it reflects the spirit of Pascal, with clean and simple design, but it is fully object-oriented. Memory allocation is auto-matic, and programmers do not normally manipulate pointers. Care has been taken to make common operations simple and concise, and the han-dling of windows is especially straightforward; programmers never have to declare handlers for events that they do not actually want to handle.Figure 48 on page 74 shows a sample program in C#. Compare it to the sample Pascal program on page 356.CA (certificate authority) an agency that issues digital certificates. See CER-TIFICATE, DIGITAL; DIGITAL SIGNATURE. cable modem	74using System; class primecheck{// Sample C# program to test whether a number is prime. static void Main(string[] args){int n, i, max; bool cont; string s;while (true){Console.Write(”Type a number (0 to quit): ”); n = Convert.ToInt32(Console.ReadLine());if (n==0) break;s = ”prime”; cont = (n > 2); i = 1;max = (int)Math.Sqrt(n); // largest divisor to try while (cont){i++;Console.WriteLine(”Trying divisor {0}”,i); if (n % i == 0) //if n divisible by i{s = ”not prime”; cont = false;}else{cont = (i < max);}}Console.WriteLine(”{0} is {1} \n”,n,s);}}}FIGURE 48. C# programcable modem a MODEM that provides computer communication over tele-vision cables (either coaxial or fiber-optic) rather than telephone lines.cache1. a place where data can be stored to avoid having to read the data from a slower device such as a disk. For instance, a disk cache stores copies of frequently used disk sectors in RAM so that they can be read without accessing the disk.The 486 and Pentium microprocessors have an internal instruction cache for program instructions that are being read in from RAM; an external cache is also used, consisting of RAM chips that are faster than those used in the computer’s main memory. See also L1 CACHE; L2 CACHE. 75	Cairo2. a set of files kept by a WEB BROWSER to avoid having to download the same material repeatedly. Most web browsers keep copies of all the web pages that you view, up to a certain limit, so that the same pages can be redisplayed quickly when you go back to them. If a web page has been changed recently, you may have to RELOAD it to see its current contents.cacls (presumably: change access control lists) a powerful console-mode command in Windows 2000 and its successors for changing permissions and security attributes of files, analogous to CHMOD in UNIX. For exam-ple, the commandcacls myfile.txt /g ”Domain Users”:Rgives all members of the group “Domain Users” permission to read myfile.txt. For full documentation type:cacls /?The cacls command is used mainly in BAT files, since you can also change permissions by right-clicking on the file or folder and following the menus. See also CHMOD; PERMISSION.CAD (Computer-Aided Design) the use of a computer for design work in fields such as engineering or architecture, with the computer’s graphics capabilities substituting for work that traditionally would have been done with pencil and paper. In order to do CAD, it is necessary to have a high-resolution monitor and a software package designed for the purpose.In order to draw a building, for example, it is necessary to enter the plans by using a graphical input device, such as a mouse or graphics tablet. There are several advantages to having the plans in the computer:1.	The computer can automatically calculate dimensions. In fact, the ability to calculate dimensions is the biggest difference between CAD programs and ordinary draw programs.2.	Changes can be made easily (e.g., adding a new wall).3.	Repetitive structures can be added easily.4.	The image can be enlarged to obtain a close-up view of a particu-lar part, or it can be shrunk to make it possible to obtain an over-all view. The image can be rotated to view it from many different perspectives. For example, the Boeing 777 airplane, first rolled out in 1994, was designed entirely on computers. Previous airplanes had been designed the traditional way with paper drawings, and then a full-scale mock-up had to be constructed to make sure that the parts would fit together in reality as they did on paper. CAD made this extra work unnecessary.CAI (computer-aided instruction) teaching any kind of knowledge to peo-ple with the aid of a computer.Cairo Microsoft’s code name for a version of Windows under development during the mid-1990s. As it turned out, no single version of Windows cakebox	76corresponds to Cairo; portions of the Cairo project appeared in versions ranging from Windows 3.1 to Windows Vista. Compare BLACKCOMB; CHICAGO; LONGHORN; MEMPHIS; WHISTLER.cakebox (humorous) the round plastic box in which bulk recordable CDs are supplied.calendar see GREGORIAN CALENDAR; JULIAN CALENDAR; JULIAN DATE; LEAP YEAR.calibration adjustment of image values to ensure faithful rendering of col-ors and gray tones when output to a printer or imagesetter. The calibra-tion loop should include your scanner, your monitor, your software, and the printer. The goal is to make sure that colors are treated identically by the scanner, the software, the screen, and the printer. See COLOR.CALL1.	a statement in FORTRAN, PL/I, some versions of BASIC, and most assembly languages, which transfers control of execution to a subpro-gram. When the subprogram ends, the main program resumes with the statement immediately after the CALL. Languages such as C, Pascal, and Java perform calls by simply giving the name of the routine to be called. In line-numbered BASIC, subroutines are called with the GOSUB command.2.	computer-aided language learning, the teaching of foreign languages to people with the aid of a computer.Call Waiting a service offered by many American telephone companies that makes your telephone beep if someone tries to call you while you are already using the telephone. You can then put the previous call on hold and switch to the incoming call. These beeps disrupt transmission of computer data by telephone. To make a call that will not be inter-rupted by beeps, dial *70, wait for a dial tone, and then dial the call in the usual way. If you are using a Hayes-compatible autodial modem, give the number 555-1212 as *70W555-1212 or *70,,555-1212.callout the line and caption marking the specific parts of a labeled illustra-tion. For examples, see Figure 298 at WINDOW on page 529.-cam abbreviation for camera, especially a digital or video camera whose images are made available by computer network. For instance, a camera connected to the World Wide Web is a webcam; a camera mounted on a tower is a towercam; and a camera strapped to the back of a horse might be called a horsecam.CAM (Computer-Aided Manufacturing) the use of computers in a manu-facturing process. For example, a computer could store a three-dimen-sional representation of an object and then control the manufacture of the object by automated machinery. Some of the principles of CAM are the same as with computer-aided design (see CAD), and sometimes a sys-tem is referred to as CAD/CAM. 77	Capability Maturity Modelcamel notation a way of combining words by running them together, cap-italizing every word except the first: thisIsAnExample. A word written this way has a low head and one or more humps, like a camel. ContrastPASCAL NOTATION. See INTERCAPS.camera-ready copy artwork or printed pages that are ready to be pho-tographed and offset printed. The camera will see only black and white, not shades of gray, so the camera-ready copy must be free of smudges, dust, and stray marks. Usually, pale blue marks do not photograph, and most other colors photograph as black.camera, digital see DIGITAL CAMERA.CAN-SPAM (Controlling the Assault of Non-Solicited Pornography and Marketing) a law passed by the U.S. Congress in 2003 (15 U.S.C. 7701) providing penalties for sending deceptive mass e-mails. The act required the Federal Trade Commission to investigate whether or not to establish a national Do Not E-mail registry, but the commission concluded that such a registry would not work and instead emphasized the need to establish a means to authenticate the origin of e-mail messages.cancel1.	to stop the execution of a command. Most dialog boxes have a Cancel button. This clears the dialog box from the screen without taking any action.2.	to send a command deleting a message from a newsgroup or other public forum. (See NEWSGROUP.) It is important to know how to do this in case you post something that turns out to be redundant or misin-formed. On Usenet, the cancellation does not take effect instantly because the cancel command has to travel to all the sites that received the original message. Normally, users can cancel only their own mes-sages, but see CANCELBOT.cancelbot (from cancel and robot; see BOT) a computer program that auto-matically cancels certain messages from a newsgroup or other public forum. Cancelbots often eliminate messages that are excessively large or are copied to excessive numbers of newsgroups (see SPAM).Canon engine see ENGINE.Capability Maturity Model a set of criteria developed by the Software Engineering Institute at Carnegie Mellon University for judging and improving the performance of a software development organization. The model recognizes five levels of maturity:1.	Initial: Developers work as rugged individualists, trying to emu-late other successful software developers.2.	Repeatable: The work is planned and somewhat predictable.3.	Defined: The work is planned in some detail with a plan that can be, and is, followed.4.	Managed: The work is not only planned but measured; managers know whether the expected level of productivity is being achieved. caps	785.	Optimizing: Based on measurements, the process is continually improved.Many software companies never get past the initial stage, which is ade-quate for highly talented individuals but leaves the overall organization very dependent on their specific talent (compare HACKER, definition 1).See also SOFTWARE ENGINEERING.caps capital letters. THIS SENTENCE IS TYPED IN ALL CAPS. Contrast with MIXED CASE; LOWERCASE.Internet tip: Don’t send e-mail or post to newsgroups with your mes-sage typed in all caps. Not only is it more difficult to read, but it seems as if you are shouting (all your words are emphasized).Caps Lock a special keyboard key which acts like the Shift key for the let-ter keys. You do not have to hold it down; when Caps Lock is on, YOUR TYPING LOOKS LIKE THIS. A common mistake is to leave Caps Lock on when you wish to type normally. Then letters you wish to be capital-ized are lowercase, but everything else is all caps lIKE tHIS. Some word processors have commands to correct capitalization errors of this sort.CAPTCHA (Completely Automatic Public Turing Test to Tell Computers and Humans Apart) an on-screen device that presents a quiz requiring a human response, to prevent an automatic BOT from gaining access to a location, such as a web page selling tickets. The CAPTCHA often appears as a graphical image of a simple arithmetic problem or a word written along a wavy curve. The automated bot only sees a graphical image and can’t provide the answer to the math problem. One purpose of the CAPTCHA is to prevent ticket scalpers from using bots to pur-chase large amounts of tickets from Internet ticket sellers. However, software advances may allow the bot to solve the CAPTCHA quiz.capture1. to cause a picture or graphic to be saved as a bitmapped image. SeeFRAME GRABBER; SCREEN SHOT.2. to divert data from a serial or parallel port to a networked printer, a print spooler, or the like.Carbon application, Carbonized application (on the Macintosh) a soft-ware package written to take advantage of new features of Mac OS X, but also compatible with Mac OS 9. Contrast COCOA APPLICATION.carbon copy a copy of an outgoing electronic mail message kept by the sender or forwarded to someone other than the recipient. (See CC; BCC; FCC.) In pre-computer days, carbon copies were made in typewriters by placing a sheet of carbon-coated paper and an extra piece of plain paper behind the main document.card1. a medium for storing data, such as a FLASH MEMORY CARD, SECURE DIG-ITAL CARD, SMARTMEDIA, or COMPACTFLASH. See CARD READER. 79	carrier2.	a printed-circuit board, especially one designed to be added to a microcomputer to provide additional functions (see Figure 49).3.	see PUNCHED CARD.FIGURE 49. Card (definition 2)card reader1.	a device that enables a computer to read FLASH MEMORY CARDs, (SMARTMEDIA, COMPACTFLASH, and the like).2.	a device that enabled a computer to read PUNCHED CARDs.CardBus the 32-bit version of the PCMCIA (PC CARD) bus. See also EXPRESS-CARD and note there.caret1.	The character ˆ, more properly called CIRCUMFLEX or HAT, which indicates exponentiation in some programming languages.2.	^ the proofreading symbol used to mark where something should be inserted. (Caret is Latin for “is missing.”)3.	the INSERTION POINT (i.e., the point on the screen where characters will appear when they are typed on the keyboard).careware SHAREWARE that requests a donation to charity rather than a mon-etary payment to the author. See FREE SOFTWARE.carpal tunnel syndrome a repetitive-use injury of the carpal tunnel (a nerve pathway in the wrist) that afflicts some typists. The main symp-toms are numbness and tingling in the hand. Stretching exercises and medication help mild cases, but sometimes surgery is necessary to relieve the pain. To prevent carpal tunnel syndrome, take a break and stretch your hands frequently. Some typists find a padded wrist support to be helpful. See ERGONOMICS.carriage return see CR.carrier a signal that has another signal modulated onto it. For example, a modem transmits data through a telephone line by transmitting a contin-uous tone as a carrier. Variations in frequency and phase of the carrier TILED WINDOWS;cartridge	80encode the binary data. DSL networks use a radio-frequency carrier to transmit data over phone lines.cartridge a self-contained, removable part of a computer, usually small and contained in a plastic case. For example, laser printers often take toner cartridges (containing toner, i.e., ink). Game machines often accept soft-ware in plug-in cartridges.cascade to arrange multiple windows so that they look like a stack of cards, with all but the top and left edges of each window hidden by the one in front of it (see Figure 50). When the windows on a screen are cascaded,you can see the title bar of every window. ContrastOVERLAID WINDOWS.FIGURE 50. Cascaded windowscascading menu a menu that leads to more menus. For an example, seeSTART MENU.cascading style sheet a set of HTML rules governing the appearance of a set of pages at a web site on the World Wide Web. Cascading style sheets use precedence rules to decide which of two commands should take effect in case of a conflict. See STYLE SHEET.CASE (computer-aided software engineering) the use of computers to help with the process of designing software.case1.	the property of being capitalized (uppercase, LIKE THIS) or lowercase (like this); so called because of the wooden cases in which printers’ type was stored in the 1800s.2.	a statement in Pascal that directs a program to choose one action from a list of alternatives, depending on the value of a given variable. Here is an example of a case statement:CASE place OF1 : writeln(’First place !!!’);2 : writeln(’Second place’);3 : writeln(’Third place’) END; 81	CcIf the variable place has the value 1, then the first writeln statement will be executed, and so on. The switch statement in C and Java and the SELECT CASE statement in newer versions of BASIC perform similar functions.case fraction a small fraction that is a single character in a font. See illus-tration at BUILT FRACTION, page 68. Contrast BUILT FRACTION; PIECE FRAC-TION. See also EXPERT SET.case-sensitive distinguishing between upper- and lowercase letters, such as A and a. For example, UNIX filenames are case-sensitive, so that MYDATA and mydata denote two different files. DOS and Windows filenames are not case-sensitive, so that MYDATA and mydata are equivalent.Names typed in C, C++, Java, and C# are case-sensitive; names typed in Pascal are not.Windows preserves the case in which filenames were originally typed, but names that differ only in case are treated as matching.Cat-3, Cat-5, Cat-5e, Cat-6, Cat-7 see CATEGORY 3 CABLE (etc.).catalog an older name for a list of the contents of a disk. See DIRECTORY.catch see TRY.Category 3 cable, Category 5 cable (etc.) a series of standards for 8-con-ductor unshielded twisted-pair network cables, which can also be used for telephone wiring. They are known in brief as Cat-3, Cat-5, and so on.Category 3 cable is for 10base-T and other networks up to 10 Mbps. Category 5 is compatible with 100base-T networking at ten times the speed, and categories 5e and up are compatible with GIGABIT ETHERNET (1000base-T). Each successive category has better high-frequency per-formance and lower crosstalk. It is always desirable to use a higher cat-egory than originally specified, since performance may improve and will certainly be no worse.See also 10BASE-T; 100BASE-T; 1000BASE-T; RJ-45 (wiring cable); PLENUM-RATED; RISER-RATED.CAV (constant angular velocity), in disk drives, a constant speed of rota-tion, regardless of whether the track being read is a long one (near the edge of the disk) or a short one (near the center). Hard disks, diskettes, and some of the newest CD-ROM drives use CAV. Contrast CLV.cc	the UNIX command that invokes the C compiler, now largely replaced by gcc (GNU C Compiler). See C; UNIX. Cc business abbreviation for “copies”; the double letter C indicates the plural. In e-mail headers, Cc: precedes additional addresses to which copies of the message should be sent. See also BCC. CCD image sensor	82FIGURE 51. CCD image sensor (in webcam with lens removed)CCD image sensor (charge-coupled device) the electronic image sensor most often used in digital cameras, video cameras, and scanners. A lens forms an image on an array of cells (Figure 52), each of which contains a photoconductive layer that accumulates a charge in proportion to the amount of light that falls on it. The accumulated electrons are then shifted from cell to cell until each cell’s value appears at the output. Scanners often use sensors in which the cells are in a line rather than a rectangular array.CCDs record only the intensity of the light, not the color. In scientific work, color pictures are obtained by taking a set of black-and-white images through different filters. In most color digital and video cameras, adjacent pixels on the CCD contain pale red, green, and blue filters, so that a color image can be obtained all at once; the color of each pixel is interpolated from those around it. In rare cases this gives inaccurate color, as when photographing a distant zebra whose black and white stripes hit pixels with different color filtration. Contrast FOVEON.Inherently, CCDs respond strongly to infrared light, but most of them contain extra layers to increase the response across the visible spectrum and cut the response to infrared. A CCD camera can often “see” the infrared beam from a TV remote control.The main limitation of CCDs is that the cells conduct some electrons even in the absence of light, in a highly random manner. This shows up when the exposure is longer than about 5 to 10 seconds; the image is covered with speckles. The CCDs used for long exposures in astronomy are cooled to low temperatures to reduce this problem, and a sample of the noise itself (a long exposure with the shutter closed) is often digitally subtracted from the image.See also CMOS IMAGE SENSOR; BAYER MATRIX. 83	CDFIGURE 52. CCD image sensorCCITT (Comité Consultatif Internationale Télegraphique et Téléphonique) former name of international organization that sets stan-dards for data communication, now known as ITU-T. See ITU-T; X.25.ccTLD (country code Top Level Domain) a two-letter code indicating the country of an Internet address. For example, the web address for Oxford University is www.ox.ac.uk; here .uk is the country code for the United Kingdom. For a complete listing of the ccTLDs, see the tables on pages 552 to 554.Most codes can be used only for sites in the countries to which they belong. However, some countries, such as Tuvalu, encourage the use of their country codes elsewhere, so long as the country’s own registrar gets the registration fees. For examples see .TV; .NU; .WS. See also GTLD.CD (Compact Disc) a type of plastic optically readable disk introduced by Philips and Sony in the 1980s to store digitized music recordings. Subsequently, CDs have become popular as a way of storing computer files (see CD-ROM). The data on CDs is encoded in microscopic grooves and is read by scanning the rotating disk with an infrared laser beam. Conventional CDs are 12 cm (4.7 inches) in diameter. An 8-cm (3.15-inch) size (“mini-disc”) is also used; it has about 1/5 the capacity. CD; CD-R; DVD; RED BOOK; GREEN BOOK; YEL-CD-E	84Unlike a diskette, a CD has its data on one long spiral track, like the groove in a phonograph record; this simplifies the design of CD players. One CD can hold 75 minutes of audio (just enough for Beethoven’s Ninth Symphony, which was the design goal) or 680 megabytes of computer data. Newer CDs have slightly more capacity, 80 minutes or 700 MB.Note the spellings disc for CDs versus disk for magnetic disks. See also RED BOOK; CD-ROM; DVD.CD-E (Compact Disc—Erasable) an older name for CD-RW.CD-R (Compact Disc—Recordable) a type of CD (compact disc) that can be recorded by the user. CD-Rs have the same capacity and are readable in the same drives as ordinary CDs (except for DVD drives—see DVD). Instead of being manufactured by pressing, CD-Rs are recorded by bleaching (“burning”) small areas of dyed plastic with a laser to create an appearance similar to the indentations in an ordinary CD.CD-Rs are somewhat less durable than pressed CDs, though still more durable than most other computer media.CD-ROM (Compact Disc—Read-Only Memory) an optical disk like an audio compact disc, but containing computer data. Audio and computer CDs are physically the same; in fact, a single CD can contain both com-puter files and music. CD-ROMs can only be read, not recorded on, by the user’s computer. CD-R and CD-RW are CD-ROM-compatible media that can be recorded on by the user.Unlike diskettes, CD-ROMs are not tied to the operating system of a specific computer (although the software that is on them may be). Any computer can read the data on any CD-ROM. CD-ROM drives are rated for their speed compared with the playback speed of an audio CD. For example, a 48 drive can read a CD 48 times faster than an audio CD would be played.CD-ROMs are a popular way to deliver digitized sound, images, and multimedia presentations that would be too bulky to put on ordinary diskettes. An entire encyclopedia or a collection of hundreds of picturescan fit on one disc. See alsoLOW BOOK; ORANGE BOOK; ISO 9660; HIGH SIERRA FORMAT; MPC; MULTIMEDIA;MULTISESSION CD; PHOTO CD.CD-ROM XA (CD-ROM extended architecture) a set of extensions to the ISO 9660 format for compact disc data, allowing sound and video to be interleaved with computer data. The earlier format required sound, video, and computer files to be on separate tracks; XA removes this restriction. Virtually all CD-ROMs are now this type.CD-RW (Compact Disc—ReWritable) a type of CD (compact disc) that can be recorded, erased, and reused by the user.The surface of a CD-RW contains an alloy that can change back and forth between a bright crystalline state and a dark amorphous state; it can be switched from one state to the other by heating it to specific temper-atures with a laser. 85	centerBecause of the different material used in them, CD-RW discs cannot be read by some early-model CD-ROM or CD-R drives.CDA see COMMUNICATIONS DECENCY ACT.CDFS the CD-ROM file system under Windows 95 and later.CE see WINDOWS (MICROSOFT).Celeron a lower-cost version of the Intel Pentium II microprocessor, with less on-chip cache memory. Before its introduction, the Celeron was code-named Covington, to the delight of the authors of this book.cell1.	a unit of information that forms a building block for a chart, database, or spreadsheet.2.	one of the sections into which a city or region is divided for cellular telephone service. Each cell is served by a different antenna tower.FIGURE 53. Cell in a spreadsheetcellular modem a MODEM that uses a cellular telephone for wireless con-nection to a computer network.cellular telephone a wireless telephone that communicates through any of a number of antenna towers, each serving a particular “cell” of the city. The user is automatically transferred from cell to cell as he or she moves around. This contrasts with earlier mobile telephones that had to be within range of a particular tower in order to work.center to cause type or other objects to appear in the middle of the line with equal amounts of space to either side.This textis set centered. Centered text is considered formal.Contrast FLUSH LEFT; FLUSH RIGHT; JUSTIFICATION. centi-	86centi- metric prefix meaning ÷ 100. Centi- is derived from the Latin word for “hundredth.” See METRIC PREFIXES.central processing unit see CPU.Centrino a set of integrated circuits made by Intel comprising the PENTIUM M microprocessor and associated components designed for use in laptop computers with wireless networking.Centronics interface a standard protocol for parallel data transmission to and from microcomputer equipment, especially printers. It was origi-nally used on Centronics printers. The PARALLEL PORT of a microcom-puter is normally Centronics-compatible. See also IEEE 1284; PROTOCOL.The connector usually used with a Centronics interface is similar to one type of SCSI connector but has 36 connections instead of 50 (Figure 54).FIGURE 54. Centronics interface connectorcertainty factor see CONFIDENCE FACTOR.certificate authority a trusted organization that issues digital certificates. (See CERTIFICATE, DIGITAL.) For examples, see www.verisign.com and www.thawte.com.certificate, digital an attachment to an electronic message using public key encryption to verify that the message truly comes from the sender it claims to come from, and has not been altered in transit. To be useful, certificates must be issued by a trusted CERTIFICATE AUTHORITY so that the recipient can check on the public key. See ENCRYPTION.Your operating system may warn you if you try to run software that was downloaded or e-mailed through the Internet and is not signed with a digital certificate. The digital certificate would help you be sure that the software has not been altered to insert a VIRUS or other MALWARE. This is important when you are downloading software from a third party, but not when you are sure you are downloading it directly from its orig-inator. Nor does the certificate prove that the software is reliable or works correctly—it only verifies the identity of the author.CGI1. (computer-generated image) a method of ANIMATION in which the computer creates two-dimensional moving images of three-dimensional objects, replacing older animation techniques that involved a series of hand-drawn or hand-edited images. 87	character set2. (Common Gateway Interface) a standard way for computer programs to generate web pages as needed, containing current information or the results of computations. Instead of a file containing the web page itself, the server contains a program that writes the web page to standard out-put. This program is often called a CGI script. It can receive input from the web page that links to it, either through environment variables or on the standard input stream. See also PHP.Note: The two definitions are easy to confuse, since animations on web pages can be generated by CGI programs (definition 1).CGM (Computer Graphics Metafile) an ANSI standard file format for graphical data (both vector and bitmap). CGM files are used mainly for exchanging data between applications..ch suffix indicating that an e-mail or web address is registered in Switzerland (the Swiss Confederation, or in official Latin,Confoederatio Helvetica). See TLD.chad the small pieces of paper that have been punched out of a PUNCHED CARD or punched paper tape. Until the 2000 U.S. presidential election, most people had never heard of chad, but in that year, the failure of some voting machines to punch neat holes in cards may have changed the course of history.chain letter a message that is intended to be forwarded from each recipient to as many others as possible. On the Internet, chain letters are very unwelcome because they waste money; the cost of delivering e-mail is borne by the recipient’s site. Many chain letters perpetrate hoaxes or pyramid schemes (see HOAX; PYRAMID SCHEME).channel1.	a radio frequency or communication path (e.g., a TV channel).2.	a high-speed data communication device used to interface IBM main-frame computers with input-output devices.3.	in desktop publishing, a set of images that will compose the final image when combined. The most common use for channels is the repre-sentation of the CMYK color separations in a paint program. Each color has its own channel. Sometimes channels can be used like layers in a DRAW PROGRAM. Selected areas can be saved to a new channel, manipu-lated separately from the rest of the image, and later recombined with the main channel.4. a discussion forum in Internet Relay Chat or a similar service. SeeIRC; CHAT ROOM.5. a web page that is constantly updated, so that viewing it is like watch-ing a television channel. See PUSH (definition 2).character any symbol that can be stored and processed by a computer. For example, A, 3, and & are characters. The ASCII coding system is one way of representing characters on a computer. See ASCII; ANSI (definition 3); EBCDIC; UNICODE.character set the set of characters that can be printed or displayed on a computer. For examples see ASCII; ANSI; EBCDIC; IBM PC; UNICODE. character string	88character string a series of characters, such as ”JOHN SMITH” or ”R2D2.” SeeSTRING.charge-coupled device see CCD IMAGE SENSOR.chat room an electronic forum in which users can communicate with each other in real time. Chat rooms are found on the Internet (see IRC) and on online services such as CompuServe and AOL.People in chat rooms use a lot of abbreviations so they can type as quickly as possible and to offer hints of their emotional reactions (see also EMOTICON). Many times the abbreviations are recognized from their context, but until you get the hang of it, a chat room can be a bewilder-ing place. For example, the words two, too, and to are almost never writ-ten out—they are replaced by the numeral 2. The numeral 4 is used to replace four and for. See Table 5 for some common phrases. See alsoELECTRONIC MAIL; INSTANT MESSAGE; LEETSPEAK; SOCIAL NETWORKING.check box a small box (in a window) that the user can turn on or off by clicking with the mouse. When on, it displays an X or check mark in a square; when off, the square is blank. Unlike option buttons, check boxes do not affect each other; any check box can be on or off indepen-dently of all the others.Figure 55. Check Boxchecked (describing Windows versions) containing extra error detection and debugging code. Checked versions of Windows are used only for certain kinds of development work. Contrast FREE.checksum a number that accompanies data transferred from one place to another and helps to ensure that the data was transferred correctly.One way to ensure correct transmission would be to transmit all of the information twice; if there were an error, the two copies would almost certainly disagree. However, this is too time-consuming to be practical. A better approach is to divide the information into small packets, such as lines of text or disk sectors, and compute a checksum for each packet. A checksum is a number that would almost certainly be different if the information were altered.A simple way to compute a checksum is to add up the ASCII codes for all the characters of data (see ASCII) and take the result modulo 256. 89	checksum (For a program that does this, see C.) Since this method gives only 256 possible checksums, it is quite possible for two different data packets to have the same checksum. However, it is very unlikely that a transmis-sion error would change a packet of information into another packet with the same checksum. Hence errors can be detected by transmitting the checksum along with each packet, and then testing whether the check-sum matches the data actually received.		TABLE 5		CHAT-ROOM ABBREVIATIONS		AFAIK		as far as I knowAFAIR		as far as I rememberAFK		away from keyboardATM		at the momentBAK		back at keyboardBBL		[I’ll] be back laterBRB		[I’ll] be right backBWAHAHAHAHA	(representation of evil laugh)CUL8R		see you laterF2F		face to faceFS		for saleFWIW		for what it’s worthGR8,	GR8T	greatIANAL		I am not a lawyerIIRC		if I remember correctlyIMHO		in my humble opinionIMO		in my opinionIOW		in other wordsIRL		in real lifeISO		in search ofISTR		I seem to rememberJ/C		just chillingL8ER,	L8TER	laterLOL		laugh(ing) out loudMWAHAHAHAHA	(representation of evil laugh)PMJI		pardon me for jumping inQT		cutieROFL		rolling on floor, laughingS/AC		“sex and age check” (please tell me		your age and gender)S/H		same hereSW :()		say what!? (gasp)TIA		thanks in advanceTHX, TNX	thanksTTFN,	TT4N	ta-ta (goodbye) for nowY		why?WAZ^		what’s up?WBASAYC	write back as soon as you can		 Chicago	90Chicago the code name by which Windows 95 was identified before its release. Compare CAIRO; LONGHORN; MEMPHIS; WHISTLER; BLACKCOMB.child an object created with the properties of another object (called the PAR-ENT). Updating the properties of the parent object affects the children, but changing the properties of the child does not affect the parent. SeeVECTOR GRAPHICS; OBJECT-ORIENTED PROGRAMMING.child process a process launched by and considered dependent on another process. See PROCESS; MULTITASKING; UNIX.chip see INTEGRATED CIRCUIT.chipset a set of integrated circuits intended to be used together. For exam-ple, many modems use a chipset made by Rockwell, and many mother-boards use an Intel chipset along with a Pentium processor.chmod UNIX command for changing file permissions. For example of its use, see PERMISSION.chroma-keying the process of digitally combining video images by the use of a subtractive background. This is the method used to show a weather forecaster in front of a set of maps. The person is videotaped in front of a blue or green background. The colored background is digitally removed (hence chroma-, meaning color), and the desired map is put in place of it. The weather forecaster watches a monitor off-camera so he or she can point to the correct spot.Chroma-keying is also used for a variety of special effects in movies.chromogenic print a picture printed photographically with colored dyes; a conventional color photograph or (much less often) a black-and-white photo produced with similar chemistry. Contrast GELATIN SILVER PRINT;GICLÉE PRINT.Church’s Thesis (or Church-Turing Thesis) the hypothesis that a TURING MACHINE (or any of its mathematical equivalents) is as powerful as a mechanical computing device can be; other devices are more efficient at particular tasks, but none of them can do anything fundamentally differ-ent. It was proposed, at different times and in different forms, by Alan Turing and by the logician Alonzo Church.CIFS (Common Internet File System) the file sharing protocol that forms the basis of Microsoft Windows networking. Formerly known as SMB (Server Message Block), it is also supported by UNIX and Linux sys-tems using the Samba software package. See SAMBA. Contrast NFS.cinnamon bun (slang) the symbol @; see AT SIGN.CIO Chief Information Officer, an officer of a business responsible for its computers and data processing.CIPA (Children’s Internet Protection Act) a law passed by Congress in 2000 (47 USC 254) requiring libraries to use blocking programs to prevent 91	class libraryaccess to Internet sites with objectionable content. The Supreme Court upheld this law in 2003 (United States v. American Library Association).circularity the problem that arises when a computer cannot finish a task until it has already finished it—an impossible situation. For example, in a spreadsheet, a circularity problem arises if you enter the formula 2*B1 into cell A1 and then enter the formula A1/2 into cell B1. In order to evaluate each of the cells the computer needs the value of the other one. Thus, it cannot proceed, and it displays a warning message instead. See Figure 56.FIGURE 56. Circularity in a spreadsheetcircumflex the symbol ˆ, either written by itself as an ASCII character, or written above a letter (e.g., ê).CISC (Complex Instruction Set Computer) a computer with many differ-ent machine language instructions. The IBM PC, 68000-based Macintosh, Pentium, IBM 370 mainframe, and VAX are CISC machines.Contrast RISC.Cisco Systems, Inc. a company headquartered in San Jose, California that is the leading provider of high-speed networking hardware. The web address is www.cisco.com.class an object type in object-oriented programming. See OBJECT-ORIENTED PROGRAMMING.Class A (describing computer equipment) approved by the U.S. Federal Communications Commission for use in industrial or business but not residential areas because of the risk of radio-frequency interference. See FCC (definition 1).Class B (describing computer equipment) approved by the U.S. Federal Communications Commission for use in residential areas because the risk of radio-frequency interference is low. See FCC (definition 1).class library a set of classes available to programmers in an object-oriented language such as Java or C++. In addition to the base classes that come with the language itself, a programmer can use and extend other classes created from other sources. Classic mode	92Classic mode an operating system mode which allows Macintosh comput-ers running OS X to run older Mac OS 9 software.clear1.	to set a flip-flop or memory location to zero.2.	to make the screen go blank.click to press one mouse button very briefly (usually the leftmost button, if there is more than one). Contrast PRESS; DOUBLE-CLICK. See also WINDOW.Figure 57. Clickclick path analysis the study of the sequence of web pages reached by vis-itors to a web site, which can be helpful in determining if visitors can find information as was intended by the web designers.clickable image a picture on a WEB PAGE or in a HYPERTEXT document that the user can select, by clicking with the mouse, in order to call up fur-ther information. See IMAGE MAP.ClickOnce deployment a method of software installation, under Microsoft.NET Framework, where a program is downloaded and installed from a web address and can automatically check the same web address for updated versions.clickworker1.	a volunteer doing tedious computer work for scientific research, usu-ally in short work sessions. For an example, see clickworkers.arc.nasa. gov/top.2.	(more generally) a worker who manages information with the aid of a computer, typically without being especially knowledgeable about or interested in computing.client1.	a computer that receives services from another computer. For exam-ple, when you browse the World Wide Web, your computer is a client of the computer that hosts the web page.2.	an operating system component that enables a computer to access a particular kind of service. For example, computers that use Netware net-works must have a Netware-compatible client installed. 93	clockclient area the part of a window in which editing or drawing actually takes place. It does not include the borders, the title, the menu bar across the top, or the scroll bars, if any.client-side application a computer program that runs on a network client rather than on the server. For instance, Java applets are client-side appli-cations; when you view a web page that contains an applet, the applet is sent to your computer and runs on it. Contrast SERVER-SIDE APPLICATION.clip art artwork that can be freely reproduced. Many of the pictures in newspaper advertisements come from clip art. Many clip art collections are available on diskettes or CD-ROMs for use with various drawing, painting, and desktop publishing programs.FIGURE 58. Clip art examplesClipboard on the Macintosh and in Windows, a holding area to which information can be copied in order to transfer it from one application program to another. For instance, the Clipboard can be used to transfer text from a word processor into a drawing program.The contents of the Clipboard vanish when the computer is turned off. Also, in most software, only one item at a time can be on the Clipboard; the next CUT or COPY command will replace the old item with a new one.See also PASTE.clipping1.	(in Macintosh OS 9) a fragment of text or graphic image that can be temporarily stored on the Desktop. This provides another way (other than the Clipboard) for transferring information from one program to another.2.	in digital audio, chopping off the tops and bottoms of the sound waves, resulting in distorted sound.clock1. the circuit in a computer that generates a series of evenly spaced pulses. All the switching activity in the computer occurs while the clock is sending out a pulse. Between pulses the electronic devices in the com-puter are allowed to stabilize. A computer with a faster clock rate is able to perform more operations per second. clone	94The clock speed of a computer is often given in megahertz (MHz) or gigahertz (GHz), where 1 MHz = 1,000,000 cycles per second and 1 GHz = 1000 MHz. Often, the clock speed is doubled or tripled inside the CPU so that high-speed signals do not have to be carried outside it. The original IBM PC had a clock speed of 4.77 MHz. See MICROPROCESSOR. 2. the circuit within a computer that keeps track of the date and time, often called a real-time clock or clock/calendar. Commonly, the real-time clock is powered by a battery and runs even when the computer is turned off.clone1.	a computer that is an exact imitation of another (e.g., a clone of the IBM PC), or a software product that exactly imitates another. In biology, a clone is an organism that has exactly the same genetic material as another, such as a plant grown from a cutting.2.	in OBJECT-ORIENTED PROGRAMMING, to duplicate an object.clone tool1.	a tool available in paint programs that allows you to duplicate areas. To use the clone tool, click on the center of the area you wish to copy. Move the cursor to where you want the new area to be. Press and hold down the mouse button (the leftmost button if there is more than one) while you paint in the image. The clone tool can be used to create a bunch of cherries out of a few, cover a gap left by deleting an area, or to put a third eye on someone’s forehead. It is not necessary to define the outline of an area or object when using the clone tool; it works from the center out as far as needed. See also RUBBER STAMP.2.	(in drawing programs) a tool that copies the properties of an object to a secondary object.FIGURE 59. Clone toolclose1. (in Windows) to exit a program and clear it from the computer’s memory. This is different from MINIMIZE in that a program reduced to an icon is still running and waiting for input from you, but a closed program has been put away. Most importantly, a minimized program is still being held in the computer’s memory. If you close a minimized program, you will regain the memory it is taking up. 95	CMOS RAM2. (on the Macintosh) to reduce a window to an icon without quitting the application software. The Close button is at the far left of the title bar. Click once to close the open window. This sense of close is analogous to MINIMIZE in Windows. To clear the program from memory, choose Quit from the File menu.This illustrates one of the most basic areas of confusion between Macintosh and Windows terminology. Be careful when talking to your cross-platform friends or you’ll both end up confused!3.	(in programming) to release a file when a program is finished using it.4.	(in electronics) to put a switch into the position that allows current to flow.Closed Beta a test of incomplete software that is only open to a small group, such as the developer’s employees. See BETA TESTING.cloud computing computing operations carried out over computers linked to the web. The users pay for computing as a service rather than pay for hardware purchases.cluster1.	a group of disk sectors that are treated as a unit for purposes of allo-cating space; an ALLOCATION UNIT. See also LOST CLUSTER.2.	a group of servers that function as a single system.cluster computing using networked computers to work on computation-ally complicated problems, often more economically than on a tradi-tional SUPERCOMPUTER. Software needs to be designed to distribute the tasks to the different machines. For an example, see BEOWULF. The clus-tered computers are part of the same organization, in contrast with GRID COMPUTING.CLV (constant linear velocity) in disk drives, a speed of rotation that varies depending on whether the drive is reading a long track near the edge of the disk or a short track near the center. Thus the disk itself passes beneath the head at a constant speed. Most CD-ROM drives use CLV.Contrast CAV.CMOS (complementary metal-oxide semiconductor) a type of integrated circuit noted for its extremely low power consumption and its vulnera-bility to damage from static electricity. CMOS devices are used in digi-tal watches, pocket calculators, microprocessors, and computer memories. See FIELD-EFFECT TRANSISTOR.CMOS image sensor an image sensor similar to a CCD, but that uses metal-oxide semiconductor (MOS) transistors for lower power con-sumption. See CCD IMAGE SENSOR; FOVEON.CMOS RAM a special kind of low-power memory that stores information about the configuration of a computer. It is operated by a battery so that it does not go blank when the machine is turned off. Compare NVRAM. CMS	96CMS1. (Conversational Monitor System) the interactive part of IBM’s VM/SP (VM/ESA) operating system for mainframe computers.The key idea behind VM and CMS is that each user has a whole sim-ulated mainframe computer all to himself. The simulated computer has a virtual disk (i.e., one large real disk file that simulates the function of a disk drive on which many small files can be stored). See also VIRTUAL MACHINE; VM/ESA.2. Content Management System, software allowing an organization to manage the creation, publication, and updating of a set of web pages and other documents that have multiple authors. The system should ensure consistency (so that the web version of a document has the same content as the paper version); it should make sure that changes are only made to the most recent version (so different authors aren’t making changes to dif-ferent versions of the document) and it must protect the documents against any changes made by unauthorized persons. Also, authors must use LOGICAL DESIGN when creating their documents so that the actual visual appearance can be determined at the time of publication.CMYK the four standard printing inks: cyan, magenta, yellow, and black. When combined, these four colors can reproduce a full-color image. If you ask your computer to make color separations for a PROCESS COLOR job, four COLOR SEPARATIONS will be generated, one for each of the stan-dard inks. See also SPOT COLOR.(You may be wondering why K is used to stand for black instead of B. It’s to avoid confusion with cyan, which is a greenish blue, and could also lay claim to B.)coaster (slang) a compact disc (CD) given away free as advertising mater-ial; if you don’t need the software, you can at least put the disc under your coffee cup.coaxial cable a cable that consists of a single conductor surrounded by insulation and a conductive shield. The shield prevents the cable from picking up or emitting electrical noise. (Contrast TWISTED PAIR.) Coaxial cables are rated for their impedance, given in ohms, which indicates how the inductance and capacitance of the cable interact. For instance, RG-58 cable is rated at 52 ohms. This impedance cannot be measured with an ohmmeter.COBOL (Common Business-Oriented Language) a programming lan-guage for business data processing, developed in the early 1960s by sev-eral computer manufacturers and the U.S. Department of Defense. As the sample program in Figure 60 shows, COBOL statements resemble English sentences, and the structure of the program requires that some documentation be included. COBOL programs are long and wordy, but easy to read, making it easy for programmers other than the author to make corrections or changes. 97	code signingIDENTIFICATION DIVISION.PROGRAM-ID. COBOL-DEMO.AUTHOR. M. A. COVINGTON.ENVIRONMENT DIVISION.CONFIGURATION SECTION.SOURCE-COMPUTER. IBM-PC.OBJECT-COMPUTER. IBM-PC.DATA DIVISION.WORKING-STORAGE SECTION.77 SUM PICTURE IS S999999, USAGE IS COMPUTATIONAL.77 X PICTURE IS S999999, USAGE IS COMPUTATIONAL. PROCEDURE DIVISION.START-UP.MOVE 0 TO SUM. GET-A-NUMBER.DISPLAY ”TYPE A NUMBER:” UPON CONSOLE. ACCEPT X FROM CONSOLE.IF X IS EQUAL TO 0 GO TO FINISH. ADD X TO SUM.GO TO GET-A-NUMBER. FINISH.DISPLAY SUM UPON CONSOLE. STOP RUN.FIGURE 60. COBOL programCocoa application (on the Macintosh) a software package written specifi-cally for Mac OS X. Contrast CARBON APPLICATION.code1. a  way  of  encrypting  information  (making  it  unreadable).  SeeENCRYPTION.2.	a way of representing information on a machine or in some physical form. For example, the bit patterns in memory can be used as a code to stand for letters and digits.3.	computer programs, whether written in machine language (OBJECT CODE) or a programming language (SOURCE CODE).code base the collection of computer program SOURCE CODE that constitutes a software product. For example, the code base of Windows Vista is derived from that of Windows NT.code page the table of information, inside DOS and its derivatives, that tells it how to display characters and interpret keystrokes. Code pages other than the usual (default) one are normally used outside the United States to accommodate the characters needed for foreign languages.code signing attaching a DIGITAL SIGNATURE to a piece of code so a person who downloads it will know its origin. If the code comes from a trusted person or organization, the user will be more likely to grant it permission to take a wider range of actions, such as writing to the local disk drive. codec	98codec (coder-decoder or compressor-decompressor)1.	a software component that enables a computer to play and/or record a particular audio or video file format. A common problem is that not all PCs have codecs for all popular file formats, and since codecs are usu-ally installed with software rather than as separate items, the user does not know what is and is not present. Occasionally, a slightly corrupted audio or video file will play successfully with one codec but not with another that should also handle it.2.	an electronic circuit for converting audio or video signals into or out of digital form.coding the process of writing an algorithm or other problem-solving pro-cedure in a computer programming language.cognitive computing the use of computers in COGNITIVE SCIENCE.cognitive engineering the practical application of COGNITIVE SCIENCE, espe-cially as it involves computers; the art of building tools to help people think. Cognitive engineering is broader than ARTIFICIAL INTELLIGENCE because its goal is to build tools, not just models.cognitive prosthesis a tool that extends the human mind, such as a calcu-lator or computer.cognitive science the scientific study of intelligence (as distinct from the study of the brain), including artificial intelligence and some branches of computer science. See ARTIFICIAL INTELLIGENCE.cold boot, cold start see BOOT.collate to place printed pages in order before they are bound into a book. Some photocopiers have the ability to collate multipage documents automatically.When printing multiple copies of a document, choose “collate” if you want the pages printed in the correct order and you don’t mind taking some extra time; the whole document will be sent to the printer more than once. If you do not choose “collate,” you will get all the copies of page 1, followed by all those of page 2, and so on; printing will be faster because the printer will receive each page only once, followed by a com-mand to print it repeatedly.collating sequence the alphabetical order of all characters representable on a computer (including digits, punctuation marks, and other special char-acters). The collating sequence is important because it is often necessary to sort (alphabetize) data that includes characters other than letters. The collating sequence of a computer is the same as the order of the numeric codes for the characters. See ASCII; EBCDIC.collector one of the three layers of a bipolar transistor. See TRANSISTOR.collision the situation in which two computers are trying to transmit on the same Ethernet network segment at the same time. Each of them will try 99	coloragain after a random length of time (not the same length of time, or they would collide again, ad infinitum). Some collisions are normal on any busy network. Constant collisions indicate that the network is over-loaded. See ETHERNET.FIGURE 61. Additive color mixingcolor the visible difference between different wavelengths of light. Fortunately, the human eye can be fooled—the color of any wavelength can be simulated by mixing light of other wavelengths. Colors on a CRT screen are produced by mixing red, green, and blue light from tiny phos-phor dots. Colors on paper are produced by mixing or alternating inks in various ways.Screen colors are specified as RGB (red, green, and blue) values in either of two ways. One way is to define a PALETTE (a working set of col-ors) and assign numbers to the colors on it. For example, a bitmap image with 4 bits per pixel can distinguish up to 16 colors. The other way is to give values of red, green, and blue for each pixel. For example, 24-bit color uses 8 bits each for red, green, and blue, requiring a total of 24 bits per pixel.Since 8 bits are used for each color, there are 28 = 256 different lev-els for that color. Each color value is often represented as two HEXADEC-IMAL digits, where 00 is the minimum value and FF is the maximum value. Here are some specific examples:Hexadecimal code	Color	FF0000	red (maximum intensity)00FF00	green (maximum intensity)0000FF	blue (maximum intensity)FFFF00	yellow (mixed red and green)FF00FF	violet (mixed red and blue)FFC800	orange000000	blackFFFFFF	white	 color channel	100In general, if the red, green, and blue values are equal, the result will be a shade of gray.Colors on paper are usually specified as CMYK (cyan, magenta, yel-low, and black) or HSB (hue, saturation, and brightness). Because the pig-ments being mixed are completely different from the phosphor dots on the screen, printed colors do not always match screen displays, even after extensive CALIBRATION.color channel a representation of color information in a color bitmap image. An RGB image has three channels: red, green, and blue. Each channel can be manipulated separately from the others.color separations camera-ready artwork that has been broken down into individual printouts, one for each color of ink that is to be printed. All the elements that should print in black will be on one sheet, and all the elements that should print in red will be on another, and so forth. Each printout will have registration marks so that the colors can be aligned properly or registered (see REGISTRATION, definition 2).color temperature a method of measuring the redness or blueness of a photographic light source by comparing it to the temperature (in degrees Kelvin) of a white-hot object that would produce the same color of light. Sunlight has a color temperature of about 5000 K; incandescent light bulbs, 2700 K to 3400 K. Color temperature measures only redness or blueness and does not account for other tints that a light source might have, such as green or purple.color/gray map a paint program filter that allows adjustment of a picture’s color balance. In grayscale mode, it can be used to adjust the exposure of an imperfect black-and-white photo.column move the operation of moving material to the left or right within a file (e.g., moving the material in columns 10–20 into columns 5–15 on several consecutive lines in one step). See EDITOR..com1.	a suffix indicating that a web or e-mail address belongs to a commer-cial entity (in any country, but mostly the United States). Along with .edu,.gov, .int, .net, .org, and .mil, this is one of the original set of Internet top-level domains. Since 2000, .com, .net, and .org have been assigned almost indiscriminately to organizations of all types. See also TLD; ICANN.2.	the filename extension used for small, nonrelocatable machine lan-guage programs in DOS. To execute the program, type the name of the file without the final “.COM”; for example, to execute AAA.COM, type aaa. Contrast EXE FILE.COM (Component Object Model) Microsoft’s architecture for building software components (i.e., packages of procedures and data structures ready for use by programmers). Many of these components are referred to as ActiveX controls (see ACTIVEX). 101	command promptCOM was introduced in the mid-1990s and provides infrastructure for OLE (Object Linking and Embedding; see OLE). COM components can be written in any major programming language. They can be used by the program to which they belong, other programs running concurrently on the same computer, or even programs running elsewhere and communi-cating through a network; the last of these is called DCOM (Distributed COM; compare CORBA). Though developed for Windows, COM has been ported to UNIX.COM+ is an extended form of COM in which more of the creation and handling of components is done automatically. The .NET Framework takes the same idea even further (see .NET FRAMEWORK).COM1, COM2 the filenames assigned to the first two serial ports on a PC-compatible computer under Windows.combine a drawing program command that merges two separate objects into one so that the whole thing can become one object. This is similar to grouping objects, but there is an important distinction. A group of objects can be treated as a single object, but the individual elements retain their separate attributes. A combined object is a single object; it has only one outline (or path) and only one fill. Interestingly, a combined object can have holes in it that you can see through. See MASK; PATH (definition 3).comma-separated values see CSV FILE.Command key the key marked with a cloverleaf-like symbol () on the Macintosh. It is used like a shift key to change the meanings of other keys or to perform special functions. For example, you can print a pic-ture of the current screen by holding down Command and Shift and pressing 4. If you press 3 instead, a picture of the current screen will be saved as a MacPaint document.command line the place where a user types in response to a single COM-MAND PROMPT, ending by pressing Enter.command prompt1.	the series of characters, such as C:\TEMP> or unix%, that indicates that an operating system is ready for the user to type a command.2.	in Windows, a CONSOLE MODE window in which the user can type commands (Figure 62). FIGURE 62. Command prompt comment	102comment information in a computer program that is ignored by the com-puter and is included only for the benefit of human readers. Various computer languages identify comments with markers such as C, ;, REM, or /* */.Comments reflect the fact that programs are written to be read by human beings, not computers. If we could write in the language that best suited the computer, we would not need human-readable programming languages such as Pascal, C, and Java.Comments can also be used to remove material from a program tem-porarily without deleting it. When you modify a program, it is a good idea to preserve the original form of the material you changed, in a com-ment, so that you can restore it or study it if the need arises.common logarithm a logarithm to the base 10. See LOGARITHM.Communications Decency Act a law passed by the U.S. Congress in 1996 making it illegal to transmit indecent material to children through the Internet. In 1997, the Supreme Court declared this law to be unconstitu-tional (Reno v. American Civil Liberties Union) on the ground that the Internet is inherently entitled to the same freedom of speech as printed media, and that there is no practical way to block material from reach-ing children while transmitting it freely to adults. See INDECENCY and cross-references there.Note that the Communications Decency Act is separate from laws against pornography, including child pornography, and such laws were not affected when it was overturned.CompactFlash a type of flash-memory non-volatile storage device com-monly used in palmtop computers, Nikon digital cameras, and other portable devices. CompactFlash cards are rectangular, about 40 mm (1.6 inches) wide, and either 3.3 mm (0.13 inch) thick (Type I) or 5 mm (0.2 inch) thick (Type II) (see Figure 63).Flash-memory CompactFlash cards interact with the computer like ATA hard disks. There are also CompactFlash Type II cards that contain an actual magnetic disk (the IBM Microdrive). Compare SMARTMEDIA; SECURE DIGITAL CARD; MEMORYSTICK; MULTIMEDIACARD; FLASH MEMORY CARD. FIGURE 63. CompactFlash card 103	complex numberCompaq a major manufacturer of IBM PC-compatible computers. Compaq’s first product, released in 1983, was the first PC CLONE (i.e., the first PC-compatible computer not made by IBM). In 1988 Compaq led a group of other computer companies in the development of a 32-bit PC bus (see EISA). In 1998, Compaq acquired Digital Equipment Corporation, manufacturers of the Alpha microprocessor. In 2002, Compaq merged with Hewlett-Packard.compatibility mode the ability of recent versions of Microsoft Windows to simulate earlier versions in order to run older software. To set the com-patibility mode for a piece of software, right-click on its icon and look for the settings under Properties.compatible1.	able to work together. For example, a particular brand of printer is compatible with a particular computer to which it can be connected.2.	able to run the same software. See PC COMPATIBILITY.compile-time error an error in a computer program that is caught in the process of compiling the program. For example, a compile-time error occurs if the syntax of the language has been violated. Contrast RUN-TIME ERROR. See also BUG.compiler a computer program that translates C, C++, BASIC, Pascal, or a similar high-level programming language into machine language. The high-level language program fed into the compiler is called the source pro-gram; the generated machine language program is the object program.complement1.	either of two ways of representing negative numbers in binary:a.	The 1-complement of a binary number is obtained by simply changing every 0 to 1 and every 1 to 0. For example, the 1-com-plement of 0001 is 1110. In this system, 0000 and its complement 1111 both represent zero (you can think of them as +0 and –0, respectively).b.	The 2-complement of a binary number is found by reversing all the digits and then adding 1. For example, the 2-complement of 0001 is 1110 + 1 = 1111. This is the system normally used on most computers, and it has only one representation of zero (0000).See BINARY SUBTRACTION.2.	the opposite of a color; the hue, which when mixed with the first hue, can give colorless gray. For example, red is the complement of green.complex number a number consisting of two parts, called real and imagi-nary. Complex numbers are written in the form a + bi, where a is the real part, b is the size of the imaginary part, and i is the square root of –1. (If b = 0, the complex number is of course equal to a real number.)Unlike real numbers, complex numbers can produce negative numbers when squared; because of this fact, all polynomials have complex roots even though some of them lack real roots. Another use of complex num- complexity theory	104bers is to represent vectors in two-dimensional space; the imaginary num-ber line is thought of as running perpendicular to the real number line.Arithmetic on complex numbers obeys the ordinary laws of algebra, as follows:(a + bi) + (c + di) = (a + c) + (b + d)i(a + bi) – (c + di) = (a – c) + (b – d)i(a + bi)  (c + di) = ac + adi + bci + bdi2 = (ac – bd) + (ad + bc)iComplex numbers are provided as a data type in FORTRAN and C++, and in mathematical packages such as Maple, but not in most other pro-gramming languages. See also MANDELBROT SET.complexity theory the mathematical study of the time (number of steps) and amount of memory needed to perform a computation.Suppose a computer is going to process n items of input. The time complexity of the calculation may be any of the following:•	Constant or O(1) if the computation takes the same number of steps regardless of how many items are to be processed. An example is copying a disk with the DOS diskcopy command, which duplicates the entire disk regardless of how much of it is actually in use.•	Linear or O(n) if the number of steps is proportional to n. An exam-ple would be copying a set of n names and addresses from one file to another, or finding the sum of n numbers, or any program that contains only one loop.•	Polynomial or O(nk) if the number of steps is proportional to n raised to some constant power. For example, if the computation involves comparing each item of input with all of the other items, it will take n2 steps. Many sorting algorithms are O(n2). A program that contains k nested loops, each with a number of steps propor-tional to n, will take time proportional to nk.•	Exponential or O(kn) if the number of steps is proportional to some constant k raised to the nth power. This is what happens if the com-puter has to try arranging n elements in all possible sequences. Exponential-time computations are generally not practical.There are some other possibilities; for example, Quicksort takes time proportional to n log n, which is less than n2.In calculating complexity, we ignore factors that become insignificant as n becomes large. Suppose a computation requires n2 +3n +5 steps. For sufficiently large n, this gets closer and closer to n2, so we say that the complexity is O(n2).Besides the time complexity, a computation may require constant, lin-ear, polynomial, or exponential amounts of memory. See also COMPUTER SCIENCE; LIMITS OF COMPUTER POWER; QUICKSORT.component any part of a larger system, either software or hardware. Reusable parts of programs are called software components. For exam-ples of how software components are used, see ACTIVEX; COM; CORBA. NATURAL LANGUAGE PROCESSING;105	computer architecturecomposite video the kind of video signal used in an analog TV set. The whole signal is transmitted on one wire. By contrast, an RGB signal has separate wires for red, green, and blue. See MONITOR.compositing the combining of bitmap images from different sources or dif-ferent objects in a single image. The opacity, or ALPHA, of each object deter-mines how it combines with objects behind it. See also CHROMA-KEYING.compression see DATA COMPRESSION.CompuServe an online information service based in Columbus, Ohio, accessible by modem from anywhere in the United States. (See MODEM.) Since 1998, CompuServe has been a subsidiary of AOL. For further information, see www.compuserve.com.computational linguistics the use of computers in the study of human language, and the study of how to make computers understand informa-tion expressed in human languages. SeePARSING.computer a machine capable of executing instructions on data. The distin-guishing feature of a computer is its ability to store its own instructions. This ability makes it possible for a computer to perform many operations without the need for a person to type in new instructions each time.Computer a FOLDER on the DESKTOP of Microsoft Windows Vista that con-tains all the disk drives, the Control Panel, and other information about the system.Ordinarily, folders are directories. The root directory of a disk drive is also a folder. “Computer” is a special folder that gives you access to the entire machine.In previous versions of Windows, this folder was called “My Computer.”computer architecture the design and internal structure of digital computers.Fundamentally, a computer is a machine that can store instructions and execute them. Thus, it consists of two major parts, MEMORY and the cen-tral processing unit (CPU), which communicate through a set of parallel electrical connections called the BUS (Figure 64). The bus also connects to input-output devices such as a screen, a keyboard, and disk drives.The CPU spends its time retrieving instructions from memory and doing whatever those instructions say. Each instruction is a pattern of bits (binary ones and zeroes, represented by electrical on and off sig-nals). When the instruction reaches the CPU, the CPU must decode (rec-ognize) it and activate the appropriate functional unit within the CPU in order to carry out the instruction. Functional units include adders, mul-tipliers, circuits to compare bit patterns, etc., all of which are built from logic gates (for an example, see BINARY ADDITION). computer architecture	106 FIGURE 64. CPU communicates with memory and other devices via busThe CPU contains REGISTERs to hold data that is being worked on. For example, in order to add two numbers, the CPU will typically retrieve the two numbers from memory into registers, perform the addition, place the result in another register, and finally store it back into memory. Figure 65 illustrates the parts of the CPU. In most present-day comput-ers, the CPU is a single integrated circuit (IC) called a MICROPROCESSOR.FIGURE 65. CPU (central processing unit) internal structure Every location in memory has an ADDRESS (i.e., a bit pattern, binary number, that identifies the location). To retrieve the contents of memory location 011000011, the CPU places the bit pattern 011000011 on the 107	computer ethicsaddress portion of the bus, activates the “read memory” line, and waits a specified length of time. The memory places the contents of that loca-tion onto the data portion of the bus so that the CPU can read it. To put data into memory, the CPU puts both the address and the data onto the bus and activates the “write memory” line. Some computers also include “read port” and “write port” lines, which are like the lines used for accessing memory except that addresses are understood as applying to input and output devices (printer ports, etc.) rather than memory.Most computers use a VON NEUMANN ARCHITECTURE, which means that programs and data are stored in the same kind of memory. Some micro-controllers use a HARVARD ARCHITECTURE, with separate memories for program and data (mainly because programs are kept permanently recorded in ROM, but data must be changeable).Programmers normally do not write CPU instructions. Instead, they write programs in a high-level language such as BASIC, C, or Pascal, and use a COMPILER to translate the programs into machine language. It is also possible to write programs in ASSEMBLY LANGUAGE, which trans-lates into machine language more directly.computer ethics the responsible use of computers and computer networks. Malicious misuse of computers is rare, but serious misjudgments by well-meaning people are unfortunately common. Some important points to remember are the following:1.	People have the same legal and ethical responsibilities when using a computer as at any other time.Slander, deception, harassment, and the like are just as wrong when done via computer as when done any other way, and they incur the same legal penalties.Using a computer without the owner’s permission is prose-cutable as theft of services (just like using any other machine with-out the owner’s permission). Damaging property or data by releasing a computer virus is also prosecutable as a crime.2.	Computers will not necessarily prevent all improper acts; users are responsible for what they do. For example, if a computer is set up incorrectly so that it lets unauthorized people use it without a password, that does not justify the unauthorized usage, just as a defective door lock does not justify burglary.3.	Some of the information stored in computers is private and confiden-tial and should not be abused. This applies particularly to credit records, educational records, and the like. Such information may also be incomplete or inaccurate because people did not correct errors that they considered inconsequential. If the information is later used for a completely different purpose, the errors can be damaging.4.	Electronic communications are not guaranteed to be private. You do not know what path your electronic mail follows or who may see it en route. Do not send credit card numbers or other confi-dential information through e-mail unless you have confirmed that it is traveling by a secure path. COMPUTER TRES-computer law	108Also, be aware that e-mail can be faked; there is no guarantee that a piece of mail actually came from the person or site shown on the header.5.  Users must respect software copyrights and licenses.The price of a piece of software is more than just the cost of the disk and manual; it’s also your share of the cost of developing the product. If people don’t pay for software, there will be no software.6.	Manufacturers, programmers, and independent consultants have responsibilities to their customers. It’s wrong to claim to be more of an expert than you really are; it’s also wrong to sell a shoddy product while concealing defects in it. Admittedly, no one can ensure that any complex piece of software is 100% reliable, but common decency requires programmers and vendors to act in good faith—when there’s a problem, do your best to correct it or at least warn the user about it. In the past, many manufacturers have tried to disclaim all responsibility for the performance of their products, but there are encouraging signs that the user com-munity will no longer tolerate this dubious practice.7.	On the Internet, you are everyone else’s guest.The cost of running the Internet is paid by the sites that receive messages, not just the sites that send them. Accordingly, you must be careful what you send out, and to whom.For  more  about  ethical  aspects  of  computer  communications  seeACCEPTABLE-USE  POLICY;  COMPUTER  LAW;  DOMAIN  NAME  POACHING;HACKER  ETHIC;  INTERNET;  MAIL  BOMBING;  NETIQUETTE;  OBSCENITY;PORNOGRAPHY; SPAM; SPOOFING; USENET.computer law laws pertaining to computers. An important principle is that computers are not exempt from the pre-existing laws. For instance, com-puter users must obey laws against fraud, misrepresentation, harassment, eavesdropping, theft of services, and tampering with other people’s property, even if the laws do not specifically mention computers.Further, many jurisdictions have specific laws againstPASS and similar acts. See also ACCEPTABLE-USE POLICY; COMMUNICATIONS DECENCY ACT; COPYRIGHT; DMCA; ECPA; GAMBLING; LICENSE; MICROSOFT ANTITRUST CHARGES; PATENT; PORNOGRAPHY; PUBLIC DOMAIN; PYRAMID SCHEME; TRADE SECRET; VIRUS.computer priesthood (1970s slang, still used) computer specialists; the experts on whom ordinary people rely for their access to the computers, as if they were priests with access to a secret part of the temple.computer science the mathematical and scientific study of the possible uses of computers. Computer science is a wide-ranging field including pure mathematics (see COMPLEXITY THEORY; HALTING PROBLEM), engi-neering (see COMPUTER ARCHITECTURE), management (see SOFTWARE ENGINEERING), and even the study of the human mind (see ARTIFICIAL INTELLIGENCE). Indeed, computer scientists often work on problems in almost any field to which computers can be applied. Computer science 109	computer securityis more than just training in the use of today’s computers and software; it includes preparation to understand the technology of the future and its theoretical underpinnings.computer security the protection of computers from tampering, physical danger, and unwanted disclosure of data. The advent of personal com-puters has made it easy for important business records or confidential data to be lost, sabotaged, or misused. Computers need protection from the following kinds of hazards:1.	Machine failure. Make backups of important files frequently. Every disk drive in the world will one day fail, losing all data.2.	Physical hazards. Protect the computer from fire, flood, and sim-ilar hazards, and store backups at a remote location. Remember, too, that the machine can be stolen. An increasing number of computer thieves are after data, not just equipment.When traveling with a laptop computer, never let it out of your possession. Many thefts occur at airport check-in counters: while you are preoccupied making your arrangements, someone can quietly steal the laptop computer you placed on the floor. Keep the computer in your hand. Stay close to the computer as it goes through the airport security check. Always bring your laptop on board as carry-on luggage. Checked baggage is treated far too roughly.3.	Operator error. It is easy to delete information accidentally. This hazard can be minimized with software that retains original files while altered copies are being made.4.	Computer tampering. Can someone come in and alter your records without your knowing it? Bear in mind that large numbers of people know how to use popular business software packages. If possible, use software that keeps records of changes—recording who made them and when—and requires validation (such as a password) to make unusual changes.5.	Malicious programming. Some computer crimes have been per-petrated by programmers who did such things as collect all the money that was lost by rounding interest payments to the nearest penny. A clever bookkeeping system run by a dishonest pro-grammer can easily conceal abuse.More recently, some people have gotten their kicks by distrib-uting destructive computer programs over the Internet. See TRO-JAN HORSE. Even more have gotten their kicks by circulating falsewarnings (see HOAX).6.	Malicious programs arriving in e-mail, often falsely described as other things. Never open a file that arrives unexpectedly, even if it’s from someone you know, unless you have confirmed what the file is. See WORM.7.	Break-ins by modem or network. Make sure you know all the pos-sibilities for connecting to your computer from elsewhere, and that you’ve blocked all access that you don’t want to allow. The computer trespass	110UNIX operating system, designed originally for use in laborato-ries where no security was needed, is generally thought to be par-ticularly vulnerable. See DICTIONARY ATTACK; WAR DIALING.8.	Be especially careful with wireless networks, and make sure all communications are encrypted. A cracker with a special antenna can access your wireless network from ten times the normal dis-tance. See WAR DRIVING; WIRELESS NETWORK.9.	Easily guessed passwords. A computer password must never be a person’s initials, nickname, child’s name, birthdate, etc., nor should it be a correctly spelled word in any language. A common way to crack accounts is to try all the words in a large dictionary, as well as all names and abbreviations that are associated with a person. Also, if a user signs onto a computer and then leaves the terminal unattended, others can tamper with it without typing the password.10.	Viruses and known software defects. Always run antivirus soft-ware, and make sure your software and operating system are kept up to date.11.	Excessive security measures. Excessive attempts to build security into a computer can easily make the computer so hard to use that productivity is crippled. In the final analysis, all computer security depends on human trustworthiness. Concentrate on securing the people, not the machine. That is, ensure that employees are trust-worthy and that strangers have no access to the machine; then give authorized users all the access they need to do their jobs effectively.See also 2600; DDOS; DENIAL-OF-SERVICE ATTACK; ENCRYPTION; ETHICAL HACKING; FINE-GRAINED SECURITY; HONEYPOT; MAIL BOMBING; PING FLOOD-ING; VIRUS.computer trespass the crime of using a computer without the owner’s per-mission (see CRACKER). Even in jurisdictions that have no specific law against it, computer trespass is illegal under pre-existing laws that pro-hibit unauthorized use of other people’s property.computer virus see VIRUS.computer vision see VISION, COMPUTER.computers, history of a story spanning many centuries. The abacus, on which information is stored by moving beads along rods, was one of the earliest calculating devices. Blaise Pascal developed an adding machine in 1642 that used toothed wheels to handle carries from one digit to the next. Charles Babbage developed the concept of a stored program com-puter when he designed the “Analytical Engine” in 1833. Unfortunately, the mechanical devices of his day could not be made to work reliably, so the “Analytical Engine” was never completed.An important data processing device, the punched card, was devel-oped by Herman Hollerith to help the U.S. Census Bureau tabulate the census of 1890. (See PUNCHED CARD.) The first electronic digital com- 111	configureputer was the ENIAC (Electronic Numerical Integrator and Calculator), which was built for the U.S. Army in 1946, largely because of the need to calculate ballistics tables. The ENIAC was programmed by plugging in cables to connect different units. In 1945 John von Neumann intro-duced the modern concept of a stored-program computer, in which the computer memory could store both programs and data.Once the concept was established, major improvements were made by developing smaller and more reliable electronic components. The ENIAC was a huge machine made with vacuum tubes. The invention of the transistor in the late 1940s made it possible to build much smaller computers that needed less cooling. Continued improvements in inte-grated circuits, which were first developed in the late 1950s, made it pos-sible to continue the miniaturization of computers.An important advance occurred in the mid-1970s when the first microcomputers were built. Previously, all computers had been large and expensive. Microcomputers are small enough and cheap enough that they can be purchased by small businesses and individuals. A micro-computer is built around a microprocessor chip, such as the 486 or Pentium, that contains the entire central processing unit on a single crys-tal of silicon. The advent of powerful, low-cost microcomputers has made the computer a common household appliance.concatenation the operation of joining two or more character strings together, end to end. For example, ”ABC” concatenated with ”DEF” equals”ABCDEF”. See STRING OPERATIONS.concurrent processing the apparently simultaneous execution of two pro-grams, where a single CPU is actually switching its attention back and forth between them very rapidly. See also MULTITASKING; PARALLEL PRO-CESSING; TIMESHARING.conferencing the use of computer networks to enable workers to commu-nicate in real time (without delay) while working together. See IRC; MUD.confidence factor (certainty factor) a truth value between 0 and 1, used to describe the reliability of a piece of information whose truth is unclear or uncertain. There are several systems of reasoning that use confidence factors; for one example, see FUZZY LOGIC. Contrast DEFAULT LOGIC, which deals with exceptions without using confidence factors.CONFIG.SYS in DOS and early versions of Windows, a file that contains information about the machine configuration, including device drivers to be loaded as the machine boots up. It is processed before AUTOEXEC.BAT.In Windows NT, 2000, XP, and their successors, the function of CONFIG.SYS has been taken over by the Registry. A separate file, CONFIG.NT, if present, is processed at the beginning of every DOS-mode program.configure to set up a computer or program to be used in a particular way. Many commercial software packages have to be configured, or installed; console	112this involves setting them up for a particular machine (including video card and printer) and for a particular user’s preferences.console1.	the main keyboard and screen of a multi-user computer.2.	a keyboard and (non-graphical) screen, or a window serving the pur-pose of such a screen.console application a Windows program that runs in CONSOLE MODE.console mode the way in which Windows runs programs that do not use win-dowing (Figure 66), including but not limited to DOS programs. A win-dow is used as a substitute for the whole screen in text mode. By pressing Alt-Enter, the user can give the program control of the whole screen.FIGURE 66. Console modeconstant a value that remains unchanged during the execution of a pro-gram. Literal expressions, such as 3.5 and ”DOLLY MADISON”, are con-stants because they always stand for the same value.constrain (in drawing programs) to restrict or limit the available move-ments or shapes. For example, when drawing a circle with a circle tool, you must hold down the Control key to constrain the rounded shape to a circle. If you let go of the constraining key too soon, you may get a fat oval rather than a perfect circle.The constrain command is also used with the rectangle drawing tool (constrains to a square) and the line drawing tool (constrains to preset angles).constructor in OBJECT-ORIENTED PROGRAMMING, a method called when a new object is created.content provider a company or organization that provides information (content) online. For example, www.cnn.com (Cable News Network) is a content provider for world news and related information. Contrast ASP(definition 2); INTERNET SERVICE PROVIDER. 113	control boxcontention see DEVICE CONTENTION.context-sensitive help information provided by a computer program when you ask for help, chosen to match what you are doing at the time. For example, a context-sensitive help key will give you information about how to edit if you press it while editing, or how to print if you press it while preparing to print.contiguous adjacent, next to each other. For instance, the states of North Dakota and South Dakota are contiguous, but Texas and Maine are not.Most computers can store a disk file in either contiguous or noncon-tiguous sectors. (See DISK.) Access is slowed if the sectors are not con-tiguous, since to get from one part of the file to another, the read/write head must jump from one part of the disk to another. See FRAGMENTATION.continuous speech speech that is spoken without pauses between words.See SPEECH RECOGNITION. Contrast DISCRETE SPEECH.contrast the range of light and dark values in a grayscale (continuous tone) image. A high-contrast image is mostly white and black with very few intermediate gray shades. A low-contrast image has little difference between the darkest darks and lightest lights. See Figure 67.Contrast is best adjusted at the time of scanning the image. If that is not possible, contrast can be adjusted in a PAINT PROGRAM. See SCANNER;HISTOGRAM.FIGURE 67. Contrast: low, normal, and highcontrast ratio the luminosity of the brightest white that can be produced by a monitor divided by the luminosity of the darkest black.control a reusable software component in Visual Basic, ActiveX, or a sim-ilar system. Many of the first controls were user interface components— check boxes, sliding bars, and the like—hence the name.control box (Windows) a small box at the left of the title bar of a window. Clicking the control box pops up a menu for controlling the size of the Control key	114window. Double-clicking on the control box closes the window and the application running in it.Control key (Ctrl or Cntl key) a special key on many computer keyboards. When it is pressed in conjunction with another key, it gives the other key a new meaning that depends on the program in use. See ASCII to see how the Control key can be used to type nonprintable control characters.control menu (Windows) a menu that appears when the user clicks on the CONTROL BOX (the box at the left of the title bar). The control menu for each window allows you to maximize, minimize, restore, resize, or close the window. See Figure 68 for illustration. See also WINDOW.FIGURE 68. Control menuControl Panel (in Windows) a group of utility programs for making set-tings that affect the computer’s operation. These range from desktop color, mouse tracking, and the like, to network communication parame-ters and printer drivers.control point see NODE.CONUS abbreviation for continental United States, usually meaning the 48 contiguous states. (Alaska is part of the North American continent but is commonly overlooked.)conversion program a program that is capable of changing a file from one format to another.For example, to use a TIFF (Tagged Image File Format) file in a web page design, the image must be converted to .JPEG or .GIF format. This can be done with a separate conversion program or by using the “Save as . . . ” command in the appropriate image-editing program. (SeeIMPORT; EXPORT; FILE FORMAT.)Note that simply changing the name of the file from myfile.tif to myfile.jpg does not convert the file type. The data contained in the file has to be reorganized by the conversion program.convolution an image processing computation described by a matrix. Suppose, for example, that you want to bring out fine detail in an image. One way to do this is to increase the difference between each pixel and its neighbors. Treating the pixels as numbers representing their bright-nesses, you can use the following convolution matrix: 115					copy										–1	–1	–1												–1	9	–1					–1	–1	–1										That means: Work through the whole image, one pixel at a time. Whenever you get to a pixel, multiply it by 9, multiply each of the sur-rounding pixels by –1, and add them together. Then replace the original pixel with that value.If all of the pixels are the same brightness, nothing changes, but if a pixel is brighter or fainter than its neighbors, the difference is exagger-ated by a factor of 9. Other convolutions can perform other special effects, such as smoothing, eliminating details smaller or larger than a certain size, and even eliminating streaks in a particular direction.When performing a convolution, the input is always from the origi-nal, unprocessed image. That is, the next pixel will not be affected by any changes made by the processing of the previous pixel.convolve to perform a convolution. See CONVOLUTION.cookie information stored on a user’s computer by a WEB BROWSER at the request of software at a web site. Web sites use cookies to recognize users who have previously visited them. The next time the user accesses that site, the information in the cookie is sent back to the site so the infor-mation displayed can vary depending on the user’s preferences. Cookies are not a security risk because they only store information that came from the web site or was sent to it by the user.The term cookie comes from a 1980s prank computer program called Cookie Monster that would interrupt users and demand that they type the word “cookie” before continuing..coop a suffix indicating that a web or e-mail address belongs to a cooper-ative (i.e., a customer-owned business [in any country]). Contrast .COM.See also ICANN; TLD.COPPA (Children’s Online Privacy Protection Act) a law passed by Congress in 1998 (15 USC 6501-6502) making it illegal for an operator of a web site or online service to collect personal information from chil-dren without parental consent. See COMPUTER LAW.coprocessor a separate circuit inside a computer that adds additional func-tions to the CPU (central processing unit) or handles extra work while the CPU is doing something else.copy1. to duplicate information in another place, leaving the original unchanged. In many spreadsheets, editors, and drawing programs, copy means either of two things:a.	to copy material from one place to another;b.	to copy material from the document being edited into a holding area, from which you can then “paste” it elsewhere. See CUT; PASTE; CLIPBOARD.2. a command that makes a copy of a disk file. See also XCOPY. copy protection	116copy protection any of numerous techniques to keep a diskette, CD, or DVD from being copied with ordinary equipment. Copy-protected diskettes were common in the 1970s and 1980s but fell into disfavor because they were incompatible with newer disk drives. A number of types of copy-pro-tected CDs have appeared recently, and similar problems may befall them. DVD technology has copy protection built in, backed up by a rather unusual copyright law (see DVD; DMCA). See also DRM.copyleft (humorous) a copyright whose owner gives permission for the product to be distributed free subject to certain conditions. See GNU.copyright (the right to copy) a legal restriction on the copying of books, magazines, recordings, computer programs, and other materials, in order to protect the original author’s right to ownership of, and compensations for reproduction of, an original work. Most computer programs are pro-tected not only by copyright but also by a software license (see SOFT-WARE LICENSE; FREE SOFTWARE).Since 1989, literary works and computer programs have been pro-tected under U.S. copyright law from the moment they are created. It is not necessary to include a copyright notice or register the copyright with the government.However, it is still prudent to include a notice of the form Copyright 1996 John Doe or © 1996 John Doe in any work to which you claim copy-right. If you think the copyright is likely to be infringed, you should also register it at the time of publication, since this increases the penalties you can collect from the infringer. In general, copies of copyrighted published works must be sent to the Library of Congress whether or not the copy-right is registered. For up-to-date information see www.copyright.gov.U.S. copyright law allows limited copying of books and magazines for private study or classroom use. However, this does not apply to com-puter programs, which can only be copied with the permission of the copyright owner, or in order to make backup copies that will not be used as long as the original copy is intact.Do not reproduce copyrighted material on web pages or anywhere on the INTERNET without the owner’s permission. Placing something on a web page constitutes republication just as if you were making printed copies. Remember that copyrights apply to sounds and pictures as well as texts. Distributing a sound bite from a movie or a picture of a cartoon character can be a copyright violation.Copyright protects expressions of ideas, not the ideas themselves. Copyrights do not cover algorithms, mathematical methods, techniques, or the design of machines (which, however, can be patented).The purpose of copyright is to encourage communication. It is there-fore paradoxical that the Digital Millennium Copyright Act prohibits the publication of certain information about copy protection schemes. SeeDMCA. See also COMPUTER LAW; PATENT; TRADE SECRET.CORBA (Common Object Request Broker Architecture) a standard set of definitions for objects to interact with each other. CORBA was created BLEND; DRAW PRO-117	country codesby the Object Management Group (see OMG). CORBA defines a standard for a layer of middleware called the ORB (Object Request Broker). The way that components interact with other components is specified by IDL (Interface Definition Language). This allows client-server computing where the clients don’t need to have any knowledge of the specific oper-ation of the component they are interacting with. For example, the client doesn’t need to know the language in which the component was written; it only needs to know the IDL specification of how the component inter-acts. For an alternative standard, see DCOM.core1.	the central part of a CPU, containing the circuitry needed to execute a single series of instructions. A CPU with more than one core can run more than one series of instructions at the same time without switching back and forth between them.2.	the essential design of a CPU, in detail; thus, two different models of CPU might be described as being built on the same core.3.	an old term for RAM, especially magnetic RAM consisting of dough-nut-shaped ferrite “cores” strung on a lattice of wires.Core Duo one of several models of Intel Pentium microprocessors that have two cores. See CORE (definition 1).Corel a corporation headquartered in Ottawa, Ontario, that introduced one of the first successful DRAW PROGRAMs, CorelDraw, in 1989. In 1996, Corel acquired the WordPerfect Office business applications. Other Corel products include a variety of computer programs and utilities such as Corel Painter, DVD MovieFactory, and WinDVD. Their web address is www.corel.com.corona wire a wire, in a laser printer, that emits a strong electric charge (a corona discharge) into the air and onto the adjacent drum. See DRUM.correspondence points points in two objects (or images) that are associ-ated with each other for blending or morphing. SeeGRAM; MORPH.cos, cosine the cosine trigonometric function. If A is an angle in a right tri-angle, then the cosine of A (written as cos A) is defined as: cos A = length of adjacent sidelength of hypotenuse The function cos(A) in many programming languages calculates the value of cos A, expressed in radians. For an illustration, see TRIGONO-METRIC FUNCTIONS.coulomb a unit of electric charge equivalent to the charge of 6.25  1018 electrons. See AMPERE.country codes See CCTLD. Courier	118Courier a typewriter-like typeface often used on laser printers. Unlike other typefaces, Courier has fixed pitch; that is, all characters are the same width. It was designed for IBM typewriters in the 1960s, but on modern laser printers, it is often unpleasantly light (thin).FIGURE 69. Courier, a fixed-pitch fontCP/M (Control Program for Microcomputers) an operating system devel-oped by Digital Research, Inc., and used on microcomputers in the 1980s. (See OPERATING SYSTEM.) The original CP/M (now called CP/M-80) was widely employed on computers that used the 8-bit Z80 proces-sor. CP/M greatly influenced the early development of DOS. See MS-DOS.CPU (Central Processing Unit) the part of a computer where arithmetic and logical operations are performed and instructions are decoded and exe-cuted. The CPU controls the operation of the computer. A microproces-sor is an integrated circuit that contains a complete CPU on a single chip.CR (carriage return) the character code that tells a printer or terminal to return to the beginning of the line; ASCII code 13. On the Macintosh, CR indicates the end of a line in a text file; UNIX uses LF, and Windows uses CRLF. See CRLF; LF.cracker a person who “breaks into” computers via the Internet and uses them without authorization, either with malicious intent or simply to show that it can be done. Compare HACKER. See also 2600; COMPUTER TRESPASS; ETHICAL HACKING; HONEYPOT.crash the sudden, complete failure of a computer because of a hardware failure or program error. A well-designed operating system contains pro-tection against inappropriate instructions so that a user’s program will not be able to cause a system crash.crawler a computer program that automatically explores the WORLD WIDE WEB and collects information; also called a spider.Cray Research, Inc. a company founded by Seymour Cray, a manufac-turer of supercomputers (see SUPERCOMPUTER). Cray’s first major prod-uct was the Cray-1, introduced in 1977, a vector processor designed for repetitious numeric calculations. See VECTOR PROCESSOR. Web address: www.cray.com. BELLS AND WHISTLES.119	crunch modeCRC see CYCLICAL REDUNDANCY CHECK.creeping featurism (slang) the practice of trying to improve software by adding features in an unsystematic way, ultimately making it less reli-able and harder to use. Comparecrippleware (slang) software that is distributed free as an incomplete or time-limited version in the hope that the user will purchase the fully functional version. See FREE SOFTWARE.CRLF (carriage return, line feed) a pair of ASCII codes, 13 and 10, that tell a terminal or printer to return to the beginning of the line and advance to the next line. Under Windows, CRLF indicates the end of a line in a text file; the Macintosh uses CR alone and UNIX uses LF alone. See CR; LF.CRM (customer relationship management) software for keeping track of past customers, sales prospects, and the like.crop factor the factor by which the image sensor of a DSLR camera is smaller than the film for which the camera’s lenses were designed. For example, on 35-mm film, each picture is 24  36 mm. If a DSLR has an image sensor half as big, 12  18 mm, it will have a crop factor of 2. Popular DSLRs actually have a crop factor of about 1.5.The crop factor effectively multiplies the focal length of the lens. A 100-mm lens on a DSLR with a crop factor of 1.5 will cover the same field of view as a 150-mm lens on a 35-mm film SLR. That is, it has a “35-mm equivalent” of 150 mm.cross-platform applicable to more than one kind of computer (e.g., PC and Macintosh).cross-post to place a single copy of a message into two or more news-groups at once. This is less expensive than posting separate copies of it in different newsgroups. It also ensures that all replies made in any of the newsgroups will be cross-posted to all of them. See NEWSGROUP.Crossfire technology allowing the use of multiple graphics cards to enhance the computer’s ability to display graphics, developed by ATI (now part of AMD). Contrast NVIDIA.crossover cable a cable with RJ-45 connectors that swap the input and out-put lines. A crossover cable can be used to connect two computers with 10base-T networking without a hub. See RJ-45 (wiring table).CRT (cathode ray tube) a glass tube with a screen that glows when struck by electrons. An image is formed by constantly scanning the screen with an electron beam. Examples of CRTs include television screens and computer monitors. See also EYEGLASSES, COMPUTER.crunch mode (slang) a work situation in which a deadline is near and everyone is working hard, keeping extended hours. Crunch mode is usu-ally the result of a mistaken estimate made by management, not a gen-uine emergency. See SOFTWARE ENGINEERING. cryptography	120cryptography the technology of encoding information so that it cannot be read by an unauthorized person. See ENCRYPTION and its cross-references.C/SC text typeset in capitals and small capitals (LIKE THIS). Sometimes written “C + SC.” See also CAPS; SMALL CAPS; U/LC. Contrast EVEN SMALLS.CSMA/CD see ETHERNET.CSS1.	See CASCADING STYLE SHEET.2.	(Content Scrambling System) an encryption-based software system developed by movie studios to prevent the copying of DVDs. See DVD;DECSS; DMCA.CSV file a text file of comma-separated values, usually with character strings in quotes, thus:”Covington, Michael A.”,”Valdosta”,4633,2.98”Downing, Douglas”,”Seattle”,1234,4.23Spaces after the commas are permitted but have no effect. This is a pop-ular way of saving the contents of a SPREADSHEET as a text file that can be read back in without losing the arrangement of the data. CompareTAB-DELIMITED.Ctrl see CONTROL KEY.Ctrl-Alt-Del a combination of keys with a special function on PC-compat-ible computers, typed by holding down Ctrl and Alt while pressing Del (Delete). Under Windows, it brings up a menu that makes it possible to kill (terminate) a malfunctioning program. (To do so, in current versions, choose Task Manager.)In Windows NT and its successors, users must also press Ctrl-Alt-Del in order to log in. For hardware reasons, only the operating system is able to respond to Ctrl-Alt-Del, so this provides assurance that when log-ging in, the user is seeing a real login prompt, not a fake screen put there by a prankster wanting to collect passwords.cubic spline a curve that connects a set of points smoothly by solving a sys-tem of cubic equations. Unlike a Bézier spline, a cubic spline is defined only by the points that the curve must pass through; it has no control points that are not on the curve.Cubic splines are the natural shapes of bent objects that are secured at particular points and are free to bend in between. The spline goes through each point smoothly, without sharp bends.Each segment of the spline (from one point to the next) is modeled by a third-degree (cubic) polynomial of the form y = ax3 + bx2 + cx + d, where a, b, c, and d depend on the endpoints of the segment and the slope that the segment should have at each end.If (x1,y1) and (x2,y2) are the endpoints and y ′1 and y ′2 are the slopes, then a, b, c, and d can be found by solving the four-equation system: 121	current loopy1 = ax13 + bx12 + cx1 + d y2 = ax23 + bx22 + cx2 + d y ′1 = 3ax12 +2bx1 + cy ′2 = 3ax22 +2bx2 + cMore commonly, the slopes are not known, but the slope at the end of each segment is set equal to the slope at the beginning of the next seg-ment. The slopes and coefficients are then found by solving a system of simultaneous linear equations (linear because x, x2, and x3 are known and can be treated as constants). Compare B-SPLINE; BÉZIER SPLINE.Figure 70. Cubic splinecue (in animation and presentation programs) an embedded code that spec-ifies when an action is to occur.Cuil (www.cuil.com) a search engine that was created to be a rival to Google. It delivers search results in tabs and menus that help the user to narrow the search.curly brackets the characters { }, also called BRACES. Contrast SQUARE BRACKETS; PARENTHESES; ANGLE BRACKETS.current the flow of electrical charge. Current is measured in amperes; 1 ampere = 6.25  1018 electrons per second = 1 coulomb per second.current directory the directory in which the computer looks for files if no other directory is specified. The current directory can be changed by cd commands in Windows and UNIX. In Windows, there is a current direc-tory on each drive, so that, for example, C:MYFILE means file MYFILE in the current directory of drive C (whereas C:\MYFILE would mean MYFILE in the root directory).To see the current directory and current drive, type cd in Windows, or pwd in UNIX.current drive in Windows and similar operating systems, the disk drive on which the computer looks for files if no other drive is specified. See CUR-RENT DIRECTORY.current loop a predecessor of RS-232 serial communication; it is occa-sionally still seen on older equipment. (See RS-232.) Do not connect cur-rent loop equipment directly to RS-232 equipment; the current loop system uses voltages as high as 100 volts and can cause damage. cursor	122cursor1.	the symbol on a computer terminal that shows you where on the screen the next character you type will appear. Cursors often appear as blinking dashes or rectangles. Many computers have cursor movement (arrow) keys that allow you to move the cursor vertically or horizontally around the screen. This ability is essential for text-editing purposes such as word processing. You can use the mouse to move the cursor quickly around the screen. Compare INSERTION POINT.2.	the mouse pointer. See also HOURGLASS.cusp node a type of NODE that marks a sudden change in the direction of the line. See Figure 71. Contrast SMOOTH NODE.FIGURE 71. Cusp nodecut to remove material from the document you are editing and place it into a holding area. See COPY; PASTE; CLIPBOARD.cyan a vivid greenish-blue color that is one of the standard printing ink col-ors. See CMYK.cyber- (prefix) see CYBERNETICS.cyber cafe an INTERNET CAFE.Cyberabad nickname for the city of Hyderabad, India, a center of high-technology industry.cybernetics the study of the processing of information by machinery, espe-cially computers and control systems; derived from Greek kybernetes meaning “helmsman”; first conceived in the 1940s. Cybernetics has evolved into computer science, operations research, and a number of other fields. The prefix cyber- on numerous computer terms is derived from this word.cyberpunk1.	an antisocial person who uses computers as a means of self-expres-sion, often performing destructive acts.2.	a genre of science fiction dating from William Gibson’s 1982 novel Neuromancer, with themes of pessimism and rebellion against a com-puter-controlled society. cyberspace the part of human society and culture that exists in networked computer systems rather than in any particular physical location. For 123	Cyrillicexample, cyberspace is where most bank accounts and electronic mes-sages reside.cybersquatting another name for DOMAIN NAME POACHING. See also UDRP.cyburbia (cyber suburbia) the community of computer users that exists in cyberspace. See CYBERSPACE; NETIZEN.cycle one oscillation of a computer’s CPU CLOCK; the shortest step into which computer actions can be divided. When two or more programs are running at once, they are said to be competing for cycles.cyclical redundancy check an error-detecting code similar to a CHECKSUM but computed with a more elaborate algorithm. Each segment of the original message is combined with additional bits to make a binary num-ber that is divisible by some previously chosen divisor.Cyclical redundancy checks are used to ensure that data is read cor-rectly from disks and other storage media. A defective CD or DVD often causes a cyclical redundancy check failure.cylinder see DISK.Cyrillic the Russian alphabet. Contrast LATIN. DAC, D/A converter	124DDAC, D/A converter see DIGITAL-TO-ANALOG CONVERTER.daemon (under UNIX) a program that runs continuously in the back-ground, or is activated by a particular event. The word daemon is Greek for “spirit” or “soul.”dagger the character †, sometimes used to mark footnotes. See also FOOT-NOTE. Also called an OBELISK or LONG CROSS.daisy-chain to connect devices together in sequence with cables. For example, if four devices A, B, C, and D are daisy-chained, there will be a cable from A to B, a cable from B to C, and a cable from C to D.daisywheel printer a printer that uses a rotating plastic wheel as a type ele-ment. Daisywheel printers were often used with microcomputers in the early 1980s. They printed high-quality text, but they were relatively slow and could not print graphics.DAS see DIRECT(LY) ATTACHED STORAGE.dash (—) a punctuation mark similar to a hyphen, but longer. On a type-writer, a dash is typed as two hyphens.Proportional-pitch type often includes one or more kinds of dashes, such as an em dash (—), which is as wide as the height of the font, and an en dash (–), which is two-thirds as wide as the em dash. Normally, the em dash joins sentences and the en dash joins numbers (as in “1995–98”).data information. The word was originally the plural of datum, which means “a single fact,” but it is now used as a collective singular.data bits a parameter of RS-232 serial communication. Either 7 or 8 bits are used for each character, preceded by a start bit and followed by a parity bit (optional) and a stop bit. See also RS-232; KERMIT.data communication the transfer of information from one computer to another. In order for communication to take place, several aspects of the communication process must be standardized. The international OSI (Open Systems Interconnection) standard (ISO Standard 7498) defines seven layers at which decisions have to be made:1. Physical layer. What kind of electrical signals are sent from machine to machine? For examples of standards on this level, see10BASE-T; RS-232; MODEM.2.	Link layer. How do the two machines coordinate the physical send-ing and receiving of signals? For examples, see HANDSHAKING;PACKET.3.	Network layer. How does one machine establish a connection with the other? This covers such things as telephone dialing and the ZIP FILE; STUFFIT;125	Data Protection Actrouting of packets. For examples, see HAYES COMPATIBILITY (com-mand chart); PACKET; COLLISION; X.25.4.	Transport layer. How do the computers identify each other and coordinate the sending of messages back and forth? This is the level at which most network protocols operate. For examples, seeTCP/IP; NETBEUI; IPX/SPX. See also PROTOCOL.5.	Session layer. How do users establish connections, log on, and identify themselves?6.	Presentation layer. What does the information look like when received on the user’s machine? The presentation layer includes file format and filename conversions and the like.7.	Application layer. How does software use the network—that is, how do application programs exchange data? The application layer does not consist of the programs themselves but, rather, the communication facilities that they use.The OSI standard does not specify what any of these layers should look like; it merely defines a framework in terms of which future stan-dards can be expressed. In a simple system, some of the layers are han-dled manually or are trivially simple.data compression the storage of data in a way that makes it occupy less space than if it were stored in its original form. For example, long sequences of repeated characters can be replaced with short codes that mean “The following character is repeated 35 times,” or the like. A more thorough form of data compression involves using codes of different lengths for different character sequences so that the most common sequences take up less space.Most text files can be compressed to about half their normal size. Digitized images can often be compressed to 10 percent of their original size (or even more if some loss of fine detail can be tolerated), but machine-language programs sometimes cannot be compressed at allbecause they contain no recurrent patterns. See alsoJPEG; MPEG; MP3.data mining the exploration of DATABASES to find patterns in the data. For instance, data mining of the sales records of a supermarket chain can reveal seasonal patterns and hidden relationships between products. The classic example is the discovery that an appreciable number of cus-tomers—presumably young fathers—are likely to buy both diapers and beer on Fridays.data processing the processing of information by computers. This term dates back to the 1960s and often describes the part of a business orga-nization that handles repetitive computerized tasks such as billing and payroll.Data Protection Act a British law protecting people from misuse of their personal information, enforced by the Information Commissioner’s Office (web address: www.ico.gov.uk). DATA STRUCTURESdata rate	126data rate see BAUD.data recovery the art and technique of recovering part or all of the informa-tion lost because of accidental deletion or damage to the storage media.The simplest kind of data recovery is to pull files back out of the Windows RECYCLE BIN or Macintosh TRASH. Special software can retrieve any deleted file that resided on an area of disk that has not yet been overwritten (see RECOVERING ERASED FILES).If the disk or other storage medium has been damaged, data recovery can still be done by technicians who can replace parts of disk drives, adjust them to read poorly recorded tracks, and the like. There are also utilities for recovering lost data on FLASH MEMORY CARDS whose directo-ries have become corrupted.data structures ways of arranging information in the memory of a com-puter. In computer programming, it is often necessary to store large num-bers of items in such a manner as to reflect a relationship between them. The three basic ways of doing this are the following:1.	An array consists of many items of the same type, identified by number. The examination scores of a college class might be repre-sented as an array of numbers. A picture can be represented as a large array of brightness readings, one for each of the thousands of cells into which the picture is divided.2.	A record (in C, a struct) consists of items of different types, stored together. For example, the teacher’s record of an individual student might consist of a name (character data), number of absences (an integer), and a grade average (a floating-point num-ber). Records and arrays can be combined. The teacher’s records of the entire class form an array of individual records; each record might contain, among other things, an array of test scores.3.	A linked list is like an array except that the physical memory loca-tions in which the items are stored are not necessarily consecutive; instead, the location of the next item is stored alongside each item. This makes it possible to insert items in the middle of the list with-out moving other items to make room. More complex linked struc-tures, such as trees, can be constructed by storing more than one address with each item.4.	An object. See OBJECT; object-oriented programming.See ARRAY; LINKED LIST; RECORD.data types kinds of information that can be represented in a computer pro-gram, such as whole numbers, floating-point numbers, Boolean values (true and false), characters, strings, and pointers. In most programming languages, the type of each variable must be declared before the variable can be used. Some languages such as Lisp, Prolog, and Visual Basic allow some or all variables to take on values of any type.In many programming languages, programmers can define their own types, either as subranges of existing types (e.g., numbers between 0 and23), or as	combining smaller units of information. In 127	database management OBJECT-ORIENTED PROGRAMMING, user-defined types can have procedures (called METHODs) associated with them.database a collection of data stored on a computer storage medium, such as a disk, that can be used for more than one purpose. For example, a firm that maintains a database containing information on its employees will be able to use the same data for payroll, personnel, and other pur-poses. See DATABASE MANAGEMENT.FIGURE 72. Databasedatabase management the task of storing data in a database and retriev-ing information from that data. There are three aspects of database man-agement: entering data, modifying or updating data, and presenting output reports. Many mainframe computers are used by businesses for database management purposes. Several software packages are avail-able for database management on microcomputers, such as dBASE and Microsoft Access, and some data management capabilities are provided with spreadsheets such as Lotus 1-2-3 and Excel. Some examples of database applications include maintaining employee lists and preparing payrolls; maintaining parts order lists and keeping track of inventories; maintaining customer lists and preparing bills for credit customers; and keeping track of the students at a school.Information in a database system is generally stored in several differ-ent files. For example, a business will often have a file of regular cus-tomers and a file of employees. Each file consists of a series of records, each representing one person or one transaction. Each record consists of several fields, with each field containing an individual data item. For example, in an employee file there would be one record for each employee, and there would be a field containing the person’s name, a field for the address, a field for the Social Security number, and so on. ONLINE TRADING.datagram	128A database management system must make provisions for adding new records (e.g., when an employee is hired); for deleting unneeded records (e.g., when an employee retires); and for modifying existing records. Some fields (such as the Social Security number) will not change; other fields (such as year-to-date pay) must be changed frequently.The main purpose of a database management system is to make it possible to obtain meaningful information from the data contained in the database. A database program can respond to brief queries on the screen, or it can present detailed printed reports in a format chosen by the user. Here are some general functions that a database management system should be able to fulfill:1.	Sort the records according to the order indicated by one specific field (e.g., sort in alphabetical order by name, or in numerical order by zip code). You should be able to designate a secondary field along which sorting will occur when there are ties in the pri-mary field. For example, if you are sorting the records by the num-ber of months the customers are overdue in their payments, you probably would like the names of all people 1 month overdue in alphabetical order, then the names of all people 2 months overdue in alphabetical order and so on.2.	Set up selection criteria that allow you to examine only the records that meet a specific condition. For example, you may wish to look only at customers who live in your city, or you may wish to look at all employees whose job title is either “delivery driver” or “warehouse worker.”3.	Count the number of records that meet a specific condition. For example, you may wish to count the number of employees who have been with the company for more than 10 years.4.	Perform calculations, such as computing the total amount owed on overdue accounts, or the year-to-date pay for each employee.5.	Connect information from more than one file. For example, a data-base system might contain an employee file that lists the job clas-sification for each employee. A separate file for each job classification would contain information on wages, fringe benefits, and work schedules that apply to all workers in that classification.See also RELATIONAL DATABASE; SQL.datagram a PACKET of information transmitted by NETWORK.daughterboard, daughtercard a small circuit board that plugs into a larger one. Contrast MOTHERBOARD.day trading the practice of buying stocks or other securities and reselling them within a day (or less) to profit from short-term fluctuations. Before the Internet, day trading was possible only by spending all your time at a stockbroker’s office; otherwise you would not see market results quickly enough to act upon them. Nowadays, day trading can be carriedout online. See VGA CONNECTOR.129	.deDazed and confused... an error message displayed by some versions of LINUX upon encountering an apparent hardware failure.dB abbreviation for DECIBEL.DB-9, DB-15, DB-25 designations for the kind of connectors commonly used on serial ports, video cards, and parallel ports respectively, with 9, 15, or 25 pins. For pictures see Figure 235 (page 432) and Figure 189 (page 352). The suffix P means “plug” and S means “socket;” thus aDB-25P has 25 pins and a DB-25S has 25 holes. SeeDB2 popular database management software from IBM (see www-306.ibm.com/software/data/db2).dBm power level in decibels relative to a level of one milliwatt; used to measure signal strength on telephone lines. See DECIBEL.DBMS (DataBase Management System). See DATABASE MANAGEMENT.DCE (Data Communications Equipment) equipment that uses RS-232 ser-ial communications, with conductor 2 for input and conductor 3 for out-put. Equipment that uses conductor 3 for input and 2 for output is called DTE (Data Terminal Equipment). A standard RS-232 cable can link two pieces of equipment only if one of them is DTE and the other is DCE; otherwise, both will try to transmit and receive on the same conductors, and a special cable that interchanges conductors 2 and 3 must be used. Most PC serial ports are configured as DTE; most modems as DCE.DCOM (Distributed Component Object Model) a Microsoft-developed standard for allowing software components to interact with each other over a network. For an alternative standard, see CORBA and COM.DDoS (Distributed Denial of Service) a DENIAL-OF-SERVICE ATTACK con-ducted through a large set of attackers at widely distributed locations. This is often done by distributing a computer virus that will turn its vic-tims into ZOMBIES that carry out the attack.DDR1.	(describing computer memory) (double data rate) term used to describe a type of SDRAM computer memory that gives faster perfor-mance by transmitting data on both the rising and the falling edges of each clock pulse. See SDRAM.2.	(Dance Dance Revolution) a popular game for the Sony Playstation, Nintendo Wii, and other game machines, in which the player dances on a platform that senses his or her movements. Introduced in 1998, it was one of the first video games to incorporate real exercise.DDR 2, DDR3 higher-speed versions of DDR SDRAM. See DDR (defini-tion 1)..de suffix indicating that an e-mail or web address is registered in Germany (in German, Deutschland). See TLD. de-Bayerization, de-Bayering	130de-Bayerization, de-Bayering the act of decoding an image from a BAYER MATRIX to a full-color picture.de facto standard a standard that is not official but is established by wide-spread usage.dead link an HTML address that is no longer valid. When a dead link is selected, the browser returns an error message.Dead links are the result of the target web page having moved to a new location, an HTML programming error (usually a mistyped file-name), or the server being overloaded. Try the link again later when the Internet is not as busy. If you still get an error message, you may want to e-mail the appropriate WEBMASTER about the dead link.dead start see BOOT.deadlock a situation in which each of two processes is waiting for the other to do something; thus, neither one can proceed. See MULTITASKING.Debian a distribution of Linux and a wide variety of free application software originated by Debra and Ian Murdock (hence the name). It is one of the most popular Linux distributions. For more information, or to download Debian free of charge, see www.debian.org. See also LINUX; UBUNTU.deblurring the use of digital image processing to correct a blurred image. In order for this to be possible, the exact nature of the blur must be known; sometimes it can be inferred from the appearance of a small, bright object in the picture. See IMAGE PROCESSING; SHARPEN.debug1.	to remove errors (bugs) from a computer program. See BUG.2.	to run a computer program one step at a time while monitoring the values of variables, in an attempt to diagnose errors. See DEBUGGER.debugger a software tool for running programs one step at a time and examining the contents of memory.DEC see DIGITAL EQUIPMENT CORPORATION.deca- metric prefix meaning 10 (= 101). Deca- is derived from the Greek word for “ten.” See METRIC PREFIXES.deci- metric prefix meaning ÷ 10. Deci- is derived from the Latin word for “ten.” See METRIC PREFIXES.decibel (dB) a unit of relative loudness or power; one tenth of a bel (a unit named for Alexander Graham Bell and now rarely used). Decibels are used in three ways:1.  to express the ratio of two power levels:first power level dB = 10 log10 second power level 131	defaultFor example, multiplying power by 2 is equivalent to adding about 3 decibels; multiplying power by 10 is equivalent to adding 10 deci-bels; and multiplying by 100 is equivalent to adding 20 decibels.2.  to express the ratio of two voltage levels:first voltage level dB = 20 log10 second voltage levelBecause power is proportional to the square of voltage, this is equivalent to the previous formula if both voltages are driving the same load impedance.3.	to describe the loudness of a sound, expressed in decibels relative to the threshold of human hearing. Clearly audible sounds range from about 20 to 100 dB; those much above 100 dB are painful to the ears.decimal number a number expressed in ordinary base-10 notation, using the digits 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, whether or not there are any digits to the right of the point. For example, 3.14 is a decimal number, and so is 314.declare to state the attributes of a variable, such as its DATA TYPE.decoder a circuit that recognizes a particular pattern of bits. Decoders are used in computers in order to recognize instructions and addresses. Figure 73 shows a decoder that recognizes the bit pattern 1101.FIGURE 73. Decoder for the bit pattern 1101decryption decoding—that is, translating information from an unreadable or secret format into a form in which it can be used. Contrast ENCRYPTION.DeCSS a program making it possible to copy DVDs encrypted by CSS. It was developed by 15-year-old Norwegian Jon Johansen, working with other hackers, who was tried but acquitted of criminal charges in Norway. See DMCA.dedicated assigned to only one function. For instance, a dedicated phone line is one that is always connected to the same equipment.default an assumption that a computer makes unless it is given specific instructions to the contrary. For example, a word processing program may start out assuming a particular default combination of margins, page length, and so on, which the user can change by issuing specific commands. ERING ERASED FILESCURRENT DRIVE.CURRENT DIRECTORY.default directory	132default directory seedefault drive seedefault logic (defeasible logic) a formal system of reasoning in which some facts or rules have priority over others. For example, statements about ostriches might have priority over statements about birds because an ostrich is a specific kind of bird. It is then possible to say without contra-diction that birds fly, but ostriches don’t fly. In classical logic, “birds fly” and “ostriches are birds” together with “ostriches don’t fly” is a contra-diction. Default logic is often used in expert systems. See EXPERT SYSTEM.Contrast FUZZY LOGIC; CONFIDENCE FACTOR. See also BOOLEAN ALGEBRA.Default.asp, Default.htm, Default.html in Microsoft’s web server soft-ware, the file name that is used for a WEB PAGE when no file name is specified in the URL. It has the same role as the more common file name index.html. See INDEX.HTML.defeasible logic see DEFAULT LOGIC.deform (3D program) to digitally manipulate an on-screen object so that it is twisted or stretched. Some programs allow you to deform objects interactively; other transformations are done with FILTERs that can distort or break up the object during an animation.degauss to demagnetize. Color CRTs need to be degaussed when they show areas of weak or incorrect color. Some monitors degauss themselves every time they are turned on.degree measure a way of measuring the size of angles in which a complete rotation has a measure of 360 degrees (written as 360°). A right angle is 90°. Contrast RADIAN MEASURE.DejaNews a SEARCH ENGINE for Usenet NEWSGROUPs, formerly located at www.deja.com but now incorporated into Google (google.com) as “Groups.” It contains permanent copies of almost all newsgroup post-ings since 1981. The name DejaNews was a pun on French déjà vu “already seen.”Del the DELETE key on a computer keyboard.delegate (in C#) a variable whose value is a METHOD; more precisely, an OBJECT that contains or points to a method, allowing one method to be passed as a parameter to another. Delegates serve the same function in C# as function pointers in C.delete to remove an unwanted item (character, word, art, file). See RECOV-for help on restoring deleted files.delimiter a character that marks the beginning or end of a special part of a computer program. For instance, /* and */ are delimiters marking the beginning and end of a comment in C. In many programming languages, quotation marks are used as delimiters to mark character strings. 133	deselectDelphi an object-oriented version of the programming language PASCAL for developing interactive programs under Windows. Delphi was designed by Anders Hejlsberg, who also developed TURBO PASCAL and C#. It is a product of Borland International. See also KYLIX.demibold a typeface weight between ordinary type and boldface. Sometimes just called demi. See WEIGHT.demon see DAEMON.denial-of-service attack a malicious attack on a computer whose purpose is to interfere with the computer’s normal functioning, rather than to gain services for oneself or steal confidential data. Denial-of-service attacks are often launched by people who are frustrated at not being able to break into a computer, or who are angry at the target computer’s users or administrators. Often the attackers do not realize they are disrupting service for everyone, not just for a single intended victim. For examples, see MAIL BOMBING; PING FLOODING. See also COMPUTER SECURITY; DDOS.deployment the act of installing software on computers.deprecated a software feature from a previous version that still exists in the current version, but the developers recommend that it no longer be used (likely because a newer feature has been introduced).depth of field the ability of a picture to show objects at different distances in focus at the same time. Depth of field is greater at smaller apertures (higher-numbered f-ratios). See F-RATIO.depth of focus tolerance of focusing errors; like DEPTH OF FIELD but refer-ring to variation in the way the camera is focused, rather than variation in the distance of the subject.DES (Digital Encryption Standard) an encryption system using 56-bit keys in a complicated 16-round substitution process. It was the U.S. govern-ment standard before the adoption of AES. See ENCRYPTION.descender the part of a character that extends below the baseline. For instance, the letter p has a descender; the letter o does not. See ASCENDER; TYPEFACE.FIGURE 74. Descendersdeselect to tell the computer you do not want to work with a particular object. There are minor differences in how different software does this, but clicking on the background or another object will usually deselect the current object. If you want to select multiple objects, you can hold deskew	134down Ctrl while clicking on the desired objects. This allows you to select as many items as you want. To deselect just one item of a group, click on it again while continuing to hold Ctrl. If you are choosing between mutually exclusive options in a dialog box, choosing one but-ton will clear the others (see RADIO BUTTONS; OPTION BUTTONS).Hint: If you wish to select all but one or two objects in a drawing, the fastest way is to “Select All” (either by using the edit menu option or by MARQUEE SELECT), and then deselect the unwanted objects.deskew to straighten; to undo the effects of a SKEW command.desktop the whole computer screen, representing your workspace. You manipulate objects (ICONs) with the mouse in much the same way that you work with papers and other objects on your physical desktop.On the Macintosh, the desktop is also a special file containing infor-mation about the arrangement of icons, the programs you are using, and the like. This information is saved whenever you shut the computer down and retrieved when you turn it on again.In Windows, the desktop is a special directory for each user. It nor-mally contains many SHORTCUTs to program files in other locations. The shortcuts are represented by files with the extension .lnk.The desktop is not identical with the ROOT DIRECTORY of a disk; it is more like a directory containing everything on the computer, including the disk drives. In Windows, the disk drives are accessed through a desk-top icon called “Computer.”FIGURE 75. Desktop (Windows)desktop publishing the use of personal computers to design and print pro-fessional-quality typeset documents. A desktop publishing program such as Adobe InDesign or QuarkXpress is much more versatile than a word processing program; in addition to typing documents, the user can spec-ify the layout in great detail, use multiple input files, have comprehen- 135	device nodesive typographic control, insert pictures, and prepare the publication for printing or electronic distribution.The distinction between word processing and desktop publishing is becoming blurred. Most word processing programs can produce ele-gantly typeset documents that include extensive graphics. The major dif-ference is that desktop publishing programs allow objects to be placed in a particular location on the page, whereas word processors treat the text and graphics linearly. This gives the desktop publisher the tools neces-sary for handling more complex works and allows more emphasis on the publication’s design.A subcategory of desktop publishing is ELECTRONIC PUBLISHING, that is to say the production of publications that are meant to be viewed on screen rather than printed. These electronic documents are distributed by computer networks (see WORLD WIDE WEB), e-mail, or CD-ROMs. An elec-tronic document may have sound, music, illustrations, animations, video clips, or HYPERTEXT links (special buttons or keywords that jump the reader to a new text or picture).Desktop, Active see ACTIVE DESKTOP./dev in UNIX, the directory that contains links to specific devices such as disk drives and serial and parallel ports.device any component of a computer that is used for input or output, such as a printer, modem, disk drive, or sound card.device contention the situation in which several computer programs, run-ning concurrently, are trying to use the same device (such as a modem or printer) at the same time. Multitasking operating systems handle con-tention in a number of different ways, such as SPOOLING output for print-ers, making a program wait until the requested device is available, or simply denying access to a device that is already in use.device driver a program that extends the operating system in order to sup-port a specific device, such as a disk or tape drive, video card, or printer.Device drivers are a very important part of Microsoft Windows. They insulate application programs from the hardware so that, for example, the manufacturer of a word processing program does not have to know what kind of printer you are going to be using, and if a new printer is invented in the future, you can use it even if it wasn’t anticipated when the program was written. Installation of device drivers usually happens automatically when hardware or software is installed; you can also add and remove device drivers from the Control Panel (see CONTROL PANEL;PLUG AND PLAY).device ID a unique name given to a hardware device for use by PLUG AND PLAY and the Windows REGISTRY.device node a directory entry, similar to the directory entry for a file but identifying a piece of hardware. Under UNIX, device nodes are found in the directory /dev. /dev/null	136/dev/null in UNIX, a “device” that is actually a place for discarding data. Anything written to /dev/null is discarded, and any software that attempts to read from /dev/null is told that no data is available. You can specify /dev/null in places where a filename is required but no data will actually be read or written. Compare BIT BUCKET.dewarp to straighten; to undo the effects of a WARP manipulation.DHCP (Dynamic Host Configuration Protocol) a protocol for assigning an IP ADDRESS to each computer automatically as it joins the network, for use as long as it remains connected, rather than assigning a permanent IP address in advance. See LEASE; PROTOCOL.Dhrystone see MIPS.DHTML see DYNAMIC HTML.dial-up connection a connection between computers established by dialing a telephone number through a MODEM.dial-up networking computer networking that relies on communication through ordinary telephone lines via MODEM.dialog box a window that appears in order to collect information from the user. When the user has filled in the necessary information or clicked on the appropriate buttons, the dialog box disappears. Figure 76 shows a dialog box containing several different kinds of elements. There is almost always an OK button for the user to click after filling in the infor-mation. See also CHECK BOX; LIST BOX; OPTION BUTTONS; TEXT BOX. FIGURE 76. Dialog box 137	digital dividedictionary attack an attempt to guess a computer password by trying every word in a large dictionary and, often, every combination of letters that would make a pronounceable word.die (plural dice or die) the individual piece of silicon containing a transis-tor or integrated circuit. A memory cache located “on die” is one on the same piece of silicon as the CPU.digerati (slang, plural) people knowledgeable about the Web and other digital communications media. (From digital and Italian litterati.)digital representing information as electrical “on” and “off” signals that correspond to binary digits and can be stored in computer memory. Digital electronics contrasts with analog electronics, which represents information with signals that vary within a predefined range. Digital sig-nals have two advantages: they can be copied exactly, without even the slightest loss of quality, and they can be further processed by computer.See also DIGITAL COMPUTER.digital audio sound represented in digital form. See DIGITIZED MUSIC; MP3;WAVE FILE.digital camera a camera that takes pictures with a CCD IMAGE SENSOR or CMOS image sensor and transmits them directly to a computer or records them on a memory card without using film.The most important specification of a digital camera is the number of PIXELs in the image. To show as much detail as a good 35-mm slide, a dig-ital image requires about 2000  3000 pixels. A much lower resolution is sufficient for snapshots. The image on a TV screen is equivalent to about 400  600 pixels. See MEGAPIXEL (table). See also CCD IMAGE SENSOR;CMOS IMAGE SENSOR; FOCAL LENGTH; FOVEON; F-RATIO; IMAGE PROCESSING.digital computer a computer that represents information in discrete form, as opposed to an analog computer, which allows representation to vary along a continuum. For example, the temperature of a room might be any value between 0° and 100°F. An analog computer could represent this as a continuously varying voltage between 0.00 and 1.00 volts. In contrast, a digital computer would have to represent it as a decimal or binary num-ber with a specific number of digits (e.g., 68.80 or 68.81).All modern, general-purpose computers are digital. Analog computer circuits are, however, frequently used in industrial control equipment.A digital computer is more accurate than an analog computer because it only needs to sense the difference between clearly distinguishable states. For example, a slight voltage fluctuation would affect the result in an analog computer, but a slight voltage fluctuation would not affect a digital computer because the computer could still easily distinguish the 0 state from the 1 state of any circuit element. For the same reason, dig-ital music reproduction (e.g., a compact disc) is more accurate than ana-log reproductions (e.g., a traditional record).digital divide the division of the world into those that have access to com-puters and the Internet and those that are too poor to afford that access. Digital Equipment Corporation	138Digital Equipment Corporation (DEC) a company whose products included the PDP-8, PDP-11, and VAX minicomputers, the VT-100 ter-minal, and the Alpha microprocessor. In 1998, Digital Equipment Corporation was aquired by Compaq, now part of Hewlett-Packard.digital film the memory cards or other storage devices used in digital cam-eras. Unlike real photographic film, the brand of digital film does not affect the quality of the picture or the amount of light required to make it. The digital memory device simply records whatever the camera gets.digital image processing see IMAGE PROCESSING.Digital Millennium Copyright Act see DMCA.digital music music signals that have been converted to numbers that are stored electronically. A musical sound wave is continuous, but it can be represented by numbers that give the amplitude of the wave at each moment in time, as long as the sound is sampled often enough to seem continuous. See SAMPLING RATE. A music CD stores sound digitally (as opposed to a traditional vinyl record, which stored sound in analog for-mat by the variations in the groove.) Music files can also be stored on a computer hard disk, but the file representing a song is too large to do this conveniently unless it has been compressed. The development of MP3 compression led to a boom in digital music as people stored music on computer hard disks and digital players such as the IPOD. However, ram-pant file sharing through services such as NAPSTER and GROKSTER led to court cases that restricted the use of these services. See also P2P; AAC;WAVE FILE; MIDI; WMA; OGG VORBIS; ITUNES.digital photo frame a device that displays one or more digital pictures in succession, over and over, without being constantly connected to a com-puter. The pictures are delivered to it on a memory card or by temporar-ily connecting to a computer using USB or network connections.digital picture frame see DIGITAL PHOTO FRAME.digital signal processing (DSP) the use of computers to process signals such as sound or video. Applications of DSP include decoding modu-lated signals (e.g., in modems), removing noise, and converting video to different formats.digital signature a way of authenticating that an electronic message really came from the person it claims to have come from. A digital signature can be encrypted with your PRIVATE KEY. The recipient can decrypt the message with your PUBLIC KEY to verify that it is really you. Nobody who does not know your private key could create a signature that would cor-rectly decrypt with your public key. However, the digital signature can-not be sent by itself, because then someone could simply copy the encrypted version. Instead, the signature needs to be sent as part of an abstract (or manifest or digest) of a particular message. The recipient can check to make sure that the hash function given in the manifest matches the hash function calculated from the message. If someone tampered 139	DIMMwith the original message (so they could attach their own message to your digital signature) the mismatch would be detected.digital television transmission of video signals to the viewer as digital data rather than as analog signals. Digital television can be delivered by cable or by antenna. In the United States, government regulators have required all full-power broadcasters to switch to digital by February 2009, although analog signals will remain available in cable systems and from low-power transmitters, and older television receivers can receive digi-tal signals by antenna through a converter box. Other countries, such as Luxembourg and the Netherlands, went all-digital as early as 2006.Digital signals can, but need not, be high-definition (see HDTV, SDTV). As with other kinds of digital communication, digital TV signals are almost useless if they are not strong enough to give perfect reception; it is no longer possible to make the best of a weak signal or poor antenna by tolerating a snowy picture and noisy audio.digital-to-analog converter an electronic circuit that converts digital information (binary numbers) into voltages at specific levels. DACs are used to generate sound and video signals. Contrast ANALOG-TO-DIGITAL CONVERTER.digital zoom a change in the field of view of a digital camera achieved by discarding outer parts of the image and using only the center. Although it is displayed enlarged, the zoomed image actually has no more resolution than if it were not zoomed, since you are still looking at the same pixels.Contrast OPTICAL ZOOM. See also RESAMPLE; INTERPOLATION (definition 2).digitize to convert input into a form that can be processed by a computer.See FRAME GRABBER; SCANNER.digitized music music represented as computer data, either by recording the sound waves themselves or by storing a musical score in digital form. See DIGITAL MUSIC.dimensions (of an array) the different directions in which elements can be counted. Each dimension corresponds to one subscript. A list of objects is a one-dimensional array, while a table is a two-dimensional array.See ARRAY.DIMM (dual inline memory module) a memory module similar to a SIMM but with different signals on the two sides of the tiny circuit board. DIMMs usually have 168 pins; compared to SIMMs, they allow more memory to be installed in fewer sockets. See SIMM. FIGURE 77. DIMM (dual inline memory module) dimmed	140dimmed not available for selection. If a menu option appears in light gray rather than black type, it cannot be chosen. For example, if you wish to align two objects, the align command will be dimmed until you have two or more objects selected. If you click on a dimmed command, nothing happens. See Figure 78.FIGURE 78. Dimmed (disabled) selections on menuDIN paper sizes see PAPER SIZES (ISO).dingbats special characters that are neither letters nor mathematical sym-bols, such as ‡ ♠ ♥ ◊ ♣ ↔. See also FLEURON.diode a semiconductor device that allows electric current to pass in one direction, but not in the other. Diodes are formed by joining two types of doped semiconductors: P type, with a deficiency of free electrons (and an excess of holes), and N type, with a surplus of free electrons. (See SEMICONDUCTOR.) The place where the two regions are joined is called the junction.Electrons can flow from N-type to P-type material, but not the other way. The diode is said to be forward-biased when the voltage across it is in the right direction to make it conduct, and reverse-biased when the voltage is applied in the opposite direction. The diode is forward biased when a negative voltage is applied to the N region and a positive voltage to the P region; then free electrons in the N region are driven to the junc-tion where they combine with holes from the P region. If a positive volt-age is applied to the N region, electrons are pulled away from the junction and no current can flow (the diode is reverse biased).See also ELECTRONIC CIRCUIT DIAGRAM SYMBOLS; LED; TRANSISTOR.DIP (dual in-line package) a plastic case with two rows of pins (Fig. 79), the way INTEGRATED CIRCUITS (ICs) were packaged in the early days of personal computers. Most ICs now come in smaller packages which mount on the surface of a printed-circuit board rather than by placing pins into holes.DIP switch a miniature switch or set of switches in the same size package as a DIP integrated circuit. See DIP. DISK SHARING; FILE SHARING; NET-141	directoryFIGURE 79. DIP (dual in-line package) integrated circuitsdir the command, under Windows command line, that makes the computer display all the files in a particular directory. For example,dirby itself lists the files in the current directory of the current disk. Here are some other examples:dir	a:	Current directory of drive Adir	a:\mystuff	Directory MYSTUFF of drive Adir	\mystuff	Directory MYSTUFF of the current diskdir	mystuff	Subdirectory MYSTUFF under the current		directory of the current diskTo see a directory of all of the files on the current drive that have names ending in .bas, typedir *.basThe asterisk acts as a “wild card,” matching all filenames.direct-to-press a method of production where a machine similar to a laser printer takes data directly from a computer to produce film or plates for a printing press without the intermediate step of printing out camera-ready copy.direct(ly) attached storage (DAS) disk drives or other storage devices that are connected directly to the computer that uses them, rather than con-nected through a network. ContrastWORK ATTACHED STORAGE.directory an area on a disk where the names and locations of files are stored. A disk can, and usually does, contain more than one directory; directories can contain other directories.On the Macintosh and in Windows, directories are called folders. Directories are pictured as tree structures or boxes within boxes (Figure 80).Directories are a way of classifying files; they do not divide the disk itself into sections. Any file can use as much space as needed, anywhere on the disk, regardless of what directory it is in. DirectX	142FIGURE 80. Directories (shown graphically)DirectX an optional add-on animation and sound subsystem for Microsoft Windows, required by certain games and other software that needs higher graphics performance than can be obtained through normal oper-ating system services. DirectX is available as a free download from Microsoft and is updated frequently. See also DXDIAG.dirty (slang) needing to be written to disk or needing to be updated in some way.disassembler a program that converts machine instructions into assembly language so that a human being can read them.disc a compact disc (CD) or digital video disc (DVD). See DISK and usage note there.disclaimer a statement absolving someone (or even oneself) of responsi-bility. The so-called “usual disclaimer” on e-mail messages is roughly, “These are not the opinions of my employer or the organization that transmits this message for me.” Many people abbreviate this to “usual disclaimer applies.” The disclaimer is unnecessary unless there is some-thing about the e-mail message that would specifically cause people to misunderstand it.discrete speech speech that is spoken with pauses between words to make it easier for a computer to recognize. Contrast CONTINUOUS SPEECH. SeeSPEECH RECOGNITION.discretionary hyphen a hyphen that is used only when the word falls near the end of a line; sometimes called a SOFT HYPHEN. By specifying where you want the word to be broken, you override the word processor’s auto-matic hyphenation. Contrast REQUIRED HYPHEN; HARD HYPHEN.disk a round, flat device for storing computer data. There are three main kinds: DISKETTEs, HARD DISKs, and OPTICAL DISKs (of which CD-ROMs are the most common kind).Diskettes are made of flexible plastic coated with iron oxide. Information is recorded on them by magnetizing the iron oxide coating 143	diskettein specific places. They were the main storage medium during the first couple of decades of microcomputer use but they are less common now. There were two sizes: 51⁄4 inches and 31⁄2 inches.Hard disks are like diskettes except that the iron oxide is coated on stiff aluminum disks. There are several layers in a single disk pack. Microcomputers come with a hard disk permanently mounted within the computer, and external hard disks can be used to back up the internal hard disk.Hard drive capacities have increased dramatically since their intro-duction. Early drives only held five megabytes of data. Modern hard dri-ves typically store hundreds of gigabytes.The iron oxide on the disk consists of microscopically small needles, each of which acts like a tiny bar magnet. Information is stored by magne-tizing these needles. The read-write head, which skims the surface of the disk, can either generate a magnetic field to magnetize the needles or detect the magnetic field of needles that are already magnetized. The binary dig-its 0 and 1 are represented by changes in the direction of magnetization.Data on disks is stored in many concentric circles, each of which is called a track. Each track is divided into sectors, which are the smallest units that the computer can read into memory in a single step. On a dou-ble-sided or multilayer disk pack, the set of tracks in corresponding posi-tions on different layers is known as a cylinder.The directory of a disk is a special area in which the computer records the names and locations of all the files on the disk. The user can create many directories on a single disk.Optical disks store information by etching a transparent plastic medium with a laser beam. See CD-ROM; CD-R; CD-RW; DVD.Usage note: This word is often spelled disc when referring to com-pact discs (CDs), DVDs, and laser discs, but is always spelled disk when referring to magnetic disks and diskettes. The difference in spelling probably reflects the European origin of the CD.disk drive a device that enables a computer to read (and, in most cases, write) data on disks. Microcomputers typically contain one hard disk drive and one or more CD or DVD drives. See DISC and DISK.disk farm (slang) a room full of disks; a large set of disk drives used by a single computer or network.disk server a computer that performs DISK SHARING. Contrast FILE SERVER.disk sharing the use of networking to allow a computer to use a disk drive that is located in another computer, and to format and control it as if it were locally attached, without relying on the server to own and manage files. Disk sharing is essentially the same thing as a storage area network (SAN). Contrast FILE SHARING, NAS.diskette a removable flexible magnetic disk on which computer programs and data can be stored. 51⁄4-inch diskettes were often called floppy disks because the entire diskette, including cover, was flexible. WIDE WEBdiskless workstation	144Usage note: The alternative term floppy disk is going out of fashion because present-day diskettes have rigid shells. See also DISK.diskless workstation a computer that has no disk drive of its own, down-loading the operating system and all software and data files through a network.display panel a small panel that displays information on a piece of equip-ment that does not have a screen. For example, the HP LaserJet 4M printer has a display panel that can display the status of print jobs.dissolve see TRANSITION EFFECT.Distort a set of Photoshop filters that twist an image. Glass, Ocean Ripple, Pinch, and Wave are examples of effects that can be applied. See Figure 81 for an example.FIGURE 81. Distortdistribute a drawing program command that places objects evenly over a defined area.FIGURE 82. Distributedistributed spread over more than one computer. For example, the WORLDis a distributed library of information.distro (slang) a distribution (a prepared, ready-to-install or ready-to-use copy) of a piece of free software such as Linux or TEX. Some free soft-ware packages are so complex that different people have prepared dif-ferent ready-to-use distributions of the same software. See FREE SOFTWARE; LINUX; TEX.dithering the representation of an intermediate color by mixing dots of two other colors. (See Figure 83.) Hardware limitations make it impossible to print or display all possible colors. Dithering is used to represent 145	DMCAshades of gray or colors on a printer or screen that cannot produce them directly.FIGURE 83. Dithering patternsDL (describing a DVD or similar disc) double-layer; recorded in two lay-ers from the same side, giving twice as much capacity as a single layer.DLL (Dynamic Link Library) a file containing a library of machine-lan-guage procedures that can be linked to programs as needed at run time. See LINK, definition 3.DLLs are used in Microsoft Windows. Their practical benefit is that programs don’t need to include code to perform common functions because that code is available in the DLL. The program is smaller, and changes can be made once to the DLL routine instead of separately to each program.DLL Hell (slang) in Windows (especially before 2000), the confusing situ-ation that results when more than one DLL file has the same name. If the DLLs are installed in a system directory, then the most recently installed file replaces earlier ones. Since nothing prevents two programmers from choosing the same file name, the conflicting DLLs can be completely unrelated. More often, the conflicting DLLs are different versions of the same file, one newer than the other; in that case, software relying on the file will start up normally, but there will be subtle malfunctions.Windows 2000, XP, and Vista take various measures to separate DLLs that belong to different programs, and to undo any modifications made to system DLLs. The .NET framework eliminates DLL Hell by tying each application explicitly to its DLLs and allowing multiple ver-sions to coexist.DLP (Digital Light Processing), a system developed by Texas Instruments for projectors that reflect light off an array of microscopic mirrors. By controlling the fraction of the time that a particular mirror reflects, the brightness of each pixel can be controlled. See www.dlp.com.DMA (Direct Memory Addressing) the ability of a peripheral device, such as a disk controller, to access the memory of a computer directly, without going through the CPU. DMA makes it possible to transfer information to or from external devices much more quickly than would be possible if the CPU had to handle every byte of information during the transfer.DMCA (Digital Millennium Copyright Act) a controversial law passed by the U.S. Congress in 1998 designed to secure copyright protection of digital works.Unlike earlier copyright laws, the DMCA prohibits not only copying, but also the use of technology to get around technical measures designed DNA	146to prevent copying, even when the copying itself is legal. “No person shall circumvent a technological measure that effectively controls access to a work protected under this title” (17 USC 1201). The law also makes it ille-gal to distribute circumvention technology but this is hard to enforce.Critics argue that the DMCA is badly flawed. It prohibits people from experimenting to learn exactly how certain software works even when that software is running on their own computers, and unlike all earlier copyright laws, the DMCA enables copyright owners to block people from reading and viewing material that they possess, not just copying from and redistributing it. See also COMPUTER LAW; DECSS.DNA1.	(deoxyribonucleic acid) the molecule that carries inherited informa-tion in all living things. DNA encodes the sequence of amino acids that form proteins. The operation of DNA is somewhat like a computer that uses three-digit base-4 numbers. In this manner, a set of DNA molecules contains the entire plans for building a plant, animal, or human being. Like computer programs, DNA contains “go to” instructions that skip over some of the material.2.	(Windows Distributed Internet Architecture) Microsoft’s model for developing distributed computing applications with Windows.DNS1. (domain name server) a server responsible for translating domain addresses, such as www.example.com, into IP (Internet Protocol) num-bers such as 127.192.92.95. Domain name servers are interconnected so that if the nearest one cannot look up a name, it will query several other servers at various locations. Normally, when a computer is attached to the Internet, the IP address of a DNS has to be given to it as part of the setup information. However, computers that receive IP addresses through DHCP are also given DNS information automatically. See alsoIANA; TLD.2. (Do Not Set) a copyediting abbreviation used to mark marginal nota-tions or special typesetting instructions on a manuscript.do the keyword, in C and related languages, to execute over and over until a condition becomes false, checking the condition at the end rather than at the beginning, so that the loop is guaranteed to execute at least once. Here is an example:/* This is Java; C, C++, C# are very similar */ int i=5;do{System.out.print(i + ” ... ”); i—;}while(i>0);System.out.println(”Finished!”); 147	documentationSee WHILE for explanation of how this works; the only difference is that even if the condition of the do loop were false at the beginning, the loop would execute once, because the test is not performed until the end.Compare REPEAT.Dock1.	to anchor; to fix into position. Many application programs allow the user to move subordinate windows, such as toolboxes, from their origi-nal position to a new location at the top, bottom, or side of the main win-dow, where they snap into place. This is called docking the toolbox.The Windows taskbar (with “Start” and icons for open applications) is normally docked at the bottom of the screen, but by dragging it with the mouse, you can dock it at the top or either side instead.2.	(in Mac OS X) a panel of animated icons at the bottom of the screen that shows the current active (running) programs, and provides a place for aliases of frequently used files and the Trash. The Dock assumes some of the functions of the Application Menu and FINDER from earlier versions of Mac OS. It is similar to the TASKBAR in Windows.docking station an accessory that gives a laptop computer additional capa-bilities when it is used at a fixed location. A typical docking station will include a charger for the laptop’s battery, connection to a larger monitor, and possibly additional disk drives or other peripherals.doctor the version of ELIZA (a simulated psychotherapist) built into the Emacs editor. See ELIZA.document a file containing a text to be printed (e.g., a letter, term paper, or book chapter) or a drawing or other piece of work that a human being is editing with the aid of the computer.document mode the normal way of typing documents that are to be printed. The word processor includes codes that indicate hyphenation, page breaks, and the like, thereby producing a special word processing file rather than a text file. See also NONDOCUMENT MODE; TEXT FILE.documentation written descriptions of computer programs. Documentation falls into several categories:1.	Internal documentation, consisting of comments within the pro-gram. (See COMMENT.) Internal documentation is addressed mostly to future programmers who may have to make corrections or other modifications.2.	Online documentation, information that is displayed as the pro-gram runs or that can be called up with a command such as help. The user should be able to control the amount of information dis-played (more for beginners, and less as the user’s experience increases). Also, help commands should be sensitive to the context in which they are invoked; for instance, typing help within an edi-tor should call up information about the editor, not the whole oper-ating system. Documents and Settings	1483	Reference cards, containing easily forgotten details for quick refer-ence. A reference card assumes that the user is already familiar with the general principles of the program. Reference cards printed on paper are becoming obsolete, but the same kind of documentation is often made available online.4	Reference manuals, setting out complete instructions for the pro-gram in a systematic way. Related information should be grouped together, and a good index should be provided.5	Tutorials, serving as introductions for new users. Unlike a refer-ence manual, a tutorial gives the information in the order in which the user will want to learn it; items are grouped by importance rather than by function or logical category.Documents and Settings the directory which, in Windows 2000 and later, normally contains a directory for each user account, containing that user’s “My Documents” folder and other data. In Windows Vista, it was renamed Users.dodge/burn tool a paint program tool that simulates the effects of tradi-tional dodging and burning methods in the photographic darkroom: the burn tool gradually increases the darkness of the area you pass the tool over; likewise, the dodge tool lightens the area.DOM (Document Object Model) a system in which a web page is viewed as a collection of objects that can be manipulated by an object-oriented scripting language such as JAVASCRIPT. See also DYNAMIC HTML.domain1. a portion of the Internet distinguished by a particular final part of the name. For instance, www.covingtoninnovations.com and ftp.covingtoninnovations.com are two servers in the domain covingtoninnovations.com, which is a subdomain of .com, its top-level domain (TLD).2. in Windows NT and its successors, a group of networked computers that share a server and a set of user accounts.domain address an Internet address in conveniently readable form, such as jones.com, as opposed to the IP ADDRESS, which consists of numbers. SeeINTERNET.domain name hijacking see DOMAIN NAME POACHING.domain name hoarding the practice of registering multiple domain names, some of which will remain unused, simply to prevent a competitor from using them.domain name poaching the practice of registering an Internet domain name with the intention of reselling the domain name rights to a corpo-ration or individual. For example, you might want to register www.ford.com for yourself in the hope that Ford Motor Company would then buy it from you. Also called DOMAIN NAME HIJACKING and CYBER-SQUATTING. See also UDRP. DOUBLE PRECISION149	double bufferingdongle a device that attaches to a computer, typically on a USB port, and must be present in order to run a particular piece of software, but has no other purpose. Dongles are used to prevent unauthorized copying. SeeCOPY PROTECTION.DoS abbreviation for “Denial of Service.” See DENIAL-OF-SERVICE ATTACK.DOS (Disk Operating System) the name of various operating systems pro-duced by various computer manufacturers, including an early operating system for the IBM 360; the disk operating system for the Apple II (Apple DOS); MS-DOS, developed by Microsoft for 16-bit microcom-puters; PC-DOS, a version of MS-DOS commonly sold with the IBM PC; and Caldera DOS (formerly DR-DOS), a third-party substitute for MS-DOS.Since 1983 the name DOS has almost always referred to MS-DOS or equivalent operating systems, as it does throughout this book. See MS-DOS.dot1.	the character . (period), often used in filenames and Internet addresses.2.	the decimal point. For example, “nineteen dot two” means 19.2 thou-sand bytes per second, a standard data transmission rate.dot-bomb (humorous) an Internet business that failed, usually in a spec-tacularly quick manner.dot-com an Internet address ending in .com; more generally, a business that operates on the Internet. See COM.dot-compost (humorous) derisive term describing the residue of failed dot-com business ventures. See also DOT-BOMB; DOT-GONE.dot-con (humorous) fraud committed on the Internet. See also HOAX; PYRA-MID SCHEME.dot-gone (humorous) a failed Internet business, the opposite of a DOT-COM.See also DOT-BOMB, DOT-COMPOST.dot-matrix printer a printer that forms characters as patterns of dots made by pressing pins onto a ribbon. These printers were widely used in the 1980s, but inkjet printers give comparable or higher quality and speed without making as much noise. See also INKJET PRINTER; LASER PRINTER.dots per inch see DPI.double in C and related programming languages, a keyword for declaringfloating-point numbers.double buffering1. in graphics, the practice of computing the next frame of an animation in memory while the previous frame is being displayed, and then copy-ing the new frame to the screen quickly. That way, the process of draw-ing each frame on the screen does not produce flicker. double-byte font	1502. more generally, the use of two buffers (memory areas) to hold data being sent to an output device. One buffer can continue to accept data while the other buffer is being copied from memory to the device.double-byte font a FONT that uses two bytes (16 bits) to represent each character, thereby allowing more than the 256 characters that could fit into a single-byte font. See UNICODE. Contrast ASCII, ANSI, and EBCDIC, which are single-byte character sets.double-click to depress the button of a mouse twice very rapidly (if the mouse has more than one button, use the leftmost one). This is usually the shortcut to open or launch the selected file. If you find it difficult to double-click quickly enough, you can adjust the mouse’s double-click speed. You’ll find the mouse adjustments in the Control Panel (Macintosh and Microsoft Windows).FIGURE 84. Double-clickdouble dagger the character ‡, which is sometimes used to mark footnotes. See FOOTNOTE for more information.double precision a way of representing floating-point numbers with about twice the number of significant digits used in earlier implementations. Double-precision data types were first implemented in FORTRAN and later in BASIC and C. Today, “double” is in fact the normal floating-point type in C and its derivatives on most CPUs. See also ROUNDING ERROR.down not available for use. A computer is said to be down when it mal-functions or when it is being tested or maintained.downlevel pertaining to an earlier version of a product. For example, Windows Server 2003 supports downlevel clients; that is, client com-puters attached to the server can run earlier versions of Windows.Contrast UPLEVEL.download to transmit a file or program from a central computer to a smaller computer or a computer at a remote site. See KERMIT; FTP.Contrast UPLOAD.downstream (describing data transmission) in a direction from the server to the client, or from the main computer to the peripheral. CompareDOWNLOAD. Contrast UPSTREAM. 151	draw programdownward compatibility the ability to work with older equipment and/or software than that for which a computer program or accessory was designed. Contrast UPWARD COMPATIBILITY.dpi (dots per inch) the number of pixels or printer dots per linear inch. The first generation of laser printers could print with a resolution of 300 dots per inch. 600- and 1200-dpi laser printers are now available. See RESO-LUTION for further details.draft quality a printout in low resolution, unsuitable for CAMERA-READY COPY, but adequate for proofing the copy and checking the placement and alignment of graphics.drag to move an object by using a mouse. To do this, move the mouse pointer to the object, and then hold down a mouse button (usually the leftmost but-ton if there is more than one), and move the mouse. The selected object will move with the mouse pointer, as if the pointer were dragging the object. When you are finished, drop the object by releasing the button.FIGURE 85. Dragdrag and drop1.	the ability to move text or graphics by dragging it to a new location with the mouse.2.	a method of opening an application program. Simply pick up a file icon and drag it to the icon of an application program that can open the file.drain one of the three regions in a field-effect transistor.DRAM (Dynamic Random-Access Memory, pronounced “D-ram”) a com-puter memory that requires a refresh signal to be sent to it periodically. Almost all computers use DRAM chips for memory. See EDO; MEMORY;RAM; RDRAM; SDRAM. Contrast SRAM.draw program a graphics program that operates in terms of lines and shapes. Unlike a paint program, a draw program treats the picture as a collection of objects, each of which will be printed as sharply as the printer can print. Thus, the sharpness of the picture is not limited by the resolution of the screen. Also, individual circles, lines, rectangles, and other shapes can be moved around without affecting other objects they overlap. However, individual pixels cannot be edited. Draw programs are sometimes called vector graphics or object-oriented graphics programs.Draw programs are preferred for drawing diagrams, while paint pro-grams are superior for pictorial artwork or correcting photographs. Some drill down	152popular draw programs include Adobe Illustrator, CorelDraw, and Macromedia Freehand. Contrast PAINT PROGRAM.FIGURE 86. Draw programdrill down (informal) to follow a series of menus or otherwise reach a piece of information through a series of steps.drive see DISK DRIVE.drive bay a space in a computer enclosure that can hold a disk drive.driver see DEVICE DRIVER.drizzle an algorithm to combine digital images while resampling them to a higher resolution, invented by Andrew Fruchter and Richard Hook for processing Hubble Space Telescope data.The drizzle algorithm gives sharper and smoother results than ordi-nary resampling. It is called “drizzle” because each input pixel is treated like a drop of water that is to be divided into smaller droplets. Its con-tent is put into output pixels in proportion to the part of its area that falls on each of them.The key step, however, is that before this is done, each drop is shrunk so that it does not cover the entire original pixel, but only its central por-tion. In effect, the drizzle algorithm resamples from spots rather than squares. As a result, the larger pixel size of the original images has less of a degrading effect on the output. See RESAMPLE.DRM (Digital Rights Management) a system for restricting the use and copying of digital versions of intellectual property (such as music and movies). Several different systems have been used, but determined copyright infringers can find ways around them (since any digital con- 153	drumtent is subject to being copied by the right hardware). Also, DRM sys-tems may be too restrictive by preventing content purchasers from exer-cising rights that are allowed under copyright law. Libraries and museums are concerned that in the future, they may not be able to play today’s media, even though suitable equipment and software are avail-able, if no one is still in business who can continue to verify the licenses. On the other hand, artists’ livelihoods are at risk when their works can be freely copied. See COPYRIGHT; DIGITAL MUSIC.drop cap (drop capital) a large capital letter that occupies more than one line at the beginning of a chapter in a book.FIGURE 87. Drop capsdrop-down menu a menu that appears when a particular item in a menu bar is selected (see Figure 88). Also called a pop-up menu. See alsoMENU BAR; PULL-DOWN MENU.FIGURE 88. Drop-down menudrum in a LASER PRINTER, the large photosensitive cylinder that receives the image and then transfers it to paper.To print a page, the drum is first given an electric charge of several thousand volts from the CORONA WIRE to make it repel toner. Then it is scanned with a laser; the points hit by the laser conduct electricity and lose their charge. Finally, fine particles of TONER are applied to the drum; they stick only in the places discharged by the effect of the laser.Then the drum rolls the image onto the transfer roller, which transfers it to the paper, and the paper is heated by the FUSER to melt the toner par-ticles and make them stick.Damage to the drum of a laser printer causes streaks and spots that recur on every page. Some printers include a new drum in every toner DIGITAL  SIGNATURE;DSA	154cartridge; a recycled cartridge may contain a previously used drum. Because the drum is photosensitive, it should not be exposed to bright light.DSA (Digital Signature Algorithm) a U.S. federal standard for digital sig-natures using public key encryption, developed in 1991 by the NationalInstitute	of  Standards  and  Technology.  SeeENCRYPTION.DSL (Digital Subscriber Line) any of several ways of transmitting high-speed digital signals over existing telephone lines (see also HDSL;SDSL; VDSL).The DSL service usually offered to homes and small businesses is known as ADSL Lite or G.lite (see ADSL) and provides maximum data rates of 1.5 Mbps upstream and 0.5 Mbps downstream and is what the rest of this article will describe.DSL uses radio-frequency signals in wires that were designed for ordinary audio. This makes it possible to provide “always-on,” constant network connections, and at the same time, provide ordinary telephone service on the same line.The strength of the radio-frequency signals falls off rapidly with dis-tance from the DSLAM (DSL Access Multiplexer) at the telephone com-pany’s central office (CO). At greater distances, or where signals are weak for other reasons, lower data rates are used, down to 384 kbps down-stream. Beyond 15,000 feet from the DSLAM, DSL service is usually not available. However, as demand for DSL increases, telephone companies are quickly installing DSLAMs in wiring boxes along major roads.The incoming phone line connects to a special DSL modem, which, in turn, is connected to a combined ROUTER and FIREWALL and then to the local area network. Ordinary telephones connect to the same line through one or more filters (see DSL FILTER).Compare T1 LINE; T3 LINE; ISDN.DSL filter a device that blocks high-frequency DSL signals from entering ordinary telephone equipment, where they might cause noise on the tele-phone, or, worse, be absorbed and weakened, interfering with DSL per-formance elsewhere in the building.The best place for a DSL filter is where the phone line enters the house. The line should split there; one branch should go through a DSL filter to the rest of the ordinary telephone wiring, and a separate branch should go through high-quality Cat 5 or better cable directly to the DSL modem.DSL filters can also be installed on individual telephones, preferably at the wall outlet.DSLAM (DSL Access Multiplexer) a device that joins a high-speed com-puter network to a set of ordinary telephone lines in a telephone com-pany central office. In order to offer DSL service, a telephone company has to install a DSLAM and give it a fast connection to the Internet, which will be shared by the DSL subscribers. 155	DuronDSLR (digital single-lens reflex) a digital camera that is also an SLR, so that the viewfinder uses the same lens that will take the picture.DSLRs normally take the same interchangeable lenses as the same manufacturer’s film SLRs, but with a different field of view (see CROP FACTOR). They are designed for highest-quality professional work.A disadvantage compared to other digital cameras is that with a DSLR, the LCD screen normally cannot display the picture until it has been taken; only the optical viewfinder works while the picture is being composed. Other digital cameras can display an electronic image con-tinuously. Some newer DSLRs have this capability which is called live focusing.DSP see DIGITAL SIGNAL PROCESSING.DSS (Digital Signature Standard) a federal standard for digital signatures using the Digital Signature Algorithm (DSA).DTD abbreviation for “document type definition.” See XML.DTE (data terminal equipment) see DCE.DTMF (dual-tone multi-frequency) the signaling system used on push-button telephones. Each signal consists of two tones transmitted simultaneously.DTV see DIGITAL TELEVISION.dual boot capable of running more than one operating system. Typically, the user chooses the desired operating system at boot-up time.dual-core having two CPU cores. See CORE (definition 1).dual-core processor two MICROPROCESSORs built into one, in a single pack-age or even on a single chip. Contrast HYPER-THREADING, which is the ability of a single processor to follow two instruction streams. The two approaches together allow one processor to do the work of four.dump to transfer data from one place to another without regard for its sig-nificance. A dump (on paper) is a printout of the contents of a computer’s memory or disk file, shown byte by byte, usually in both hexadecimal and character form. Dumps are usually very hard to read and are used only when there is no other convenient way to get access to the data. Large-scale copying of files from disk to tape, or vice versa, is some-times referred to as dumping.DUN see DIAL-UP NETWORKING.duplex1.	printing on both sides of the paper.2.	communication in two directions. See HALF DUPLEX; FULL DUPLEX. Duron a high-speed Pentium-compatible microprocessor made by AMD.Compare ATHLON. dusty deck	156dusty deck (slang) an ancient, poorly understood computer program that goes back to the days of punched cards; something that is obsolete but has to be kept usable because someone needs it. Compare LEGACY.Dutch auction1.	an auction in which several items, all alike, are being sold at once to the highest bidders. Dutch auctions are popular on eBay and similar online auction services. See EBAY.2.	an auction in which, instead of asking for bids, the would-be seller gradually lowers the asking price until someone accepts it. This is rarely done online.duty cycle the percentage of the time that a piece of equipment is in use or powered on. For example, if the lights in a room are on 8 hours out of every 24, they have a 33.3% duty cycle.DVD (Digital Versatile Disc, originally Digital Video Disc) an optical disc similar to CD-ROM but with much greater capacity (4.7 GB single-layer, 8.5 GB double-layer). Normally, DVD drives also read CDs. In 2006, an even higher-capacity disc of the same general type, the Blu-Ray disc (BD), was introduced. See BLU-RAY DISC.DVD was introduced in order to store a complete, digitized feature-length movie on a single disc. However, despite “video” in the name, DVD can store any kind of computer data. See also CD-ROM.DVDs can be 8 or 12 cm in diameter (the same as the two sizes of CDs), single- or double-sided, and single- or double-layered. Data DVDs store sets of computer files, which can contain any type of data. Video DVDs, playable in consumer DVD players, are the same types of disc but with a different filesystem designed specifically for video play-back.Video DVD technology includes some controversial measures to pro-tect the copyrights of movies and music. Because movie copyrights have different owners in different countries, some discs include region codes to control where the disc can be played. Region codes are also built into the firmware of DVD drives, and regardless of software, most drives will not play a disc from a different region. With some drives, the region code can be changed a few times (in case the drive is sold or used in a country other than the original market), but repeated changes are not possible. Region codes are not encrypted or secret.More controversial is the Content Scrambling System (CSS), an encryption-based security system. The effect of CSS is to make illegally copied discs unusable, because although the data can be copied, the keys needed to decrypt it do not survive the copying process. This prevents people from making backup copies of their own discs.An unusual provision of the Digital Millennium Copyright Act makes it illegal to circumvent the CSS algorithm or provide others with tools or information to do so (see DMCA). Despite this, the algorithm has been cracked and posted on the Internet (see DECSS). 157	dxdiagDVD-R a type of DVD designed to be recorded with studio equipment and played in ordinary DVD drives, whether or not they are compatible with home-recorded discs. DVD-R discs are not erasable or rewritable.Compare CD-R.DVD-RAM an older type of recordable, erasable, rewritable DVD. DVD-RAM drives can read ordinary DVD discs, but not vice versa. ContrastDVD+RW; DVD-RW.DVD-ROM a non-erasable DVD, typically one containing computer files rather than audio or video programs. See DVD.DVD+R, DVD-R two types of user-recordable DVDs with similar capacity and performance but different technical specifications, designed to be readable in ordinary DVD drives. Many DVD drives record and play both. DVD+R and DVD-R discs cannot be erased or rewritten. Compare CD-R.DVD+RW, DVD-RW two types of user-recordable and erasable DVDs with similar capacity and performance but different technical specifica-tions. Like DVD+R and DVD-R discs, they are designed to be readable in ordinary DVD drives. Like CD-RW discs, DVD+RW and DVD-RW discs can be erased and rewritten. Compare CD-RW.DVI1.	(Digital Visual Interface) the newer type of connector for linking computers to monitors and projectors. It carries digital signals as well as the same analog signals as a VGA connector. The connector is roughly rectangular and has room for up to 29 pins, one of which is wide and flat (Fig. 89). Contrast VGA CONNECTOR.2.	device-independent output from TEX or LATEX, which can be printed on any printer using the appropriate DVI program.3.	(Digital Video Interface) Intel’s file format for storing video on disk.FIGURE 89. DVI video connector.DVR digital video recorder, a device that records video on digital media, usually DVDs.dweeb (slang) an unsophisticated, untidy, obnoxious person.dxdiag (directx diagnostics) a utility program for testing DirectX under Windows. To run it, go to the Start Menu, choose “Run...” and type dxdiag. See DIRECTX. CASCADING STYLE SHEETS.dyadic operation	158dyadic operation an operation on two numbers (operands). For example, addition, multiplication, subtraction, and division are all dyadic opera-tions because each of them operates on two numbers. Negation is not a dyadic operation because it operates on only one number.dye-sublimation printer a type of color printer that gives excellent color images. Dye-sub printouts appear to be continuous-tone images like photographs. Actually they are composed of tiny dots (like laser or ther-mal-wax printouts), but the dots of dye have spread together. The intense colors, glossy finish, and lack of apparent halftone dots make these printouts especially suitable for fine art prints or presentations.dynamic HTML enhancements to HTML that allow the display of a page to change in response to user actions, such as mouse movements, with-out having to reload the page from the server. A page is viewed as a set of objects (see DOM) whose appearance can be changed by scripts (see JAVASCRIPT) in response to user actions such as mouse clicks. For exam-ple, a page can contain headings that are supported by detailed text that will only be visible when the user clicks on the headings.Unfortunately, some of the advanced features have not yet been stan-dardized, so different browsers will not always display the same result.See alsodynamic IP address an IP address that is assigned to a computer when it actually connects to a network and is not necessarily the same from one session to the next. Contrast STATIC IP ADDRESS.dynamic link library see DLL.dynamic RAM see DRAM.dynamic range the ratio between the smallest and largest signals that a sys-tem can handle. For instance, if an analog-to-digital converter can digitize signals from 2 millivolts to 200 millivolts, it has a dynamic range of 100:1.The dynamic range of an audio system or piece of music is often given in DECIBELs. Dynamic ranges that involve brightness are often given in terms of F-RATIOs or as logarithms to base 10. For instance, if a film scan-ner can handle a slide or negative whose dark areas transmit only 1/1000 as much light as the bright areas, then its dynamic range of 1000:1 can be described as the logarithmic value 3.0, because log10 1000 = 3. EXPONENTIAL NOTATION.ELECTRONIC COMMERCE.EXPONENTIAL NOTATION.159	earEe (in mathematics) an important number whose value is approximately 2.71828. The reason e is important is that the function ex is its own deriv-ative. In many programming languages, the function exp(x) computes ex. If y = ex, then x is the natural logarithm of y.E seee- prefix meaning electronic, especially when applied to terms in the context of the Internet or World Wide Web. See E-MAIL; E-ZINE; and E-COMMERCE for examples.Usage note: Spelling of e-words is not yet standardized, and the new words are often spelled without hyphenation (e.g., email, ezine).e-commerce seeE format seee-mail (noun) electronic mail; (verb) to send a message or file by electronic mail. See ELECTRONIC MAIL.Usage note: The spelling e-mail, with the hyphen, is now widely pre-ferred. The older spelling is email. Either way, the noun and verb are spelled alike.e-mail broadcasting the sending of the same e-mail message sent to many people from one source. Newsletters and SPAM advertisements are both e-mail broadcasts. See also MAILING LIST. Compare FAX BROADCASTING.e-tail  retail  sales  conducted  on  the  Internet.  For  an  example,  seeAMAZON.COM.e-zine, ezine (slang) electronically published magazine (i.e., a magazine published on a web page or the like). See WORLD WIDE WEB.ear1.	the small stroke on the right side of the letter g.2.	a small box of information on either side of a headline or masthead. In newspapers, an ear is commonly used for the weather forecast. FIGURE 90. (A) Ear, definition 1 (B) Ears, definition 2 Easter egg	160Easter egg (slang) a hidden part of a computer program. Easter eggs are usually activated only by a bizarre series of actions—then the user is treated to an amusing presentation that usually includes the names of the development team. The actions necessary to see an egg are very complex and would never be performed by a casual user of the program; one has to be looking for the Easter egg. Information about Easter eggs is often spread on the World Wide Web.eBay (www.ebay.com) an online auction house established in 1995 and headquartered in San Jose, California. By acting as an auctioneer, eBay enables individuals to buy and sell almost anything through the World Wide Web.An online auction has several advantages over a conventional one. Bids on an item can be collected for several days, typically a week, rather than having to be delivered all at once. Perhaps more importantly, the actual bidding can be done by computer. Would-be buyers specify their maximum bids, but the computer places actual bids that are just high enough to outbid the other bidders. Finally, the computer can search quickly through thousands of item descriptions. See also BID; DUTCH AUC-TION; RESERVE PRICE.EBCDIC (pronounced ”ebb-see-dik”) (Extended Binary Coded DecimalInformation Code) the numeric representation of characters on IBM mainframe computers. (Contrast ASCII and UNICODE, which are used on most other computers.)ebook, e-book1.	a book distributed electronically (online or on CD-ROM or similar electronic media) rather than on paper. See PDF.2.	a special-purpose handheld computer for reading texts of books.EC1.	abbreviation for ELECTRONIC COMMERCE.2.	abbreviation for European Community.ECC see ERROR-CORRECTING CODE.ECC RAM random-access memory (RAM) that uses an ERROR-CORRECT-ING CODE to recover automatically from single errors in data storage, and to detect and report more serious errors.echo to send information back to where it came from. With computers, this refers to two things:1.	When communicating by modem, a computer echoes typed char-acters if it sends them to its own screen as well as to the other com-puter. If you can’t see what you’re typing, turn echoing on; if what you type appears twice, turn echoing off.2.	In Windows and UNIX, the echo command sends a message to the screen; for example, echo Hello there! 161	editor writes “Hello there!” on the screen. In a .BAT file, the command echo off tells Windows not to print commands on the screen as they are executed. You can prevent the echo off command itself from being displayed by prefixing it with @, like this:@echo offIn fact the @ prevents display of any command, not just echo.Eclipse a free, open-source interactive development environment (editor, compiler, and debugger) for Java and other programming languages. For further information, see www.eclipse.org.ECPA (Electronic Communications Privacy Act) the main U.S. law against wiretapping and other interception of private electronic commu-nications, whether they are transmitted by wire, radio (including wire-less network), or other means. It was passed in 1986 and superseded a number of earlier laws (18 USC 2510).Critics point out that the ECPA does not require the sender of a mes-sage to encrypt (scramble) it to make it private. Thus, private messages can still be intercepted deliberately or even by accident. For example, first-generation analog cellular telephones were assigned to frequencies formerly occupied by UHF TV channels, and they used the same kind of modulation as TV sound. Thus, telephone calls could be picked up on old television sets. Similarly, radio technicians tracking down sources of inter-ference could find themselves hearing things that are illegal to listen to.On the whole, however, the ECPA is an essential part of the laws pro-tecting computer networks and communication systems from tampering and eavesdropping. It is one of the laws most commonly violated by crackers. See COMPUTER LAW; CRACKER; WIRELESS NETWORK.edge detect a paint program filter or image processing technique that out-lines the edges of objects. See Figure 91.FIGURE 91. Edge detectEDI see ELECTRONIC DATA INTERCHANGE.edit to examine a file and make changes in it, usually with the aid of anEDITOR.editor a computer program that enables the user to create, view, and mod-ify text files. EDO	162EDO (extended data out) a type of dynamic random-access memory (DRAM) that holds its output on the BUS until the beginning of the next bus cycle. This enables the computer to retrieve data from memory in one bus cycle instead of two. (To further gain speed, memory is attached to a fast bus that connects directly to the CPU, rather than the slower bus that connects to expansion cards.) EDO DRAM is often used with Pentium processors. Contrast SDRAM..edu a suffix indicating that a web or e-mail address belongs to a university-level educational institution (in any country, but mostly the United States). Along with .com, .gov, .int, .net, .org, and .mil, this is one of the original set of Internet top-level domains. Contrast .COM. See also TLD; ICANN.EEPROM (Electrically Erasable Programmable Read-Only Memory) a type of memory chip whose contents can be both recorded and erased by electrical signals, but do not go blank when power is removed. (It is called “read-only” because the recording process is too slow to be used more than occasionally.) EEPROM contrasts with permanently recorded ROM chips and with EPROMs that can be programmed electrically but cannot be erased electrically. See EPROM; ROM.effective megapixels the number of megapixels actually used for the image in a digital camera. Contrast GROSS MEGAPIXELS.efficiency the conservation of scarce resources. In order to measure effi-ciency, you have to decide which resource you want to conserve. For example, one program might be more “efficient” than another if it uses less memory, and another program might be more “efficient” in terms of speed; the question is whether you would rather conserve memory or time.With computers, some of the most important resources are:1.	computer execution time;2.	computer memory capacity;3.	auxiliary storage capacity (i.e., disk space);4.	programmer’s time.The general rule is: it is more important to work to conserve a resource if it is more scarce. With early computers, which were very slow and had limited memory (compared to computers available now), it was more important to write programs that would not require much memory and would not require as many steps for the computer to exe-cute. Now that computers are faster and have more memory, it is often the case that the programmer’s labor is the most scarce resource. This means that it is more efficient to write software in a way that simplifies the programmer’s job, even if it uses more computer time and memory. An added benefit is that if the programmer’s job is simplified, errors (bugs) are less likely.egosurfing the practice of entering one’s own name into a SEARCH ENGINE to see how many times it turns up. By doing this, one of the authors dis-covered that there is a professional boxer named Michael Covington. 163	electronic data interchangeEIA (Electronic Industries Alliance) an American organization that promotes industrial standards. On older computer equipment, “EIA” often marks an RS-232 or EIA-232D serial port. The EIA web site is at www.eia.org.EIA-232D the new official designation for the RS-232 standard for data communication. See RS-232.EICC (Electronic Industry Citizenship Coalition) a coalition of companies promoting socially responsible practices (web address: www.eicc.info).EIDE (Extended Integrated Device Electronics) a newer type of IDE disk drive and controller that allows a larger number of sectors per track and thereby overcomes the original limit of 528 megabytes per drive.EISA (Enhanced Industry-Standard Architecture) a standard 32-bit bus for IBM PC-compatible computers using the 386, 486, or Pentium micro-processor. EISA was developed by a group of competitors as an alterna-tive to IBM’s Micro Channel, retaining more compatibility with the original (ISA) bus. It has been largely superseded by PCI.In general, EISA computers can use ISA as well as EISA cards. The extra contacts on the EISA card edge connector are in a second row above the contacts that correspond to those on ISA cards.See ISA; BUS; PCI.eject1.	to remove a diskette or similar storage device from a computer.2.	to tell a computer that a diskette, CompactFlash card, or other storage device is about to be removed. The Eject operation tells the computer to finish writing data to the device immediately. The drive may or may not physically eject the storage device.electronic circuit diagram symbols graphical symbols used in schematic diagrams of electronic circuits. Examples are shown in Figure 92 on page 164. See also AND GATE; LOGIC CIRCUITS; NAND GATE; NOT GATE;OHM’S LAW; OR GATE; PARALLEL; SERIAL; TRANSISTOR.electronic commerce (EC) the carrying out of business transactions by computers. For example, computers at a store can monitor inventory lev-els and automatically order more merchandise when it is needed. Electronic commerce also includes transactions where there is a human participant, but the process is highly computerized, such as making pur-chases over the Internet. See also ELECTRONIC DATA INTERCHANGE; E-TAIL.Electronic Communications Privacy Act see ECPA.electronic data interchange (EDI) the transfer of information between organizations in machine-readable form in order to carry out business transactions. Electronic data interchange is becoming popular because it minimizes the errors that can occur if the same information has to be typed into computers several times. See OASIS. electronic document	164FIGURE 92. Electronic circuit diagram symbolselectronic document a document intended to be read as it is displayed on a monitor. An electronic document can use HYPERTEXT to create an inter-active environment for the reader. It can also use special effects such as animation, sounds, and music. Unlike with traditional printed docu-ments, there is no extra cost for full color. WEB PAGEs are a type of elec-tronic document; so are catalogs, documentation, and MULTIMEDIA presentations distributed on CD-ROM. AFAIK; BTW;165	ELIZAelectronic mail (e-mail) the transmission of messages by computer from one person to another. Messages are saved until the recipient chooses to read them. E-mail is much more convenient than ordinary mail or tele-phone calls because it arrives immediately but does not require the recip-ient to be present, nor does it interrupt anything else the recipient may be doing. Messages are easily printed out, saved on disk, or forwarded to other people.All users of e-mail should be aware that backup copies of the messages can be saved and forwarded to others and that perfect privacy cannot be guaranteed. Contrast CHAT ROOM; INSTANT MESSAGING; NEWSGROUP. Seealso SPAM.For abbreviations commonly used in electronic mail, seeFWIW; IANAL; IMHO; IRL; RYFM; TIA; YMMV.See also ELECTRONIC DATA INTERCHANGE; EMOTICON; FLAME; LOCAL-AREA NETWORK; WIDE-AREA NETWORK.electronic paper a display screen that has similar characteristics to ordi-nary paper.electronic publishing1.	the creation, manufacturing, and distribution of paperless documents. Examples of electronic documents are CD-ROM encyclopedias and web pages. Each of these new formats brings new challenges and technical problems, but all need the skill of someone who knows how to work with type and how to produce a pleasing combination of graphics and text.2.	the use of specialized computer-controlled equipment in the publish-ing and printing industries. Desktop publishing may be considered part of this trend, but electronic publishing encompasses the use of equip-ment not readily available to the mass market (powerful workstation class computers and digital presses, for example). Electronic publishing is superseding traditional methods of PREPRESS production.electrostatic printer a printer that operates by using an electric charge to deposit toner on paper. Laser printers are electrostatic printers.element one of the items in an ARRAY or LIST.elephant’s ear (slang) the symbol @; see AT SIGN.ELIZA a computer program developed by Joseph Weizenbaum of M.I.T. in 1966 to demonstrate that it is easy to make computers seem intelligent. ELIZA carries on a conversation with the user in the style of a psy-chotherapist, but it actually responds only to certain patterns of words in the input, ignoring the rest. For example, if the user mentions “mother,” ELIZA might reply, “Tell me more about your mother.”A version of ELIZA is built into the Emacs editor (Figure 93). To run it, press Esc X and type doctor and then press Enter. See EMACS.ELIZA passes the TURING TEST in a crude way, thereby demonstrating that human-like intelligence is easily faked. See also ARTIFICIAL INTELLIGENCE. ellipsis	166FIGURE 93. Eliza in actionellipsis typographic convention of using three dots (. . . ) to indicate the trailing off of a thought. In Windows, the ellipsis is typed by holding down Alt while keying 133 on the numeric keypad. Macintosh users can type an ellipsis by holding down Option and typing : (colon). Note that the three dots are actually one character. The spacing is different than simply typing three consecutive periods (Ellipsis . . . ; Three periods ...).The ellipsis also has an important function in the menu system of Microsoft Windows. The appearance of ‘. . . ’ after a menu item means that a dialog box will appear when that command is selected.em dash a long dash (—). See DASH.EM64T see X64.Emacs (originally for “editor macros”) a TEXT EDITOR that originated as a set of MACROs written for another text editor by Richard M. Stallman in the 1970s. Today, Emacs is one of the most successful products of the GNU project (see GNU). It is distributed free of charge from www.gnu.org/ software/emacs in versions for numerous operating systems.Emacs is somewhat complex to use, but very powerful. It can edit files of any size, and they need not be text files, since non-text charac-ters are represented as octal codes. Emacs features an elaborate system of context-sensitive help. A Lisp-like programming language is built in so that users can define new editing operations. See also ELIZA.embedded font a FONT that is included within a file to ensure faithful repro-duction of the formatted document.embedded Linux any version of Linux that is used in an embedded system.embedded object an object included in your file that was created in another software package and that still maintains a LINK to the other soft-ware. If the object is changed in the original software, it will be updated in the second file. See OLE for more details. 167	encryptionembedded system a computer that forms part of a larger machine of some other kind. The microprocessor that controls an automobile engine is an example. Embedded systems must usually be extremely reliable. They must also respond to events in real time (i.e., as they happen) without undue delay. See MICROCONTROLLER; REAL-TIME PROGRAMMING.EMC electromagnetic compatibility, the protection of equipment from electromagnetic interference. See RFI PROTECTION.emitter one of the three layers of a bipolar transistor. See TRANSISTOR.emoticon a typewritten symbol for a facial expression, often used in elec-tronic mail. For example, :) denotes a grin (look at it sideways), ;-) means “winking,” and =:-O means scared or surprised.Some emoticons are written so that they are viewed upright rather than sideways.ˆ-ˆ   smile0.0	surpriseˆ.ˆ;	distress (with drops of sweat)emulation the process of achieving the same results as if you had a differ-ent machine than the one you’re actually using. For example, VT-100 emulation means making a computer act exactly like a VT-100 terminal. Emulation is different from simulation, which involves imitating the internal processes, not just the results, of the thing being simulated.emulsion the coated surface of photographic film. Normally, a photo-graphic negative looks right (not flipped) if the emulsion of the negative faces away from the person viewing it. Some graphics software offers output choices of “emulsion up” and “emulsion down” to produce flipped images for systems in which a photographic negative is going to be used backward.en dash a short dash (–). See DASH.Encapsulated PostScript (EPS) a file format that is widely supported by different computers, printers, and software. Most desktop publishing soft-ware supports the importation of Encapsulated PostScript files, thus pro-viding a common denominator for exchanging files. See also POSTSCRIPT.encoding a way of interpreting binary data as representing characters. The term is used particularly in the Microsoft .NET Framework, which sup-ports several formats of UNICODE and many national variations on ASCII.encryption the act of converting information into a code or cipher so that peo-ple will be unable to read it. A secret key or password is required to decrypt (decode) the information. More and more confidential data is being sent along computer networks, so it is becoming increasingly important to develop ways to send information over computer networks securely.For example, suppose we wish to send this message: HELLOGOODBYE encryption	168One way to encrypt it is to replace each letter with the letter that comes 10 places later in the alphabet, so that letter 1 (A) becomes letter 11 (K), letter 2 (B) becomes letter 12 (L), and so forth, starting over at A when we go past Z, like this:Plain: ABCDEFGHIJKLMNOPQRSTUVWXYZEncrypted: KLMNOPQRSTUVWXYZABCDEFGHIJMathematically speaking, we change letter n to (n + 10) mod 26. Here mod stands for modulo and refers to the remainder after division by 26. For example, letter 20 (T) is shifted to 30, which becomes 30 mod 26 = 4, which is the letter D. Using this method, our message becomes:ROVVYQYYNLIOThe recipients can easily decrypt the message as long as they know the algorithm (each letter is shifted by a certain number of places) and the key (in this case, 10).Unfortunately, this algorithm is so simple that it would be easy for a spy to crack the code. There are only 25 possible keys (a key of 26 would have no effect, and a key of 27 or higher would have the same effect as a lower one). It is easy to check all 25 possibilities: Trying key: 1 Trying key: 2 Trying key: 3 Trying key: 4 Trying key: 5 Trying key: 6 Trying key: 7 Trying key: 8 Trying key: 9 Trying key: 10 Message decodes as: QNUUXPXXMKHN Message decodes as: PMTTWOWWLJGM Message decodes as: OLSSVNVVKIFL Message decodes as: NKRRUMUUJHEK Message decodes as: MJQQTLTTIGDJ Message decodes as: LIPPSKSSHFCI Message decodes as: KHOORJRRGEBH Message decodes as: JGNNQIQQFDAG Message decodes as: IFMMPHPPECZF Message decodes as: HELLOGOODBYE In this case the spy can stop after the tenth try.To make the code harder to crack, we can use a longer key. For exam-ple, say that the first letter of the message will be shifted 10, the second will be shifted 8 letters, the third will be shifted 17, and so on. If you use a key with 8 numbers, then you can repeat the pattern after every 8 let-ters (i.e., the ninth letter will be shifted the same as the first letter, the tenth letter will be shifted the same as the second letter, and so on). The longer the key is, the harder it will be for the spy to try all possibilities. If you can design it so that the time required to check all possibilities exceeds the lifetime of the universe, you’re probably safe from this kind of attack. Even if you can design it so that the expense of cracking the code is greater than the benefit the spy would receive by cracking your code, you’re probably safe.However, there are other means of attack. Not all letters of the alpha-bet are used with equal frequency. A spy can program a computer to make a guess for the length of the key; collect all letters encrypted with a particular part of the key; and then check the frequency of encrypted 169	encryption letters, guessing that the most frequently appearing letter represents E, and so on. That guess may not be right, but guessing with this system will likely proceed much faster than guessing all the possibilities. This kind of attack is easier if the message is longer, but it won’t work for numerical data where the digits are equally likely to appear.Another likely means of attack would be to attack the key itself. If the spy gets hold of the key, it will be easy to decrypt all the messages. If a lot of people are sending messages to lots of other people, it is hard for them to deliver the keys to the recipients of the messages without letting them fall into the wrong hands.One way to solve this problem is with public key encryption. In this approach, each person has both a public key (which everyone knows) and a private key (which is kept secret). If Alice is sending a message to Bob, then the message will be encrypted using an algorithm that is based on Bob’s public key. Anyone can use this key to encrypt a message to Bob, but it can only be decrypted using Bob’s private key.Here is one example of how this can work, using the algorithm devel-oped by Whitfield Diffie and Martin Hellman in 1976. Alice and Bob agree on two numbers: n = 37 and g = 7. (In reality, n and g would be much larger than this.) Each of them has a private key, which we’ll call a and b, respectively. Alice and Bob generate their public keys A and B using the formula:Public key = g (private key) mod nThus:Alice’s private key a = 8Alice’s public key A = 78 mod 37=	5,764,801 mod 37=	16Bob’s private key b = 6Bob’s public key B = 76 mod 37=	117,649 mod 37=	26Alice now generates another key K to use for the actual message using this formula:K = Ba mod n=	268 mod 37=	208,827,064,576 mod 37=	10This key is known as the session key. Now she can encrypt the message. For example, if she is sending the message HELLOGOODBYE, it will be encrypted as shown at the beginning of this entry.When Bob receives the message, he will calculate the session key using a very similar formula: end	170K = Ab mod n=	166 mod 37=	16, 777, 216 mod 37=	10Notice that this is the same value even though it is calculated from different numbers using a different formula. This works because of the following mathematical identities:(a  b) mod n = [(a mod n)  (b mod n)] mod n ac mod n = (a mod n)c mod nabc = (ab)c = (ac)babc mod n = (a mod n)bc mod n=	[(a mod n)b]c mod n=	[(a mod n)c]b mod nabc mod n = (a mod n)bc mod n = (ac mod n)b mod n abc mod n = (a mod n)bc mod n = (ab mod n)c mod nTo calculate the private key (equivalent to c), given the public key and the session key, you need to solve an equation of this general form:k = j x mod nIf n happens to be a large prime number, it is very difficult to discover the value of x even if you know the values of k, j, and n. Thus, large prime numbers play a crucial role in public-key encryption. In practice, when computers are used for encryption, the calculations are usually car-ried out directly on the binary digits of the data, using a key given as a binary number. A longer key provides greater security, but the calcula-tion process becomes more complicated.All this presumes that you can get people’s public keys reliably so that you can be sure you’re really using Bob’s public key when you send messages to Bob. Since public keys are not secret, all you need is a trust-worthy database in which you can look up people’s public keys.Until 2000, the U.S. government regulated the export of strong encryption software in the same way that it regulates the export of weapons. This regulation dated from the 1940s, before general-purpose digital computers existed; encryption machines at that time were con-sidered to be military devices.See also AES; DES; DIGITAL SIGNATURE; HASH FUNCTION; ONE-WAY FUNC-TION; PGP; RSA ENCRYPTION; SSL.end1. keyword that marks the end of a particular program structure in sev-eral programming languages. In BASIC, the END keyword tells the com-puter to stop executing the program. In Pascal, END marks the end of blocks of statements that start with BEGIN. GREEN PC.171	envelope2. the key on your keyboard that takes your cursor to the end of the cur-rent line. Some word processors use Ctrl-End as a keyboard shortcut to take you to the end of the document.end-of-file mark a symbol that indicates the end of a file. For example, in CP/M, all text files ended with ASCII character 26 (Ctrl-Z) because the computer did not otherwise keep track of the exact length of the file, only the number of disk sectors. In Windows, Ctrl-Z is often used the same way even though the computer knows exactly where the file ends whether or not an end-of-file mark is present. The UNIX end-of-file mark is Ctrl-D (ASCII 4).end user the person ultimately intended to use a product, as opposed to people involved in developing or marketing it.Energy Star a set of guidelines proposed by the U.S. Environmental Protection Agency in 1992 to reduce the amount of electricity consumed by personal computers. An Energy Star-compliant computer consumes less than 30 watts of power when idling (i.e., when turned on but not in use) and switches automatically into low-power mode if several minuteselapse without any keyboard activity. Seeengine1. the part of a computer program that implements a special technique; see INFERENCE ENGINE, MONTE CARLO ENGINE, SEARCH ENGINE.2. the printing mechanism of a laser printer, not including the computer control circuitry. Many laser printers use an engine made by Canon in Japan.ENIAC (Electronic Numerical Integrator And Calculator) one of the first electronic computers, built at the University of Pennsylvania in the mid-1940s. It contained about 18,000 vacuum tubes. Initially, the ENIAC was programmed by plugging cables into circuit boards. Today, one of the Internet nodes at the University of Pennsylvania is named eniac but is, of course, not the same machine.Enter key the key on a computer keyboard that you press at the end of each line in order to send the contents of that line into the computer. On most keyboards, the Enter key is the same as the Return key. However, IBM 3270-series terminals make a distinction: the Return key starts a new line, but the Enter key sends the contents of the whole screen to the computer.Under windowed operating systems, pressing the Enter key is usually equivalent to clicking on the currently selected icon or other highlighted item.enumerator a device driver or operating system component that identifies all hardware devices of a particular type. See BIOS ENUMERATOR.envelope1. (in a draw program) the imaginary outline enclosing an object. You can edit the envelope, turning it from a rectangle into a curved shape, and thereby distorting everything inside it. environment	1722. (in engineering) the limits imposed by physical or technical con-straints (called an “envelope” because they can be envisioned as sur-rounding an area on a graph). “Pushing the envelope” means working close to, or at, the limits.FIGURE 94. Envelope manipulationenvironment1.	the display and human interface provided by software. On a com-puter, an environment defines what you can do with the computer. For instance, the operating system, a word processor, and a spreadsheet pro-vide (at least) three different environments that respond to different com-mands. For example, if you type a word processing command while you are in the operating system environment, or vice versa, the command will not be understood.2.	(in Windows and UNIX) a data area in which you can store informa-tion for use by programs. To put information there, use the set com-mand; for example, under Windows,set prompt=$p$gtells the computer to display the current disk and directory (e.g., C:\MYDIR>) when it is ready for a command. To see the contents of the environment area, type set by itself.EPIC see IA-64.EPROM (Erasable Programmable Read-Only Memory) a type of memory chip that can be programmed electrically and erased by exposure to ultraviolet light. See also ROM; PROM; EEPROM.EPS see ENCAPSULATED POSTSCRIPT.Epson a prominent Japanese manufacturer of printers and other computer peripherals, distributed in the United States by Epson America, Inc., of Torrance, California. More information about Epson can be found at www.epson.com.The Epson MX-80 dot-matrix printer received wide acceptance dur-ing the early 1980s and set many standards to which other manufactur-ers subsequently adhered. Today, Epson makes high-quality color inkjet printers and other computer peripherals.equalize a paint program filter that adjusts the brightness range of a picture so that all levels of brightness become equally common. If some bright- 173	ergonomicsnesses are not used (because the picture is too bright or too dark, or because of a contrast problem), the equalized picture will often look much better. Equalizing can dramatically improve the appearance of objects that are nearly the same brightness as their background.erase the command that erases a file or set of files from disk in Windows and other operating systems. See also RECOVERING ERASED FILES.Eraser a paint program tool that removes colors from a picture, leaving the background color in its place (the background can be transparent). The eraser is used by holding down the mouse button (the leftmost if there is more than one) and dragging the eraser tool. You can adjust the size and shape of your eraser to suit your needs. Some programs will even adjust how well the eraser works; it can erase thoroughly or just lighten the color. See NATURAL MEDIA.FIGURE 95. EraserNote: tool erases to transparent background. (Erased material is replaced by the background color.)ergonomics the science of designing machines and working environments to suit human needs (from the Greek words meaning “the study of work”). An ergonomically designed machine is one whose design is based on the scientific study of human requirements such as vision, pos-ture, and health risks. After all, the most important part of a computer system is the human being who is operating the computer.Ergonomics goes beyond considering your comfort. Smart workers know that they need to work efficiently. When you work efficiently, you can get more done. Here are some things you can do:•	Desk. Your computer desk should be deep enough to comfortably accommodate all of your equipment. If the system unit keeps threat-ening to dump the keyboard in your lap, you may not have enough room. Consider putting the system unit on the floor or to the side of the monitor.Check the height of your desk. Is it too tall for you to type com-fortably? You may want to attach a keyboard drawer. This lowers the keyboard to a more comfortable level and gives you a storage place for the keyboard.•	Chair. Your chair is most vital to the health and well-being of your back. You should choose a chair that has adjustments for height and good lumbar support. Try to find a chair that lets you adjust the tilt error	174of the seat because it helps to periodically change the seat tilt dur-ing a long work session.•	Monitor. The monitor is one of the big-ticket items when you pur-chase your computer system. Ergonomically speaking, you do not want to skimp here. LCD (Liquid Crystal Display) monitors are superior to CRT (Cathode Ray Tube) monitors because they do not flicker. If you are still using an older CRT monitor, set the refresh rate to at least 70 Hz; it may save you a headache. Make sure you are comfortable with the height and tilt of the screen. You may need a special pair of glasses for working at the computer. (See EYE-GLASSES, COMPUTER.)•	Mouse and keyboard. The big risk is CARPAL TUNNEL SYNDROME, a condition that creates numbness or a buzzing feeling in your hands. Prevention is the key. You should keep your wrists straight when typing; don’t allow them to bend. Some people enjoy a cushioned wrist rest for their keyboard. If using the mouse gives you any dis-comfort, try using another pointing device such as a TRACKBALL.•	Lighting. To prevent glare on the screen, do not place your com-puter opposite a window. Overhead lighting should be soft (not as bright as it would be for reading).•	Posture. Good posture is important. Try to imagine that an invisible string is pulling your head up and back in line with your spine. Be relaxed rather than stiff. Sit with your feet in front of you; if they don’t reach the floor, your chair is too high or you need a footrest. Take frequent stretching breaks.error a malfunction; a situation in which a computer cannot follow its instructions, or in which recorded data cannot be retrieved correctly. In computing, error does not necessarily denote a mistake made by a human being. See ERROR MESSAGE; HARD ERROR; SOFT ERROR.error-correcting code any method of encoding data that allows small errors to be corrected. Contrast CHECKSUM, CYCLICAL REDUNDANCY CHECK, and PARITY, which are techniques for detecting errors but not cor-recting them.A simple error-correcting code would be to send each message three times, and if some part of the message does not come out the same in all three copies, let the majority rule. In order to be uncorrectable, an error would have to corrupt two of the three copies, not just one. Even then, you would know that an error had been made.Practical error-correcting codes are more concise and are based on binary matrix arithmetic.error message a message that indicates that a computer cannot do what is requested or that some part of the software or hardware is defective. Error messages range from “You can’t divide by zero” to “The disk drive isn’t working.” They do not necessarily mean that the user of the com-puter has made a mistake. See also ERROR.error trapping see TRAPPING. 175	ESRBeSATA (external SATA) a hardware implementation of the ATA disk drive protocol for connecting external disk drives to computers. It is similar to SATA but uses different connectors. See SATA.escalate to transfer a customer’s help request from the person who origi-nally handled it to someone more highly trained, and/or to mark it as more urgent.escape code a code that indicates that the following character is to be han-dled specially (e.g., as a printer control code), or a code that stands for a character that cannot otherwise be typed. For example, in HTML, the characters < > mark the beginning and end of a command, so if you want them to appear on the screen, you have to type them as the escape codes &lt; and &gt;, respectively.Escape key a key on a computer keyboard that has a special meaning depending on what software is being used. In many programs and under Microsoft Windows, the Escape key means “get out of where you are now and get back to where you were before” (e.g., back out of a menu without making any of the choices on it). The Escape key transmits ASCII character code 27, which is a character originally used to send special messages to devices. See also ESCAPE SEQUENCE.escape sequence a special sequence of character codes that cause a screen or printer to perform some action (e.g., changing type style) rather than displaying the characters.ESDI (Enhanced Small Device Interface) a standard introduced by Maxtor in 1983 as an interface for hard disks. It has largely been superseded by IDE and SCSI. See IDE; SCSI.ESRB Entertainment Software Rating Board (www.esrb.org) a non-profit, independent organization established in 1994 that reviews entertainment software and web sites and assigns standardized ratings based on suit-ability for children, as well as descriptions indicating the amount of violent and/or sexual content. ERSB labels can be found on most com-mercial software titles. The ratings are:EC	“Early childhood”	Age 3 and upE	“Everyone”	Age 6 and older; comic violence,		no sexK–A	“Kids to adults”	Equivalent to E, obsoleteT	“Teen”	Age 13 and older;		limited violence, suggestive themesM	“Mature”	Age 17 and older;		violence, sexual themesAO	“Adults only”	Graphic sex and/or violenceRP	“Rating pending”	Not yet ratedContrast ICRA, which uses ratings self-assigned by the authors; seePARENTAL CONTROLS. /etc	176/etc in UNIX, a directory that contains system configuration information; often pronounced “et-see.”Ethernet a type of local-area network originally developed by Xerox Corporation. Communication takes place by means of radio-frequency signals carried by a coaxial cable. The name “Ethernet” apparently comes from “aether,” the 19th-century name for the medium through which light waves were thought to travel. See LOCAL-AREA NETWORK;DATA COMMUNICATION.On the physical level, there are four types of Ethernet connections. Thin-wire Ethernet uses RG-58 coaxial cable. Twisted-pair Ethernet is similar but uses a pair of unshielded wires. Conventional baseband Ethernet uses a thicker coaxial cable about 3⁄8 inch (0.9 cm) in diameter, and broadband Ethernet modulates the whole Ethernet signal on a higher-frequency carrier so that several signals can be carried simultaneously on a single cable, just like cable TV channels. See 10BASE-2; 10BASE-T; 100BASE-T.The control strategy of Ethernet is called CSMA/CD (Carrier Sense, Multiple Access, Collision Detection). Each computer listens to see if another computer is transmitting. If so, it waits its turn to transmit. If two computers inadvertently transmit at the same time, the collision is detected, and they retransmit one at a time.Ethernet systems use many software protocols, including TCP/IP, IPX/SPX, and NetBEUI. See MAC ADDRESS; NETBEUI; PROTOCOL; TCP/IP.ethical hacking the practice of breaking into computers without malicious intent, simply to find security hazards and report them to the people responsible.The concept of “ethical hacking” is questionable because most people do not want strangers trying to break into their computers, no matter how benign the motives. Malicious CRACKERs almost always claim to be “eth-ical hackers” when caught. We do not allow strangers to attempt “ethi-cal burglary.” Experiments to test the security of a system should only be done with the advance permission of the victim.Eudora a pioneering e-mail program, widely available for PC and Macintosh computers, distributed free of charge from www.eudora.com. It was developed by Steve Dorner starting in 1988. He named it after the writer Eudora Welty (1909–2001), who wrote a short story, “Why I Live at the P.O.,” about the importance of mail.EULA abbreviation for end-user license agreement, the agreement that the user of a piece of software is required to accept when installing it. SeeLICENSE.Euro the common European currency introduced in 1999 to replace national currencies in Austria, Belgium, Finland, France, Germany, Ireland, Italy, Luxembourg, the Netherlands, Portugal, and Spain. More information about the European currency is available from the European Union at www.europa.eu.int and the European Central Bank, which manages the currency, at www.ecb.int. 177	EX+The Euro symbol is shown in Figure 96. In Windows, this can be typed by holding down Alt and typing 0128 on the numeric keypad. However, to display and print this symbol, users of versions prior to Windows 2000 may need to download updated fonts from www.microsoft.com.FIGURE 96. Euro currency symbolEuropean paper sizes see PAPER SIZES (ISO).EV1. (exposure value) in photography, a number that measures the effect of f-ratio and shutter speed together, as exposure adjustments. For exam-ple, an exposure of 1/250 second at f /8 is equivalent to 1/125 second at f /11; each of these is EV 14. Higher EV numbers correspond to shorter exposures or exposures at smaller apertures (higher-numbered f-stops). Adding 1 to the EV is equivalent to cutting the exposure in half. Thus, 1/250 second at f /11 is EV 15.Cameras often have “EV +/–” adjustments to deliberately increase or decrease the exposure. Here +1 means to expose more than the meter indicates, and –1 means to expose less than the meter indicates.See also F-RATIO.2. (Extended Validation) a type of digital certificate with additional verifi-cation requirements to ensure that the certificate holder is really who they claim to be. (Web address: www.cabforum.org). See CERTIFICATE, DIGITAL.EVDO (Evolution Data Optimized) a system for wireless broadband (web address: www.evdoinfo.com). Contrast HSDPA.even smalls type that is set in all small capital letters, with no lower case letters, LIKE THIS. The cross-references in this book are set in even smalls.Contrast C/SC.event-driven programming programming in which the computer spends its time responding to events rather than stepping through a prearranged series of actions. Computers that control machinery are almost always event-driven. So are computer programs that run under graphical user interfaces such as the Macintosh operating system or Microsoft Windows. Such programs respond to events such as the user choosing an item on a menu or clicking the mouse on an icon. See GRAPHICAL USER INTERFACE; OBJECT-ORIENTED PROGRAMMING; VISUAL BASIC; WINDOW.evil twin a malicious wireless hot spot that seems legitimate, but is designed to trick unwitting users into revealing personal information.EX (describing items for sale) “excellent,” i.e., fully functional and undam-aged. Particularly in the used-camera trade, EX applies to reliable, work-ing equipment that shows some visible wear.EX+ (describing items for sale) better than EX (i.e., fully functional and only slightly worn). Compare LN, LN–. exa-	178exa- metric prefix meaning 1,000,000,000,000,000,000 (1018). Exa- is derived from the Greek word for “beyond” or “outside.” See METRIC PREFIXES.Excel a popular SPREADSHEET program, originally released in 1985 by Microsoft for the Macintosh, and later adapted for Microsoft Windows.exception a situation that prevents a computer program from running as specified, such as unexpectedly reaching the end of a file or trying to divide by zero. See also TRY; UNHANDLED EXCEPTION.Exchange Server popular Microsoft software for electronic mail and other collaboration. Users run client software such as Outlook on their machines, which connect to the Exchange software on a server. See www.microsoft.com/exchange/evaluation/whatis.mspx.exclusive-OR gate see XOR GATE.exe file a file with EXTENSION .exe, containing an executable machine-lan-guage program for Windows. To execute it, simply double-click on it; right-click on it and choose Run; or type its name at a command prompt.Most application programs are distributed as .EXE files. Most compil-ers translate source code into .EXE files. See COMPILER. Contrast BAT FILE; COM (definition 2).Caution! Do not run .exe files received via e-mail because they are almost certainly viruses.execute to do what an instruction says to do. A computer alternates between a fetch cycle, when it locates the next instruction, and an execute cycle, when it carries the instruction out. See COMPUTER ARCHITECTURE.executive size a size of paper sometimes used for stationery in the United States, 71⁄4  101⁄ 2 inches (18.4 cm  26.7 cm).EXIF (exchangeable image file format) a standard way of including META-DATA in JPEG and some other file formats, mainly to preserve information about the digital camera and the camera settings used to create an image. The EXIF standard is maintained by the Japan Electronics and Information Technology Association (JEITA, www.jeita.or.jp). Unofficial but useful information about EXIF is maintained at www.exif.org.exit to clear an application program from memory; to QUIT. Most software prompts you to save changes to disk before exiting. Read all message boxes carefully. Compare CLOSE.exp the function, in many programming languages, that calculates the value of ex. See E.Expansion in computer games, a supplement to the original game that adds features and content. It is important to note that expansions usually do not include the original game software, which must be purchased separately.expert set a FONT that includes a full set of accented vowels, ligatures, small caps, and other special characters (such as an extended group of CASE START BUTTON179	exponentFRACTIONs). It is assumed that someone using such a font will have the know-how and the software to be able to set the special characters. Not every type-face has a matching expert set; you may have to take this into consideration when selecting a typeface for a particular job or when purchasing fonts.FIGURE 97. Expert set (Minion typeface, partial font)expert system a computer program that uses stored information to draw conclusions about a particular case. It differs from a database, which merely calls up stored information and presents it to the user unchanged. Expert systems are widely used to troubleshoot defects in machines; they have also been used successfully to diagnose diseases or recom-mend manufactured products.Every expert system consists of three parts: (1) a user interface, which is a way of communicating with the user through such devices as menus, commands, or short-answer questions (see USER INTERFACE); (2) a knowl-edge base containing stored expertise; and (3) an inference engine, which draws conclusions by performing simple logical operations on the knowledge base and the information supplied by the user. See alsoARTIFICIAL INTELLIGENCE; DEFAULT LOGIC; FUZZY LOGIC; PROLOG.exploit1.	(noun) a way of breaching the security of a system or using features that should be inaccessible. Often written and pronounced sploit.2.	A piece of software designed to make it easy for a large number of would-be hackers to take advantage of such a software flaw.Explorer the part of Windows that is used for exploring directories, files,and desktop menus. To access it, right-click on the	andchoose “Explore.” (See Figure 98 on page 180.)exponent a number or letter that indicates repeated multiplication. Thus the exponent n in the expression an means to multiply n number of a’s together. For example:32 = 3  3 = 9 45 = 4  4  4  4  4 = 1,024106 = 10  10  10  10  10  10 = 1,000,000Also, a2 = a  a is called a to the second power, or a squared. The num-ber that when multiplied by itself gives a is called the square root of a(written as a ). That means a  a = a. For example, 9 = 3, since 3  3 = 9. exponential function	180FIGURE 98. Explorer (Windows)exponential function a function of the form y = ax, where a can be any pos-itive number except 1 and is called the base of the function. The most commonly used exponential function is ex. See E.exponential notation (scientific notation, E format) a way of writing very large or very small numbers conveniently. For example, 2,500,000 can be written as 2.5  106 or (in E format) 2.5E6 or 2.5E+6. For very small numbers, the exponent is negative; thus 0.003 = 3.0  10–3 = 3.0E-3.export to save a file in a format other than the application program’s native format. Many word processing and graphics programs have the ability to export to several different formats. Look under the “Save As. . . ” dialog box for the available file formats.Because the export process is a type of file conversion (instead of a simple copy operation) there is the possibility of a loss of image quality or text formatting.ExpressCard a type of add-on card for laptop computers introduced by the Personal Computer Memory Card International Association (PCMCIA) in 2003 to replace the earlier CardBus (PC Card, PCMCIA Card) stan-dard. ExpressCards are much faster, since they combine USB 2.0 and PCI Express communication.Note: An ExpressCard slot and a CardBus (PC Card) slot look alike from outside the computer, but if you insert the wrong kind of card, no connection is made (and no damage occurs). A CardBus card is 2.1 inches (54 mm) wide. An ExpressCard can be the same width, but it narrows to 1.6 inches (34 mm) at the connector end, and many ExpressCards are that width along their entire length.expression a series of symbols that can be evaluated to have a particular value. For example, 2 + 3 is an expression that evaluates to 5.Extended Industry Standard Architecture see EISA. 181	external viewerextends in C++ and Java, a keyword indicating that a class inherits all of the functionality of another class, and then adds additional data or meth-ods. Instead of extends, C# uses a colon (:). For example, all program-mer-defined Java applet classes include a declaration similar to this:class myapplet extends AppletThis allows the class you write (myapplet) to include all of the features defined in the standard class Applet.extension1.	anything that adds capabilities to an existing system. For example, optional components of the Macintosh operating system are called extensions.2.	the part of a filename following the period, in Windows and other oper-ating systems. For example, the filename myfile.txt has .txt as its extension.The purpose of extensions is to indicate the type of file, but it is impor-tant to realize that the extension does not actually cause the file to be of a particular type; you can rename any file to have any extension, but when you do, your software may no longer recognize it for what it is.Some file extensions have standard meanings; see Table 6 on page 182. See the individual entries in this book for more information on some of the more important types.A practical problem arises when the same extension is used by differ-ent software packages for different purposes. For example, .tex denotes both a TEX word processing document and a Corel Draw texture. When this happens, an extension may end up associated with the wrong piece of software. See ASSOCIATE for information on how to change the soft-ware that is associated with a particular extension.Prior to Windows 95, all extensions could be no more than three char-acters. Newer file extensions can be longer.FIGURE 99. Extensions, hidden (top) and visible (bottom).In Windows, it is up to the user whether extensions are displayed or hidden (Fig. 99). The choice is under Tools, Folder Options, in any win-dow displaying files or folders.Note that you can be tricked by a filename with two extensions. If someone sends you a file named virus.txt.exe and extensions are hid-den, you will see the name as virus.txt and think it is a text file, but if you open it, it will actually execute as a program.external viewer see VIEWER. external viewer	182TABLE 6COMMON WINDOWS FILENAME EXTENSIONS.ai		Adobe Illustrator subset of .eps.asc		ASCII text file.bak, .bk		Backup copy of a file that has been edited.bas		BASIC program file.bat		Batch job (file of commands, DOS or Windows).bmp		Bitmap graphics file.c		C program file.cdr		Vector graphics (CorelDraw).class		Java bytecode file.com		Command file (smaller version of .exe).cpp		C++ program file.cs		C# program file.doc, docx		Document file (ASCII or Microsoft Word).dll		Dynamic link library.eps		Encapsulated PostScript graphics.exe		Executable file (machine-language program).gif		Bitmap graphics file (GIF format).hlp		Help file.htm, .html		Hypertext Markup Language.ico		Icon (Windows).ini		Initialization file (configuration settings).java		Java program source file.jpeg, .jpg		Compressed graphics (popular on the Web).log		Log of installation or usage (various software).mak		Makefile (Visual Basic and other environments).lnk		Windows shortcut.mid, .midi		MIDI digitized music file.mp3		MP3 digitized audio file.pas		Pascal program file.pdf		Portable Document Format (images of printed pages).prj		Project file (various compilers).pl		Perl or Prolog program.ppt, .pptx		PowerPoint presentation.ps		PostScript printable file.pst		Outlook e-mail archive file.raw		Image file.rtf		Rich Text Format word processing file.scr		Screen saver (in .exe format).swf		Shockwave file.tex		TeX document.tif, .tiff		Bitmap graphics file (TIFF format).ttf		TrueType font.tmp		Temporary file.txt		ASCII text file.wav		Sound wave file.wks, .wk2,	.wk3	Lotus 1-2-3 or Microsoft Works worksheet.wma		Windows Media audio (music).wp, .wpd,	.wp6	WordPerfect document.xls, .xlsx		Excel worksheet file.zip		ZIP compressed file		 183	eyedropperextranet a network using Internet protocols that allows a company to share information with other companies (such as suppliers and customers) but with security features preventing access to others. See VPN; PROTOCOL.Contrast INTRANET.Extreme Programming (or eXtreme Programming, abbreviated XP) a programming methodology introduced by Kent Beck and others in 1999.The key idea is never to write a long computer program without know-ing whether it will work. Instead, build every program out of small pieces that can be tested individually. This often requires writing substitutes (STUBs) for unfinished routines so that the rest of the program can be tested.Extreme Programming also includes other good management practices, such as encouraging teamwork and keeping working hours reasonable. Nothing in Extreme Programming is radically new or “extreme;” much of it reflects the way the best programmers have always worked. See SOFTWARE ENGINEERING.Despite the abbreviation XP, Extreme Programming has no specific connection to Microsoft Windows XP, as far as we can determine.extrude a special effect provided by drawing programs that creates a three-dimensional shadow. It looks as if the type (actually any object) has been squeezed out from a cookie gun.FIGURE 100. Extruded typeeyedropper a tool available in paint programs that allows you to match a color in the existing picture, and cause it to become the active color (Figure 101). All you have to do is click the eyedropper on the area of color you desire and that becomes the selected color. You can sample for your primary, secondary, and background colors.FIGURE 101. Eyedropper toolIf at first this tool seems senseless, consider what would happen if you were working on a digitized 24-bit color photograph. There are literally millions of colors available in this format—how are you going to find the right one to extend that background shade over that telephone line? Or how are you going to remember which of those colors you were using yesterday? The eyedropper will let you pick up the right color to use. See24-BIT GRAPHICS; COLOR. eyeglasses, computer	184eyeglasses, computer eyeglasses for viewing a computer screen two or three feet away. Most eyeglasses are designed for vision at a great dis-tance or for reading at about 18 inches (46 cm). Neither of these is suit-able for looking at a computer screen. Moreover, the screen cannot be seen properly through the dividing line or transition region of bifocals. In addition, the slight fuzziness of screen images causes some people’s eyes to strain as they try to focus. As a result, many eyeglass wearers think the computer has harmed their vision, although in fact there is no evidence that computer work (or any other kind of close work) harms the eyes.Computer screens emit tiny amounts of ultraviolet (UV) light, and special glasses are available that block this. However, there is much more UV in ordinary sunlight than in the image on a computer screen, so UV-blocking glasses are probably more beneficial outdoors than in the office. FUNCTION KEYS.185	fair useFF keys seef-ratio the focal length of a lens divided by the clear aperture (diameter) through which light enters. The adjustment for f-ratio is called theF-STOP.The f-ratio determines the brightness of the image formed by the lens; lower f-ratios produce brighter images. Thus, a camera with an f/1.8 lens requires much less light to take a picture than a camera with an f/ 8 lens, even with the same film or electronic image sensor.The brightness of the image is inversely proportional to the square of the f-ratio. That is why f-stops on lenses are often numbered as powers of 2 : f/2, 2.8, 4, 5.6, 8, and so on. Each f-stop gives half as bright an image as the next larger (lower-numbered) one.To increase or decrease exposure n “stops” means to increase it or decrease it by 2n. Thus a one-stop decrease means to cut the exposure in half, and a two-stop decrease means to cut it to 1⁄4 of its original value.The rated f-ratio of a lens refers to its widest opening; smaller open-ings (higher f-ratios) are provided as an automatic or manual adjustment. The actual front glass element is much larger than the opening that the light must pass through. The f-ratio of a ZOOM lens generally varies as the focal length is changed.See also A; DEPTH OF FIELD; EV; FOCAL LENGTH; P; S; TV.f-stop the adjustment for selecting the F-RATIO of a lens. Choose a smaller aperture (higher-numbered f-stop) for greater DEPTH OF FIELD.fabric network interconnections.Facebook a social networking site (www.facebook.com). Because Facebook was originally restricted to school campuses, it is the most popular service with students.facsimile see FAX.factorial the product of all the integers from 1 up to a specified number. The factorial of a number n is symbolized by an exclamation point: n!. For example:2! = 2	 1			= 23! = 3	 2	 1		= 64! = 4  3  2  1		= 245! = 5	 4	 3  2	 1	= 120fade see TRANSITION EFFECT.fair use in copyright law, a limited kind of use of copyrighted material that does not require the copyright holder’s permission. For example, quoting a few sentences from a book and acknowledging the source is fair use. fanfic	186The essential characteristic of fair use is that it does not decrease the market for the original; it may in fact increase it. Fair use does not per-mit reproduction of a complete essay, poem, or other copyrighted work, nor does it extend to music, artwork, or software. See COPYRIGHT.fanfic (fan fiction) stories written by fans of a television series, movie, or book. Fanfic makes use of the established fictional world, expanding or enriching the original story so that it more closely matches the author’s interpretation and imagination. The tradition emerged as soon as there were stories to fuel imaginations, and fan fiction is now widely pub-lished on the Internet.FAQ (Frequently Asked Questions) a file of often-needed information in question-and-answer format. Many Usenet NEWSGROUPs have, or for-merly had, their own FAQ files. These are collected at www.faqs.org and other sites and can easily be found by using a SEARCH ENGINE.FAT (File Allocation Table) the part of the disk that contains information about the sizes and locations of the files. In Windows, a FAT file system is a file system that is compatible with DOS, as opposed to a Windows NT native file system (NTFS). See also FAT32.FAT16 the original form of the FAT file system used by DOS and by Windows 95 and its predecessors. Contrast FAT32.FAT32 (File Allocation Table, 32 bits) a modified form of the FAT file sys-tem that was introduced in Windows 98 and some late releases of Windows 95. FAT32 allows each disk to be divided into a larger number of clusters (allocation units); thus, space can be allocated in smaller units and used more efficiently. More importantly, FAT32 supports disk drives larger than 2 gigabytes.favorites recorded addresses on the WORLD WIDE WEB. Web browsers nor-mally let the user record the addresses of frequently visited web pages in order to go directly to them in the future without having to type the full web address or use a search engine. Also called bookmarks.fax (originally an abbreviation for facsimile) a method of transmitting copies of paper documents over telephone lines by converting the appearance of the document into an electronic signal. The output looks much like a photocopy. Computers can send and receive fax signals by using suitable software and a fax modem. A fax document consists of a BITMAP image, not a file of characters.fax broadcasting sending the same message by fax to multiple recipients, one after another. Compare E-MAIL BROADCASTING. See also JUNK FAX; SPAM.fax modem a MODEM that can transmit FAX messages as well as computer-to-computer connections. 187	fiber optics FCC1.	(Federal Communications Commission) the agency of the U.S. gov-ernment that regulates all equipment that produces radio-frequency sig-nals, including computers. The FCC issues two levels of approval for computers: Class A (suitable for use in industrial or business areas) and Class B (suitable for use in the home). See RFI PROTECTION.2.	business abbreviation for file carbon copies, a copy of an electronic mail message that is kept by the sender. Compare BCC; CC.FCS (Final Customer Shipment) the stage at which a product has com-pleted the beta-testing phase and is available to be shipped to customers.FDD (floppy disk drive) a diskette drive.Fedora the continuing freeware project derived from Red Hat Linux. SeeRED HAT.feedback1.	a rating and/or comment given to help members of an online com-munity determine if the rated member is trustworthy. Auction sites such as eBay or review sites like Epinions depend on user feedback to func-tion effectively.2.	a phenomenon that occurs when a control device uses information about the current state of the system to determine the next control action. For example, when a thermostat controls the temperature in a house, it needs to know the current temperature in the house before it decides whether to turn on the furnace. Thus, information about the temperature “feeds back” into the device that controls the temperature. The thermostat, heater, and air temperature form a feedback loop. See LOOP (definition 2).3.	an unwanted squeal that occurs when a microphone picks up its own output from speakers. A single vibration can then go through the system over and over, producing an endless, raucous tone.femto- metric prefix meaning ÷1,000,000,000,000,000. Femto- is derived from the Danish word for “fifteen” (because it signifies 10_15). SeeMETRIC PREFIXES.fencepost error an OFF-BY-ONE ERROR; a programming error caused by doing something one less, or one more, time than necessary. So called because a person who is asked how many fenceposts, one foot apart, are needed to build a 10-foot fence, is likely to answer “ten” rather than the correct “eleven.”FET see FIELD-EFFECT TRANSISTOR.FF (form feed) the character code (ASCII decimal 12, Ctrl-L) that causes a printer to eject the current sheet of paper and start a new one. Compare LF (line feed).fiber optics cables that carry light rather than electrical energy. Fiber-optic cables are made of thin fibers of glass. Large amounts of data can be field	188carried by a single fiber-optic cable. Unlike wire cables, fiber-optic cables are not subject to crosstalk or electromagnetic noise, and they cannot be tapped into (e.g., by an eavesdropper) without producing a noticeable drop in signal level.field1.	a portion of a record in a database, containing one piece of informa-tion. For instance, in an address list, the zip code might be stored in a 10-character field. See also DATABASE; RECORD.2.	a place where information can be typed on the screen, such as one of the cells in a spreadsheet. See SPREADSHEET.3.	a region of space where an electrical, magnetic, or gravitational effect is present.field-effect transistor (FET) a transistor in which the flow of current from source to drain is controlled by a charge applied to the gate. This charge attracts electrons into the area between source and drain or repels them away from it, thus changing its semiconductor properties. No current actually flows into the gate (in practice, there is a tiny current, on the order of 10–12 ampere). Thus, field-effect transistors consume little power and can be packed very densely on integrated circuit chips.MOSFETs (metal-oxide-semiconductor FETs) have an insulating layer of metal oxide between the gate and the rest of the transistor. They consume the least power of all kinds of transistors. See CMOS; INTE-GRATED CIRCUIT; TRANSISTOR.FIFO (first-in-first-out) a QUEUE (definition 1); a data structure or memory device from which items are retrieved in the order in which they were stored. Contrast LIFO.fifth-generation computers computers built with advanced large-scale integrated circuits that break out of the traditional Von Neumann archi-tecture by making extensive use of pipelining and/or vector processing.See also COMPUTER ARCHITECTURE; PIPELINE; VECTOR PROCESSOR. This includes the current Pentium microprocessor, but the term “fifth genera-tion” is no longer widely used. It was popular in the 1980s when power-ful computers were forseen but not yet built.file a block of information stored on disk, tape, or similar media. A file may contain a program, a document, or a collection of data (such as a mailing list). A file need not occupy a contiguous block of disk space. See BINARY FILE; DATABASE MANAGEMENT; DISK; EXTENSION; RECORD; TEXT FILE.file compression see DATA COMPRESSION.file format a way of arranging information in a file. Almost every com-puter program has one or more file formats of its own; for example, WordPerfect documents are not in the same format as Microsoft Word documents, and similar programs from different manufacturers cannot necessarily process each other’s files. There are three reasons why file formats are diverse: 189	fill1.	Different programs handle different kinds of data (text vs. pictures vs. spreadsheets, for example).2.	Different programmers simply pick different ways of doing the same thing. Sometimes, inventing a new format is a point of pride, or is necessary to avoid infringing someone else’s copyright or patent.3.	Even when the end result is the same, the way different programs achieve it may be very different. For example, a Windows Paintbrush picture is a bitmap (a large grid of dots), but a CorelDraw picture consists of vector graphics (instructions to draw lines or shapes in particular positions). The two kinds of pictures are very different from the computer’s point of view.Many programs have the ability to import (bring in) files that are not in their own format. But the format of the imported file may not be very well suited to the way the program works, result-ing in a loss of quality or partial loss of information (disappear-ance of italics or footnotes, loss of graphics resolution, inability to edit the imported material, or the like). It is also possible to export files to a format other than the usual one, but again, loss of information may occur. See SAVE AS... . See also CONVERSIONPROGRAM; EXTENSION.file management system software allowing you to keep track of your com-puter files. The term is typically used to refer to products that include fea-tures that go beyond the file management capabilities provided with the operating system. The system should keep track of when changes are made and who makes them; make sure that you don’t have two people trying to make changes to a file at the same time; and provide ways of indexing and searching so a specific file can be found easily. See also CMS.file permissions see PERMISSION.file server a computer that performs FILE SHARING. Contrast DISK SERVER.file sharing the use of networking to make files available to computers other than the one that owns and manages them. With file sharing, every file exists on the server, which knows it by file name and manages its space. File sharing is essentially the same thing as NETWORK ATTACHED STORAGE (NAS). Contrast DISK SHARING, STORAGE AREA NETWORK.filename the name of a file. See also 8.3 FILENAME; EXTENSION; FILE.Usage note: Many publishers prefer to write file name as two words.filesystem a method of using a disk, optical disc, or tape to store data in files. Different filesystems are used for different operating systems and media. For examples see CDFS; FAT32; NTFS.fill (in graphics programs) the color of an object. Fills may be of a uniform tone, or they may contain shades that gradually change from one color to another. A fill may even be a pattern. See also LINEAR FILL; UNIFORM FILL. film, digital	190FIGURE 102. Fillsfilm, digital see DIGITAL FILM.filter1.	in paint programs, a tool for modifying the image. See IMAGE PROCESSING and its cross-references.2.	a program that reads a file, byte by byte, and creates another file from it in some way. For example, the Windows sort command can be used as a filter. If you typedir | sort | moreyou are sending the output of the dir command through the sort com-mand, which acts as a filter to put the lines in alphabetical order; then the result is sent to the more command to be displayed on the screen. This technique originated in UNIX. See UNIX.3.	a program that intercepts incoming e-mail, newsgroup messages, web connections, and so on, and blocks those with objectionable or unwanted content. Compare KILL FILE. See PARENTAL CONTROLS.4.	a program that translates files from one format to another when called by the import or export command in a word processor or graphics program. See CONVERSION PROGRAM; DSL FILTER; EXPORT; IMPORT.5.	in electronics, a device that blocks certain signals or frequencies. SeeANTIALIASING; RFI PROTECTION.6.	a device that blocks light of certain wavelengths or polarizations. Filters are used in front of computer screens to reduce glare.7.	a material that removes dust particles from air, sometimes used in front of a computer’s cooling fan.8.	in Windows programming, a set of patterns that match desired file-names. For example, *.c;*.h is a filter that picks out files whose names end in .c and .h.finally see TRY.find1.	the operation of searching a file or web page for a particular word or string of characters. In many editors and web browsers, this is done by typing Ctrl-F.2.	a UNIX command that searches directories to find files with particu-lar attributes. For example, this command starts in the current directory and searches all subdirectories to find files whose name starts with pas: find . -name ”pas*” -ls 191	FirefoxIn Windows, a similar function is available by choosing Search on the start menu.3. a Windows command that finds all lines in a text file that contain a particular character string; a less powerful version of the UNIX grep command. For example, this command will display all the lines in myfile.txt that contain the word birthday:find ”birthday” <myfile.txtTo find the lines that do not contain birthday, add the option /V imme-diately after find. Compare GREP.find and change, find and replace see SEARCH AND REPLACE.Finder the part of MAC OS that enables the user to explore the contents of disk drives, launch programs, and open files.fine-grained security a security model allowing the user to control the specific level of access that a particular program has to the computer. For example, a fine-grained security system would allow downloaded programs from certain providers to have read/write access to specified directories; read only access to other directories; and no access to other directories. This is generally better than an all-or-nothing approach to security, which forces the user to choose between crippling the func-tionality of a downloaded program by preventing it from having any access to the local machine, or else risking a breach of security by giving the downloaded program complete access. See also SANDBOX.finger a UNIX command that provides you with information about users of your own or other machines. For example, if you type fingersmith@gizmo1.ai.uga.edu your computer will connect with GIZMO1 (the host computer’s name) at the University of Georgia and look for a user named Smith; if one exists, you will get that person’s full name and e-mail address, along with some other information depending on the exact version of the operating system.Because the finger command has been abused (to collect addresses for junk e-mailing or even to deliberately overload a machine with requests), many larger UNIX systems no longer answer finger queries.See COMPUTER SECURITY.fingerprint reader a device that identifies a computer user by detecting the person’s fingerprint pattern, and can be used instead of passwords to allow access to computer services. Microsoft sells one such device for use with Vista.FiOS a service from Verizon providing high-speed fiber optic connections to homes.Firefox a popular free Web browser introduced in 2004 by Mozilla (www.mozilla.com/firefox). On June 17–18, 2008, version 3 of Firefox set a world record for most number of downloads of a piece of software firewall	192in a 24-hour period, and it continues to gain popularity. See BROWSER, MOZILLA.firewall a link in a network that relays only data packets clearly intended and authorized to reach the other side. Firewalls are helpful in keeping com-puters safe from intentional hacker attacks and from hardware failures occurring elsewhere. They can be implemented in hardware or software.Since 2005, a software firewall has been built into Windows. Software firewalls are also included with many ANTIVIRUS packages.FireWire a high-speed serial bus standard more formally known as IEEE 1394, and similar in function to USB, but faster. The name FireWire is a trademark of Apple; the same bus is sometimes known as i.Link, a Sony trademark.FireWire is most often used to interface video cameras to computers, but it has many other uses. The original version (IEEE 1394 and, with an improved software interface, 1394a) had a maximum speed of 400 megabits per second. The newer version, IEEE 1394b, achieves 800 megabits per second or more.See also BILINGUAL (definition 2).firmware software (i.e., computer programs) that is stored in some fixed form, such as read-only memory (ROM) or FLASH MEMORY. ContrastSOFTWARE; HARDWARE.first-generation computers the computers that were built in the late 1940s and early 1950s, using vacuum tubes as switching elements.first-person shooter a type of computer game in which the player pretends to shoot a gun from the perspective of the person doing the shooting.fish tape a tool for pulling cables through inaccessible spaces. A fish tape is a very long metal strip with a hook at the end which can be used to grab a wire or another fish tape, somewhat like catching fish with a hook on a line.fishing1.	the act of pulling a cable through the interior of a wall, or through some other inaccessible space, by using a FISH TAPE; more precisely, the act of moving a fish tape around to try to catch a cable or another fish tape.2.	the act of trying to obtain personal information by setting up a fake version of a trusted web site; normally spelled phishing. See PHISHING.fit text to path a draw program command that warps the baseline of a line of text so that it follows the shape of a specified line (PATH). See Figure 103 for an example.fix a solution to a software defect; typically a new version of a program issued in order to correct a problem. Compare PATCH. 193	flash memoryFIGURE 103. Fit text to pathfixed disk a disk drive that cannot be removed from the computer in nor-mal use. See HARD DISK.fixed-pitch type type in which all letters are the same width (e.g., I is the same width as M). Most typewriters and older printers and computer screens use fixed-pitch type. Also called MONOSPACE.Contrast PROPORTIONAL PITCH.FIGURE 104. Fixed pitch vs. proportional pitchfixed-point number a number in which the position of the decimal point is fixed. For example, amounts of money in U.S. currency can always be represented as numbers with exactly two digits to the right of the point (1.00, 123.45, 0.76, etc.). Contrast FLOATING-POINT NUMBER.flame (slang) an angry, ill-considered e-mail message or newsgroup post-ing. See ELECTRONIC MAIL; NEWSGROUP.flame war (slang) an angry, uninformative quarrel in a NEWSGROUP or other electronic discussion forum.Flash a program developed by Macromedia, Inc. to produce multimedia web content and presentations. To see Flash animations, viewers must first download the Flash player from Macromedia (www.macromedia.com/ downloads). One of the most powerful features of Flash is the ability to script an interactive experience for the viewer. Entire web sites are now being presented as Flash instead of HTML.flash drive a device that works like a disk drive but uses FLASH MEMORY as the storage medium. For the most common type see USB FLASH DRIVE.flash memory a type of EEPROM that can only be erased in blocks; it can-not be erased one byte at a time. In this regard it resembles a disk drive that is divided into sectors. Flash memory is usually used for storing MEMORY CARDflash memory card (flash card)	194larger amounts of data, like a disk; EEPROM is used for small amounts of data, such as machine configuration. Contrast EEPROM. See also FLASHand references there.flash memory card (flash card) generic term for a card containing flash memory for non-volatile storage. For examples see COMPACTFLASH;MEMORY STICK; MULTIMEDIACARD; SECURE DIGITAL CARD; SMARTMEDIA.flat-file database a database like a relational database except that it has only one table. See RELATIONAL DATABASE.flat-panel monitor a flat, thin computer screen like that of a laptop com-puter, using LCD technology. Contrast FLAT-SCREEN MONITOR.flat-screen monitor a computer screen that is flat. Normally, flat-screen monitor denotes a conventional cathode-ray tube with a flat front, as opposed to a thin LCD panel, which is called a FLAT-PANEL MONITOR.flatbed scanner a scanner in which the object to be scanned is held flat against a piece of glass. See SCANNER.flavor1.	(slang) a variety or type of something. For example, CD-ROM, CD-R, and CD-RW might be described as three “flavors” of compact disc.2.	(in early object-oriented programming experiments at MIT) an inher-itable object class.fleuron a decorative typographic ornament. See Figure 105 for examples. Fleurons may be used for purely decorative purposes or to mark the beginnings of paragraphs.FIGURE 105. Fleuronsflip-flop an electronic circuit that can switch back and forth between two states (called 0 and 1) and will remain in either state until changed. Flip-flops are the basic component of which CPU registers are composed.Figure 106 shows how to construct a flip-flop from two NAND gates. It has two possible states: state 1 (in which output 1 = 1) and state 0 (in which output 1 = 0). (Output 0 is always the opposite of output 1.)If both inputs are 1 when the flip-flop is first powered up, it will set-tle into one state or the other, at random. Bringing the “set” input momentarily to 0 will put the flip-flop into state 1, and bringing the “reset” input to 0 will put the flip-flop into state 0. Whenever both inputs are 1, the flip-flop stays in whatever state it was already in. Thus, a flip-flop is a 1-bit memory.More elaborate flip-flops include control circuitry so that the data to be stored in them can be delivered through a single input. A 16-bit CPU register consists of 16 flip-flops side by side. 195	flowchartFIGURE 106. Flip-flop constructed from NAND gatesflip horizontal a command that creates a mirror image of the original object. The image still appears right-side-up, but left and right are reversed.FIGURE 107. Flip horizontal; flip verticalflip vertical a command that turns an image upside down, but maintains the image’s left-right orientation, just like a reflection in still water. This is not equivalent to rotating the object or defined area 180 degrees. (Try it and see.)floating illustrations illustrations that should appear near, but not neces-sarily at, specified positions in a text. For instance, many of the tables in this book are floating illustrations; they appear near the articles that refer to them, but not between particular words. The page-layout software places the illustrations wherever it is convenient to put them.floating-point number a number in which the decimal point can be in any position. For instance, a memory location set aside for a floating-point number can store 0.735, 62.3, or 1200. By contrast, a fixed-point mem-ory location can only accommodate a specific number of decimal places, usually 2 (for currency) or none (for integers). Floating-point numbers are often written in scientific notation, such as 4.65E4, which means 4.65  104 = 46,500. See DOUBLE PRECISION; REAL NUMBER; ROUNDING ERROR. Contrast FIXED-POINT NUMBER.floppy disk see DISKETTE.floppy disk drive a disk drive for diskettes (floppy disks).FLOPS floating-point operations per second, a measure of computer speed.flowchart a chart consisting of symbols and words that completely describe an algorithm (i.e., how to solve a problem). Each step in the flowchart is followed by an arrow that indicates which step to do next. flush	196The flowchart in Figure 108 shows how to calculate the cube root of a number a using Newton’s method, where x is the guess for the cube root of a, and δ indicates how accurate the result must be. The procedure will follow around the loop until |x3 – a| < δ.“Start” and “stop” statements are written with ovals, action statements are written with squares, and decision statements are written in diamonds. A decision statement asks a yes-or-no question. If the answer is yes, the path labeled “yes” is followed; otherwise, the other path is followed.Writing a flowchart often helps to solve a complex programming problem, although flowcharts are seldom used now that structured pro-gramming has become popular. A flowchart is often much bulkier than the program it describes. See STRUCTURED PROGRAMMING.FIGURE 108. Flowchartflush1.	flat against a margin, as in FLUSH RIGHT and FLUSH LEFT.2.	to finish an output operation by emptying the buffer in which the information is stored while waiting to be output. flush left an arrangement of text with each line starting at the same hori-zontal position, making a neat left edge. Flush left, ragged right means 197	FOAFthat the length of the lines are allowed to vary. The right edge looks like a torn piece of paper—in other words, ragged.This isan example of some flush-left type.Flush left is how type is ordinarily arranged on a typewriter.Contrast CENTER; FLUSH RIGHT; JUSTIFICATION.flush right an arrangement of text with each line of type ending at the same horizontal position. The beginnings of the lines are irregular, but the right margin is smooth.This is an example of flush-right type.Flush-right alignment is seldom used except in charts or tables.Contrast CENTER; FLUSH LEFT; JUSTIFICATION.fly-in see TRANSITION EFFECT.fly-out menu a secondary menu that appears to the side when you select an item on the primary menu.FIGURE 109. Fly-out menusFM synthesis (frequency modulation synthesis) a technique of synthesiz-ing musical sounds by using one waveform to modulate (vary) the fre-quency of another waveform. FM synthesis is an older technique for generating electronic music. It produces a wide variety of sounds, but it does not imitate conventional musical instruments as closely asWAVETABLE SYNTHESIS does. Contrast WAVETABLE SYNTHESIS.FOAF (slang) friend of a friend, an unidentified source of a piece of infor-mation. See HOAX. CROP FACTOR; F-STOP; ZOOM (definition 2).focal length	198focal length the distance from a lens to the focal plane, or in the case of a multi-element lens, the focal length of a simple lens that would form the same size image.A longer focal length produces a larger image and covers a narrower field. On a 35-mm camera, a “normal” lens has a focal length of about 50 mm; wide-angle lenses are 20 to 40 mm, and telephoto lenses are 80 to 200 mm or longer.Electronic image sensors are usually much smaller than film, and they require shorter-focal-length lenses to cover the same field. The focal length of such lenses is sometimes specified as “35-mm equivalent,” (i.e., the focal length that would cover the same field of view on 35-mm film). Typically, the true focal length is written on the lens, and the 35-mm equivalent is used in advertising.The focal length of a ZOOM lens is variable (Figure 110). A lens marked “3 zoom” has a focal length that is three times as long at max-imum as at minimum.See alsoFIGURE 110. Focal length (7–14 mm) and f-ratio (f / 3.5–f / 4.8) (both variable on this zoom lens)focus the part of a DIALOG BOX that is ready to receive input from the key-board. The focus is generally indicated by highlighting with a special color or with an extra dotted line around a BUTTON.folder a directory (on the Macintosh or in Windows). See DIRECTORY.font a complete collection of characters (including upper- and lowercase letters, numerals, punctuation, ligatures, reference marks, etc.) in a con-sistent style and size. Desktop publishing programs and word processors let you use more than one font in a single document. When you switch to italics or to bold or to a larger size, you are changing the font even though you may still be in the same TYPEFACE.Some fonts are specifically designed to be printed at a particular size. For instance, if you have 8-point Dutch Roman and 12-point Dutch Roman on your laser printer font cartridge, you cannot use intermediate sizes such as 9-point. If the largest font you have is 72 points, then you will not be able to set 96-point type. Current systems use SCALABLE FONTs that can be used at any size. See also TRUETYPE FONT; TYPE 1 FONT. 199	forFIGURE 111. Font (Times New Roman)foot the bottom of the page. Contrast HEAD.footer text that is placed at the bottom of each page of a printed document. Contrast HEAD (definition 2).footnote a short comment placed at the bottom (FOOT) of a page that pro-vides a citation or insightful comment to the text.* Some word proces-sors cannot handle footnotes properly, placing them at the end of the chapter or article rather than at the foot of the page where they belong.Footnotes should be numbered consecutively throughout an entire book, or beginning again for each chapter or page. Sometimes footnotes are not numbered, but are referenced by a traditional set of symbols. They should be used in this order: ASTERISK (*), DAGGER (†), DOUBLE DAGGER (‡), SECTION SIGN (§), and PILCROW (¶).footprint the amount of space on a desk that a device takes up. For exam-ple, a laptop computer has a smaller footprint than a full-size PC. A smaller footprint is desirable because it leaves more space for other items on your desk. See also REAL ESTATE.for a keyword that identifies one type of loop in several programming lan-guages. A for loop causes a certain variable, known as the loop variable, to take on several values in sequence. For example, this Java code prints out the whole numbers from 1 to 10:/* This is Java: */for (int i=1; i<=10; i++){System.out.println(i);}A for loop can be used either to obtain repetition or to make the values of the variable available for some purpose. For example, here is how to calculate the sum of the whole numbers from 1 to 100 in Java:*This is a footnote. Notice that the asterisk in the footnote matches the asterisk in the text. The typeface used for the footnote is smaller than the font used for the body copy. A horizontal rule or extra vertical space should be used to separate the footnote from the text. force quit	200int t=0;for (int i=1; i<=100; i++){t=t+i;}System.out.println(t);Although the statements within the loop can examine the value of the loop variable, they should not attempt to change it, since the result of doing so is unpredictable.In C, C++, Java, and C#, the for statement is more general; it takes the formfor (init; test; step) {statements}where init is a statement that is executed once, before beginning the loop; test is a condition that must be true at the beginning of every pass through the loop; step is a statement executed at the end of each pass; and statements are the statements within the loop, if any. Unlike its BASIC and Pascal counterparts, this kind of for statement need not step a number through a series of values; it can be used for many other kinds of repeated actions, because init, test, and step can be statements and tests of any kind. Compare DO; WHILE.force quit (Macintosh) to escape from a frozen or hung program by press-ing the Option, Command (), and Escape keys simultaneously. This brings up a dialog box listing all active applications. Choose the name of the frozen program and click ”Force Quit.”See FREEZE UP; HANG; CRASH. Compare TASK MANAGER.form a screen display in which the user is expected to type in information and then press Enter or click OK. Forms enable readers of WEB PAGEs to send information to the host site.form factor the size and shape of a piece of equipment or material. Diskette drive form factors include half-height and full-height. Motherboard form factors include AT (like the original PC AT), “Baby AT” (a smaller board that mounts in the same case), and ATX (a newer type of motherboard introduced by Intel and others in 1996).form feed the character code (ASCII decimal 12, Ctrl-L) that causes a printer to eject the current sheet of paper and start a new one. Compare LF (line feed).form, HTML a web page that allows users to enter data, which can then be sent back to the server. Here is an example of the HTML code for a form where users can enter an e-mail address into a text field, use a list box to select either the deluxe, special, or standard version; use radio buttons to indicate which region they are from; use a check box to indicate whether they are a first-time visitor or not; and enter a brief message into a text area. 201	form, HTML The ACTION command of this form will simply e-mail the result of the form to the specified address. It is also possible to specify a CGI script as the ACTION. This script needs to be specially written to process the results of the form once they are sent back to the server. The text that is not enclosed by < > are labels; see Figure 113, which shows how this form appears on the screen. Each element of the form needs to have a name by which it is identified to the server; in this example, the names are EMAIL, VERSION, NEWVISITOR, REGION, and MESSAGETEXT. Finally, the form has a submit button for the user to click when ready to send the information, and a reset button to clear information if desired.For another example of an HTML form, see JAVASCRIPT.<HTML><HEAD><TITLE>Form Example</TITLE> </HEAD><BODY><H2>Example of an HTML Form</H2><FORM METHOD=POST ACTION=”MAILTO:yourcompany@xyz.com”> Enter your e-mail address:<INPUT TYPE=”TEXT” NAME=”EMAIL” WIDTH=25> <BR>Choose which version you prefer: <SELECT NAME=”VERSION”><OPTION SELECTED> Standard <OPTION> Special<OPTION> Deluxe </SELECT><BR>Indicate which region you are from:<BR><INPUT NAME=”REGION” TYPE=”RADIO” VALUE=”North”>North <INPUT NAME=”REGION” TYPE=”RADIO” VALUE=”South”>South <INPUT NAME=”REGION” TYPE=”RADIO” VALUE=”East”>East <INPUT NAME=”REGION” TYPE=”RADIO” VALUE=”West”>West <BR>Are you a new visitor?<INPUT TYPE=”CHECKBOX” NAME=”NEWVIS” VALUE=”yes”> <BR><TEXTAREA COLS=40 ROWS=5 NAME=”MESSAGETEXT”> Type your message here,as many lines as you want...</TEXTAREA><BR><INPUT TYPE=”SUBMIT”VALUE=”Click here to submit message”> <INPUT TYPE=”RESET”VALUE=”Click here to clear all data”> </FORM></BODY></HTML> FIGURE 112. Form, HTML (source code) formal parameter	202FIGURE 113. Form, HTML (as displayed on screen)formal parameter a name used within a function or subroutine for a value that will be passed to it from the calling program. Contrast ACTUAL PARAMETER.format any method of arranging information that is to be stored or dis-played. The word format can therefore refer to many different things relating to computers. Three of its most common uses are as follows:1.	The format of a file (or of any storage medium) refers to the way the information is stored in it. See FILE FORMAT.2.	To format a disk is to make the computer record a pattern of refer-ence marks on it, which is usually done at the factory. Formatting a disk erases any information previously recorded on it.3.	In spreadsheets and programming languages, the format of a data item determines its appearance (how many digits are displayed, whether there is a dollar sign, etc, whether to use italics, etc).formatting the special codes in a document that indicate italics, underlin-ing, boldface, and the like. Formatting information is lost when a file is saved as ASCII text.FORTH a programming language invented by Charles Moore around 1970. FORTH is noted for requiring few machine resources and giving very rapid execution.In FORTH, programmers define their own statements in terms of sim-pler statements. FORTH is a threaded interpretive language, meaning that a program is represented in the computer as a list of addresses of subroutines, each of which is composed of addresses of other subrou-tines, and so on until the primitive operations of the language are reached. Because it has little to do except read addresses and jump to 203	FORTRANthem, a FORTH interpreter can run very fast; in fact, there is little dif-ference between an interpreter and a compiler for FORTH.C FORTRAN IV program toC find sum of integers 1-100 INTEGER I, SUMSUM=0DO 1 I=1, 1001 SUM =SUM + IWRITE (6,2) SUM2 FORMAT(1X,I5) STOPENDFIGURE 114. FORTRAN IV programFORTRAN (Formula Translation) a programming language developed by IBM in the late 1950s, the first major programming language that allowed programmers to describe calculations by means of mathemati-cal formulas. Instead of writing assembly language instructions such asLOAD AADD BMULTIPLY CSTORE Dthe FORTRAN programmer writesD = (A+B)*Cwhere * is the symbol for multiplication. Almost all subsequent pro-gramming languages have adopted this useful feature.FORTRAN has existed in several versions, of which the most impor-tant are FORTRAN IV (USASI Standard FORTRAN, 1966) and FOR-TRAN 77 (ANSI FORTRAN, 1977). Figure 114 shows a sample FORTRAN IV program.Comments begin with C in column 1; all other statements begin in column 7, though they may have numbers in columns 2–5. Any line with column 6 nonblank is treated as a continuation of the previous statement.The FORMAT statement in the example specifies that the output should consist of a single blank, followed by an integer occupying 5 character positions. The blank is not actually printed; it serves as car-riage control and tells the printer to print a new line in the normal man-ner. This is a relic of early IBM printers that used the first character of each line as a control code for various printer functions. The number 6 in the WRITE statement refers to the printer—another relic of early IBM computer architecture.FORTRAN variable names contain up to six letters or digits; the first character must be a letter. Variables can be declared as INTEGER, REAL (floating-point), DOUBLE PRECISION (like real, but with more significant digits), COMPLEX (see COMPLEX NUMBER), LOGICAL (Boolean), or (in FOR-TRAN 77) CHARACTER. If a variable is not declared, it is assumed to be integer if its name begins with I, J, K, L, M, or N, or real otherwise. forward	204Compared to FORTRAN IV, FORTRAN 77, and FORTRAN 90 include more sophisticated input-output statements, especially for file handling, the ability to process character-string data, and block-struc-tured IF-ELSE-ENDIF statements to facilitate structured programming.See STRUCTURED PROGRAMMING.forward to send a piece of e-mail on to another person, with an indication that you received it and forwarded it. Contrast BOUNCE (definition 2).forward button on a web browser, the button that returns you to where you were before you clicked on the “back” button.forward one a graphics program command that moves the selected object up one in the stacking order. See also BACK ONE; DRAW PROGRAM; SEND TO BACK; SEND TO FRONT; VECTOR GRAPHICS.forward one; bring forward comparable commands that send the selected object down one layer. See also ARRANGE; BACK ONE; BRING FORWARD;BRING TO FRONT; DRAW PROGRAM; SEND BACKWARD; SEND TO BACK; TO BACK; TO FRONT.fountain fill a fill that is composed of two or more colors. In between the colors is a smooth blending. You are able to rotate the angle of the fill (linear) or have the blend radiate from a central point (radial). Figure 115 shows a sampling of different fountain fills. See also UNIFORM FILL. Also called a GRADIENT FILL.FIGURE 115. Fountain fillsfourth-generation computers computers built around integrated circuits with large-scale integration. Some people view these computers as being advanced third-generation computers.Foveon a type of high-resolution CMOS image sensor developed by Foveon, Inc. (www.foveon.com) and National Semiconductor Corporation. Unlike earlier color image sensors, the Foveon has separate layers sensitive to red, green, and blue light, so the image color is sensed accurately in every pixel, and no light is discarded in colored filters. SeeCCD IMAGE SENSOR; CMOS IMAGE SENSOR. Contrast BAYER MATRIX.The name Foveon presumably alludes to fovea, the Latin name for the central, color-sensitive part of the retina of the human eye.FPS1.	(Frames Per Second) the rate at which a moving image is generated or transmitted. All moving images are actually sequences of still pic-tures. See ANIMATION.2.	abbreviation for FIRST PERSON SHOOTER. 205	fragmentationfractal a shape that contains an infinite amount of fine detail. That is, no matter how much a fractal is enlarged, there is still more detail to be revealed by enlarging it further. Fractals are common in nature; they are the shapes of clouds, coastlines, broken rocks, the edges of torn pieces of paper, and the like. Fractals are important in generating realistic images of everyday objects on the computer.Figure 116 shows how to construct a fractal called a Koch snowflake. Start with a triangle and repeatedly replace every straight line with a bent line of the form:The picture shows the result of doing this 0, 1, 2, and 3 times. If this were done an infinite number of times, the result would be a fractal.FIGURE 116. Fractal built from a triangle (the Koch snowflake)fragmentation a situation in which many files on a hard disk are broken into little chunks and stored in physically different areas of the disk plat-ter. This can dramatically slow down access time and create long waits while files are being read or written.A disk becomes fragmented when many files are created and erased on it over a long period of time. Files are recorded continuously on an empty disk, but as files of various sizes get erased, empty spots develop that gradually become filled in with parts of newly recorded files. Eventually, when the user creates a large file, the file will be given space in many dis-connected places all over the disk (see Figure 117 for an illustration).To defragment a disk in Microsoft Windows, open “Computer,” right-click on the disk, and choose Properties and then Tools.FIGURE 117. Fragmentation(dark areas show sectors occupied by one file) frame	206frame1.	one in a succession of pictures in a video or animation. When dis-played rapidly one after the other, successive frames give the impression of movement. See ANIMATION.2.	a boxed area in a web page. See FRAME, HTML.3.	a boxed area that is to contain text or a graphic (WORD PROCESSING,PAGE LAYOUT SOFTWARE).frame grabber an accessory device that takes an image from a video camera, VCR, or other video input and digitizes it, creating a bitmap image. A video image consists of frames (successive pictures) transmitted at the rate of 30 per second, and the frame grabber must grab and digitize one of them.frame, HTML an area in a web page that scrolls independently of the rest of the web page. A web page can be divided into multiple frames. For example, one frame can include a navigation bar that always stays on the screen as the user moves around the text of the page that appears in the other frame.Here is an HTML file that divides the browser window into two frames. The two files mentioned in it, navbar.html and main.html, are the source files for the separate frames.<HTML><frameset rows=”100%, *” cols=”35%,100%”> <frame src=”navbar.html” name=”NavigationBar”scrolling=auto><frame src=”main.html” name=”main” scrolling=auto></frameset></HTML>Here is file navbar.html, which produces the navigation bar in the left-hand frame:<HTML><BODY><P><A TARGET=”main” HREF=”main.html#sec1”> Section 1</A><P><A TARGET=”main” HREF=”main.html#sec2”> Section 2</A><P><A TARGET=”main” HREF=”main.html#sec3”> Section 3</A></BODY></HTML>The following file, main.html, produces the main frame:<HTML><BODY><H1><A NAME=”sec1”> Section 1</A></H1>The rest of the text for section 1 would go here <H1><A NAME=”sec2”> Section 2</A> </H1>The rest of the text for section 2 would go here <H1><A NAME=”sec3”> Section 3</A></H1>The rest of the text for section 3 would go here </HTML></BODY> 207	free softwareFigure 118 shows the appearance of this page when displayed by the browser. If the right-hand frame did contain a large amount of text, the user could scroll through it while the links on the left-hand side always stayed in the same place, making it easier to jump to other sections of the document.FIGURE 118. Frames, HTMLframing error an error that occurs when an asynchronously transmitted character appears to contain the wrong number of bits.See ASYNCHRONOUS; RS-232. Framing errors usually result from trans-mitting or receiving at the wrong speed.free (describing Windows versions) not containing the extra error checking and debugging code that would be in a CHECKED version. Most comput-ers run a “uniprocessor free” version of Windows, which is a version that runs on a single CPU and does not contain extra error checking. ContrastCHECKED.free software several kinds of computer software that can legally be copied and given free of charge to other users:1.	Public-domain software. This is software that is not covered by any kind of copyright. Few substantial public-domain programs exist, but the term public-domain is often used incorrectly to refer to other kinds of free software.2.	Software that is copyrighted but is distributed free with permission of the copyright owner. Two famous examples are the Linux oper-ating system and the TEX typesetting program (see LINUX; TEX). Many UNIX utilities are also in this category.3.	Shareware. This is copyrighted software that can be distributed free to anyone, but that requests or requires a payment from satis-fied users directly to the author. Shareware is often misleadingly described as “free.” Free Software Foundation	208Free Software Foundation see GNU.Freescale the company that was formerly the microprocessor division of Motorola. See MOTOROLA.freeware see FREE SOFTWARE.freeze date a date after which the specifications for a software project can-not be changed. See SOFTWARE ENGINEERING.freeze up to stop responding to input; to HANG. A frozen computer shows no activity on screen or on the front panel of the computer system and will not respond to the keyboard. Many times, the mouse cursor disap-pears. In Windows, control can often be regained by pressing Ctrl-Alt-Del and letting the operating system close the offending software. Macintosh users can often escape from a frozen program by pressing Option-Command-Esc and then selecting the FORCE QUIT button.friend a fellow user of a SOCIAL NETWORKING SITE with whom you have some sort of relationship. Usually, both users must confirm that they want to be friends, and there are restrictions on contacting or viewing personal information of other users if you are not friends.On social networking sites, “friends” are often just a contact list of passing acquaintances, and the term rarely indicates a close relationship.friendly name the most familiar or meaningful of several names denoting the same thing. For example, a networked printer might be known as \\gizmo1\ttya-lj4 and as Office Printer. The latter is its friendly name.front end a computer or a program that helps you communicate with another computer or program. For example, supercomputers usually do not communicate with their users directly; instead users submit pro-grams through another computer called the front end.front side bus see FSB.FS online abbreviation for “for sale.”FSB (front side bus) the BUS by which a CPU communicates with its fastest input-output devices. It is rated for speed in megahertz (MHz) or million transfers per second (MT/s). The latter rating is higher because there is usually more than one data transfer per clock cycle.FTP (File Transfer Protocol) a standard way of transferring files from one computer to another on the Internet and on other TCP/IP networks. (See TCP/IP.) FTP is also the name of any of various computer programs that implement the file transfer protocol.When you connect to a remote computer, the FTP program asks you for your user name and password. If you do not have an account on the computer that you have connected to, you can use anonymous FTP to retrieve files that are available to the general public. In that case the pro- HALF DUPLEX.INTERNET; PROTOCOL; WORLD WIDE WEB.209	full duplexcedure is to give anonymous as your user name and then type your e-mail address in place of the password. Also remember to use the command binary if the file you are transferring is anything other than plain ASCII text. Figure 119 shows an example of an anonymous FTP session.C:\> ftpftp> open ai.uga.eduConnected to ai.uga.edu FTP server. ftp> userUser name: anonymousPassword: yourname@your.site.addressUser ’anonymous’ logged in. ftp> cd /pub/pc.utilities ftp> binaryftp> dir pkzip.exe ahed.zip pred.zipftp> get pred.zippred.zip transferred, 17895 bytes in 2.5 seconds ftp> quitFIGURE 119. FTP session (user’s commands are in italics)You can also retrieve files by FTP using a web browser. For example, to retrieve the file whose name is filename in directory pub/directory-name on host ftp.cdrom.com, give the URL as:ftp://ftp.cdrom.com/pub/directoryname/filenameIf you need to specify an account name and password, do this:ftp://userid:password@zzzzz.com/directoryname/fiLenameMost browsers will prompt you for the password if you leave it out. Ordinarily, FTP does not work through certain routers and firewalls.For the solution, see PASSIVE FTP.See alsoFTP site a computer that makes files available for downloading by FTP.FUD (slang) (fear, uncertainty, and doubt) the tactic of trying to make cus-tomers afraid of adopting a rival product by creating doubts about its future.full adder a logic circuit that accepts three one-digit binary numbers (two addends and one digit carried from the previous column) and produces two outputs, a sum output and a carry output. See BINARY ADDITION.full duplex communication in two directions at the same time. For instance, an ordinary telephone is a full duplex device; you can hear theother person while you are talking. Contrast BOOLEAN QUERY; SEARCH ENGINE.full-text search	210full-text search the act of searching through every word in a set of docu-ments to retrieve information you are interested in. This is a slow but thorough way to use a computer to search through web pages, court records, scholarly journals, or other material to retrieve items that youare interested in. See alsofunction1.	(in mathematics) a value that depends on one or more arguments in such a way that, for any particular set of arguments, the function has only one value. For example, the positive real square root of a number is a function of that number. The sum of two numbers is a function of the two numbers. A function need not exist for all possible arguments; for example, negative numbers have no (real) square roots.2.	(in computer programming) a subprogram that acts like a mathemat-ical function: given a particular set of argument values, the function returns a unique result.In C, C++, and Lisp, all procedures (subroutines) are called functions, though they need not return a value (see PROCEDURE; VOID). In Java and C#, procedures and functions are called METHODs and are always attached to classes, though not always to specific objects.function keys (F keys) keys labeled F1 to F12 on PC keyboards, and sim-ilar keys on other kinds of computers. Their function depends on the software being run. See also PROGRAMMABLE FUNCTION KEY.fuser in a laser printer, the hot roller that warms the toner particles that have been deposited on the paper, to melt them and make them stick. SeeDRUM.fuzzy logic a formal system of reasoning developed by Lotfi Zadeh in which the values “true” and “false” are replaced by numbers on a scale from 0 to 1. The operators and, or, and the like are replaced by proce-dures for combining these numbers. See BOOLEAN ALGEBRA.Fuzzy logic captures the fact that some questions do not have a sim-ple yes-or-no answer. For example, in ordinary English a 6-foot-high man might or might not be described as tall. In fuzzy logic, a 6-foot man might be tall with a truth value of 0.7, and a 7-foot man might be tall with a truth value of 1.0. A problem with fuzzy logic is that there is often no clear way to come up with the numbers that are used as truth values. For an alternative, see DEFAULT LOGIC.Fuzzy logic is often used in expert systems; it nicely bridges the gap between logical inference and mathematical modeling. See CONFIDENCE FACTOR; EXPERT SYSTEM.FWIW online abbreviation for “for what it’s worth.” 211	gammaG<g>, *g* e-mail abbreviation indicating a grin.g2g chat-room abbreviation for “[I’ve] got to go.”gadgets in Windows Vista and in Google Desktop, add-ons to the sidebar often done by developers other than Microsoft or Google, which provide additional functions such as displaying the weather in your area or pro-viding a place for notes.gain the amount of amplification in an audio amplifier or similar circuit. Adjusting the gain is one way to control the loudness of the sound from a sound card.gambling a popular, but often illegal, online form of recreation. Internet casinos are World Wide Web sites that offer games of chance, with the ability to place bets and receive money. Users are often under the impression that this activity is beyond the reach of local laws, but actually, gambling laws do not make any exemption for computers. U.S. federal law specifically prohibits placing bets “by wire” unless the gam-bling is legal at the locations of both parties.A further difficulty is that there is no way for the casino operator to tell whether the players are human beings. Some of them could be com-puter programs, simulating the behavior of a human being at a web browser, and meanwhile collecting detailed information about the work-ings of the casino—the ultimate card sharp, so to speak. The feasibility of doing this has been demonstrated by Paul Apostolik at The University of Georgia, although, for legal reasons, no actual money was wagered.See COMPUTER LAW.Usage note: In the computer world, gaming does not mean gambling. It means playing computer games for recreation.gamer a devoted fan of computer games. Gamers and the software they love best are responsible for pushing the limits of computer hardware further than most other groups.gaming cafe a small business resembling an INTERNET CAFE but with an emphasis on computer games. Games are often networked and visitors can play against each other.gamma (γ)1. in computer graphics, a measure of the nonlinear response of a video screen. The brightness of a pixel on the screen is not proportional to the brightness value sent to it from software. Instead, the actual brightness B isB = aV γwhere a is a constant, V is the brightness value, and the exponent γ is usually about 2 to 2.5. If gamma is known, it can be corrected so that all parts of the image are displayed with the specified brightness. gamma testing	2122. in photography, a measure of contrast. Normally, the gamma of film is about 0.7 and the gamma of photographic paper is about 1.4. When multiplied together, the two give a gamma of 1 so that the picture has the same contrast as the original subject.gamma testing a third stage of software testing sometimes performed after beta testing but before commercial release. In gamma testing, the soft-ware is believed to be complete and free of errors, but the manuals and packaging may not yet be in final form. See also ALPHA TESTING and BETA TESTING.gamut1.	in music, the range of notes in the scale.2.	the range of colors that can be reproduced by a screen, printer, or other device.There is no way to reproduce all visible colors with a finite number of inks or phosphors. Properly mixed primary colors will give all hues, but not all at high saturation. A color is “out of gamut” if it is more sat-urated (brilliant) than the available inks can reproduce.The gamut of an RGB computer monitor is very different from that of a CMYK printer (see CMYK). Vivid reds and blues on the screen are often unprintable with cyan, magenta, and yellow ink; other purples and ruby-reds are printable but not displayable on the screen.See COLOR and cross-references there. See also HSB; CMYK.Gantt chart a diagram that shows the schedule for a series of tasks. See example at PROJECT MANAGEMENT.garbage collection clearing out objects that are taking up space in memory but are no longer in use by a program. In Lisp, Prolog, Java, and C#, garbage collection happens automatically. In C++, there is no garbage collection; the programmer must specifically release the memory taken up by an object when it is no longer needed.gas plasma display a computer display that works on the same principle as a neon light; the display is usually orange on black. Gas plasma screens were used on early laptop computers. They are bright and easy to read, but they consume considerably more electricity than LCD displays. A plasma is a glowing, ionized gas.gate1. one of the three parts of a field-effect transistor. See FIELD-EFFECT TRANSISTOR.2. any of several logic circuits. See AND GATE; LOGIC CIRCUITS; NAND GATE; NOR GATE; NOT GATE; OR GATE.gateway a link between different computer networks.GB see GIGABYTE.GC see GARBAGE COLLECTION. 213	gigabyte gcc see CC.geek (slang) an enthusiastic computer specialist; a person with an intense interest in computers to the exclusion of other human activities. The term is not normally an insult. Compare NERD.gelatin silver print a conventional black-and-white photograph. (Black-and-white photographic paper uses a gelatin emulsion containing silver compounds that need not be replaced with colored dyes, though they can be.) Contrast CHROMOGENIC PRINT; GICLÉE PRINT.Genuine Advantage a Microsoft anti-piracy program requiring users to validate their software. A key for the copy of the software is matched with a key for the hardware in an online database to prevent that copy from being used on other computers. The system will let people know if they have unknowingly bought counterfeit software, but a validation failure due to a network outage disables genuine copies of the software.gibi- proposed metric prefix meaning 1,073,741,824 (230), the binary counterpart of giga-. See METRIC PREFIXES.gibibyte 1,073,741,824 bytes.giclée print a picture printed on an inkjet printer, usually on coated paper to resemble a photograph. (In French, giclée means “sprayed.”) ContrastCHROMOGENIC PRINT; GELATIN SILVER PRINT.GIF (Graphics Interchange Format) a file format developed by CompuServe for storing bitmap images on disk. (See BITMAP.) GIF images can have up to 65,536  65,536 pixels and 256 colors. GIF is a common format for images on the World Wide Web. Compare JPEG; TIFF.gif89a a GIF with the ability to play a short animation. They are very pop-ular on web pages because, unlike FLASH, gif89a’s don’t require a plug-in for the animation to play. Many programs can save a series of drawings to be an animated gif. One such is Adobe’s Image Ready.You can have too much of a good thing, however. Esthetically speak-ing, a web page with too many animated gifs is confusing and way too busy.giga- metric prefix meaning 1,000,000,000 (109) or, in rating computer memories and disks, 1,073,741,824 (= 10243). Giga- is derived from the Greek word for “giant.” See METRIC PREFIXES.Gigabit Ethernet see 1000BASE-T; ETHERNET.gigabyte (GB) approximately one billion bytes. With computer memories, one gigabyte is always 230 = 1,073,741,824 bytes = 1024 megabytes. With disk drives, a gigabyte is sometimes understood as 1000 megabytes. See also MEGABYTE. FREE SOFTWARE; EMACS.gigahertz	214gigahertz (GHz) one billion cycles per second, a unit of frequency equal to 1000 megahertz; a measure of the frequency of a radio signal or the clock speed of a computer. See CLOCK; HERTZ; MEGAHERTZ; MICROPROCES-SOR; RF; WIRELESS COMMUNICATION.GIMP (Gnu Image Manipulation Program) a freely distributed paint program for UNIX and Windows, distributed from www.gimp.org. See also GNU.Glide a service providing online software allowing users to access their data from any machine (web address: www.transmediacorp.com).glitch erroneous response that occurs inside a computer because signals that are supposed to be simultaneous actually arrive at slightly different times. Software errors are occasionally called glitches. See BUG.G.lite a popular, inexpensive type of ADSL telephone-line Internet con-nection that provides data rates up to 1.5 Mbps downstream and 0.5 Mbps upstream. See ADSL; DSL; and cross-references there. The name is a nickname for the standard’s official designation, G.992.2.Global Positioning System see GPS.global variable a variable that can be recognized anywhere in a program.Contrast LOCAL VARIABLE.glue logic additional, relatively simple logic circuits needed to connect one major part of a computer to another. For example, many microprocessors require some glue logic between the CPU and the memory. See LOGIC CIRCUITS.glyph any printable character; the printed appearance of a character.Gmail a free e-mail service provided by Google, with a large amount of space for messages and the ability to search through past messages.GNOME (GNU Network Object Model Environment; the g is usually pro-nounced, like “g’nome”) the most popular graphical desktop environ-ment for Linux, based on the X WINDOW SYSTEM and similar in look and feel to Windows 95 and 2000. GNOME is part of the GNU project. SeeGNU. Compare KDE.GNU a project led by Richard Stallman of the Free Software Foundation. GNU stands for “Gnu is Not UNIX”; the g is usually pronounced, like “g’noo.” The GNU project’s original goal was to develop a freely dis-tributed substitute for UNIX, but Linux (the leading free UNIX) evolved into a somewhat separate project. The most important GNU products are the Emacs editor and the GNU C Compiler.GNU software is copyrighted and is distributed free subject to certain conditions (typically, you must distribute it complete and intact, with source code). This set of conditions is sometimes called “copyleft” (i.e., a copyright with the opposite of its usual function) and also applies tosome other free software, such as Linux. See also 215	Google DesktopGO TO, GOTO a statement in Fortran, BASIC, and other programming languages that transfers execution to another place in the program. For example, here is a BASIC program that prints “Hello” 1000 times:10 LET X=020 PRINT ”HELLO”30 LET X=X+140 IF X<1000 THEN GO TO 20Use of the GO TO statement has been strongly discouraged since the 1970s, when E. W. Dijkstra and others discovered that it was very error-prone. If a program contains GO TO statements, it’s hard to tell, by look-ing at it, exactly how it will work, because execution can jump from any point to any other point. Instead, statements such as FOR, WHILE, and REPEAT are much easier for the programmer to get correct. See STRUC-TURED PROGRAMMING.gold, golden1.	(describing a disk or CD) ready to be duplicated and sold to the public.2.	(describing a software product) in its original manufactured state; thus “Windows 2000 Gold” is Windows 2000 as supplied on CD-ROM, without any subsequent updates downloaded.gooey pronunciation of GUI. See GRAPHICAL USER INTERFACE.Google a popular web search service that has expanded into many other areas (web address: www.google.com). Other search engines preceded Google in sending software SPIDERs (or webcrawlers) to automatically look at the text of a web page, and then follow all the links on each page to find other pages. In this way the search engine could create an index to the web, allowing people to enter words and then find a list of web pages including those words. However, that list might be very long, and it might contain irrelevant results. One reason Google became popular is because it devel-oped a system to rank pages, based on the number of other pages that link to them. Also, it attempts to determine the relevance of the text on a page to the user’s query, so that more relevant results will be placed near the top of the list. Another reason for Google’s success is its large investment in computer hardware so its searches operate with great speed.Google raises revenue largely through advertising, and it has expanded into other services, such as shopping, e-mail, mapping, and digital imaging. In 2008, it settled a lawsuit with publishers and authors over a plan to scan and index the text of a large number of books from major libraries. (Author’s Guild v. Google).The word Google is also now used as a verb. For example, “I’ll Google that” means “I’ll search for that on the Web.”Google Desktop a program provided by Google that allows the user to eas-ily search through files, and provides a sidebar where gadgets can be installed. See GADGETS. ILLEGAL OPERATION.PROTOCOL; WORLD WIDE WEB.Google Docs	216Google Docs a service provided by Google to easily share information that includes spreadsheets and presentations that can be edited by a group of people, such as co-workers.Google Earth an application provided by Google that allows the user to browse satellite images of the world by street address.Google whacking (slang, humorous) the sport of trying to find a word so obscure that it occurs on just one of the many web pages indexed by Google.Gopher a hypertext protocol that was used in the early 1990s, before HTTP. The name “gopher” is a pun on “go for” (i.e., go and get things).Seegotcha (slang)  a  pitfall;  a  feature  that  leads  to  mistakes.  CompareMISFEATURE..gov a suffix indicating that a web or e-mail address belongs to a govern-ment organization. Along with .com, .edu, .int, .net, .org, and .mil, this is one of the original set of Internet top-level domains. Contrast .COM. See also ICANN; TLD.GPF (General Protection Fault) an error that arises in Microsoft Windows when a program tries to access a location in memory that is not allocated to it. GPFs are usually caused by programming errors such as uninitial-ized pointers. Software that produces GPFs is defective or improperlyconfigured. See alsoGPS (Global Positioning System) a network of satellites allowing users with portable GPS devices to determine precise locations on the surface of the Earth. The portable GPS device measures the exact time taken for signals to reach it from at least four different satellites; from this, the instrument can compute its location.GPS was developed by the U.S. Department of Defense for military purposes, but it has plenty of civilian applications. Hikers and campers can use it to plot their position on maps. GPS-equipped automobiles can call up computerized maps showing their exact position. Police cars can transmit their exact position to the dispatcher automatically. Boats and ships can use GPS for practically effortless navigation.To keep GPS from being accurate enough for enemy military use, the Department of Defense originally introduced constantly varying errors that limited GPS’s accuracy to about 300 feet (100 m). This system of intentional errors was called Selective Availability. When Selective Availability was turned off in May 2000, the accuracy became approxi-mately 100 feet (30 m).GPU (Graphical Processing Unit) an integrated circuit that assists the CPU with graphics-intensive calculations.gradient fill a fill that is composed of two or more colors. In between the colors is a smooth blending. You are able to rotate the angle of the fill SPELL CHECKER.217	graphics(linear) or have the blend radiate from a central point (radial). See alsoFOUNTAIN FILL; UNIFORM FILL.FIGURE 120. Gradient fill tool (from Adobe PhotoShop)grammar checker a part of a word processing program that flags sentences that violate rules of grammar. For example, in Microsoft Word, if you type “They is happy,” the word “is” will be marked with a wavy green underline to indicate that it is not grammatical. However, grammar checkers cannot always be relied on to identify proper usage for your particular document.See alsograndfather tape a tape that is two generations older than the current data; for example, a backup tape from two days ago if backups are done daily.See also HARD DISK MANAGEMENT.graph1.	a graphical display of information, designed to make it easier for the reader to interpret and understand numerical data. See BAR GRAPH and PIE CHART for examples.2.	in mathematics, a set of points connected together in a specific way.See TOPOLOGY.graphical user interface (GUI) a way of communicating with the computer by manipulating icons (pictures) and windows with a mouse. Before GUIs became widespread, it was common for computers to operate in a mode where only text (no graphics) could be displayed on the screen.Xerox developed a machine with a graphical user interface in the 1970s, but the first widely used GUI machine was the Apple Macintosh in 1984. The release of Microsoft Windows 3.0 in 1990 led to wide-spread use of a graphical user interface on IBM PC-compatible comput-ers. For examples, see MACINTOSH; WINDOWS.graphics the use of computer output devices, such as screens, printers, and plotters, to produce pictures. The applications of computer graphics include publishing, education, entertainment, and the visualization of computed data (e.g., graphs of mathematical functions).There are two basic ways to tell a computer how to draw a picture. In vector graphics, the computer is told to put a real or imaginary pen in a particular position and then draw a line a certain distance in a certain direction (or draw a line to another specific point). The alternative is graphics card	218raster graphics or bitmap graphics, in which the screen or plotting area is divided into a rectangular array of points (called pixels, from “picture cells”), and the computer is told what color each point should be. Computer screens and printers are raster devices, but some graphics soft-ware uses vector-style instructions internally so that all lines will appear as sharp as possible on the device actually used to display them.The amount of fine detail that a particular graphics device can show is called the resolution. The resolution of a printer is usually given in dots per inch; the resolution of a screen is given as the size of the whole image. A good laser printer can print 1200 dots (pixels) per inch, so that an 8  10-inch picture is a 9600  12,000 array of pixels and contains far more detail than the screen can display.FIGURE 121. Graphical user interface (Windows)graphics card a video card that can display graphics as well as text. All modern video cards are graphics cards. See SVGA; VGA.graphics tablet an alternative to a mouse. A graphics tablet consists of a pressure-sensitive pad on which you draw with a special pen called a STYLUS. This is much more natural than attempting to draw with a mouse. See DRAW PROGRAM; PAINT PROGRAM.grayed displayed in gray type and not available for selection. If a menu option appears in light gray rather than black type, it cannot be chosen (see Figure 78 on page 140). Menu selections are grayed whenever it is impossible to do the thing they call for (e.g., “Save” may be grayed if there is no data to be saved). See DIMMED.graylist a spam filtering method in which message delivery is delayed for senders not on the WHITELIST or BLACKLIST. The message is delivered if it is re-sent (in the hope that spammers won’t re-send and legitimate senders will). 219	Green PCFIGURE 122. Graphics tablet (Photo courtesy of Wacom Technology)grayscale1.	a series of boxes filled with a range of black tints from pure white to 100% black. A grayscale is used to test a printer, monitor, scanner, or printing press.2.	(scanner terminology) the range of grays in an image as measured by the scanner.3.	a description of any image that contains shades of gray as well as black and white.Greek the alphabet used in ancient and modern Greece, Α Β Γ ... Ω and α β ’γ δ ... ω. Greek letters are often used as mathematical symbols. For the complete Greek alphabet, see page 6. Contrast CYRILLIC; LATIN.greeking the use of random letters or marks to show the overall appearance of a printed page without showing the actual text. With computers, greeking is used when the page is displayed too small for the text to be readable on the screen.FIGURE 123. GreekingGreen Book the Philips/Sony standard for multimedia interactive compact discs (not including personal computer software).Green PC a personal computer that draws little electrical power when idle, even though still turned on. A Green PC typically stops spinning its hard CLUSTER COMPUTING.ENERGY STAR.Gregorian calendar	220disk and shuts down power to the monitor if several minutes elapse withno keyboard activity. See alsoGregorian calendar the calendar system presently in use, introduced by Pope Gregory in 1582 and adopted in England in 1752 and in Russia in 1918. It is exactly like the Julian calendar except that years divisible by 100 are not leap years unless they are also divisible by 400. The Gregorian calendar thus follows the earth’s revolution around the sun more accurately than the Julian calendar did. See LEAP YEAR.grep the UNIX command that reads a text file and outputs all the lines that contain a particular series of characters. For example, the commandgrep abc myfilereads the file myfile and outputs every line that contains “abc.”Instead of specifying the exact characters to be searched for, you can give a regular expression that defines them. For example,grep [bB]ill myfileoutputs all the lines that contain either “bill” or “Bill.” See REGULAR EXPRESSION.The origin of the word grep is disputed, but it may be an abbreviated editing command, g/re/p, where re stands for “regular expression,” g means “global search,” and p means “print” (i.e., display all lines that match the search criteria). Grep programs have been written for other operating systems, such as Windows. Compare FIND.grid a feature of various draw programs and paint programs that allows lines to be drawn only in certain positions, as if they were drawn on the lines of graph paper. The grid makes it much easier to draw parallel and perpendicular lines, lay out diagrams, and avoid irregular breaks. However, when the grid is turned on, there are positions in which you cannot draw. See also DRAW PROGRAM; PAINT PROGRAM.grid computing the process of solving computationally complicated prob-lems by distributing parts of the problem to unused capacity on a widely dispersed set of machines that are connected to the Internet.For examples, see SETI@HOME; TERAGRID; and www.grid.org. For con-trast, seegrid system a way of standardizing the layout of many related pages, such as the pages of a multipage document. The designer first draws a grid that will define the possible positions of columns, horizontal divisions, and pictures. Not all the possibilities of the grid are used on any single page, but the grid ensures that column positions do not vary haphazardly, and thereby makes the pages look related.griefer (slang) a person who plays a multiplayer game or participates in other online group activities for the purpose of making other people mis-erable. Griefers do not play to win; they do not defeat their opponents ELECTRONIC CIR-221	guidelinefairly. They play to lose, and they dish out insults, misinformation, and harassment in the process.Grokster a file sharing service found liable for inducing its users to violate copyright law, in the case Metro-Goldwyn-Mayer Studios et al. v. Grokster (545 U.S. 913 (2005)), decided by the Supreme Court in 2005.See also NAPSTER.gross megapixels the total number of megapixels on an image sensor, whether or not all of them are actually used in taking pictures. ContrastEFFECTIVE MEGAPIXELS.grounding the establishment of a uniform reference voltage level across several pieces of electrical equipment that are connected together.In any electrical device, “ground level” is the voltage level to which all other voltages are compared. In most computers, the ground level is connected to the ground pin (the third, rounded pin) of the power plug, and the power line then connects it to the earth itself, thereby assuring that the ground level for all machines is the same. This helps prevent cables from picking up noise or emitting radio-frequency interference. Italso reduces the danger of damage from lightning. SeeCUIT DIAGRAM SYMBOLS; POWER LINE PROTECTION; RFI PROTECTION.groupware software that makes it easy for a group of people to work on the same data through a network, by facilitating file sharing and other forms of communication. Lotus Notes is an example of widely used groupware.GTG chat-room abbreviation for “[I’ve] got to go.”gTLD (generic Top Level Domain) an identifier such as .com or .edu at the end of a U.S. web address; see www.iana.org/domains/root/db/# for the complete list of possibilities. See also CCTLD; TLD.GUI see GRAPHICAL USER INTERFACE.GUID (Globally Unique Identifier) a 128-bit number used by Microsoft WINDOWS to identify a user, software component, or other entity.GUIDs are most often written as groups of hexadecimal digits in braces, such as:{79376820-07D0-11CF-A24D-0020AFD79767}Windows includes an algorithm to generate GUIDs based on an encrypted version of the user’s MAC ADDRESS, which in turn is guaranteed to be unique. (See MAC ADDRESS.) Thus, anyone running Windows who has an Ethernet adapter can create GUIDs that are known to be unique in the entire world. Computers without an Ethernet adapter can generate GUIDs that are likely to be unique but not guaranteed to be.guideline a nonprinting line that aids in aligning text and other objects in a draw program or page layout program. Some programs allow you to turn guiltware	222on a snap-to-guidelines feature that causes the guidelines and objects to have a magnetic attraction for each other. See also SNAP-TO-GRID.guiltware persistent NAGWARE; software that repeatedly asks for a mone-tary contribution and tries to make you feel guilty if you don’t pay.Contrast CAREWARE; FREEWARE.gunk (slang) any undesirable thing that degrades the performance of a computer, such as physical dust, obsolete software, or spyware. Gunk includes well-intentioned utilities that waste CPU time constantly mon-itoring the status of a modem, network card, or disk drive, as well asVIRUSES, ADWARE, and other MALWARE.gutter the blank space between columns of type or between pages of a book. 223	hafniumHh4x LEETSPEAK for “hacks.” See HACK, especially definition 4.hack1.	to modify, especially in an improvised way: “This version of the pro-gram has been hacked to run under UNIX instead of Windows.”2.	to program a computer, either tediously or enthusiastically: “We spent the whole night hacking.”3.	to break into a computer system or otherwise do mischief: “We’ve been hacked.” See also ETHICAL HACKING.4.	a clever programming technique: “This hack enables a console-mode program to change the title bar of its window.”When someone in an online game is accused of “hacks,” it means that he or she is suspected of using software bugs or a third-party program to achieve results that the game designers did not intend.hack attack (slang) a sudden inspiration or compulsion to work on a com-puter program. Despite what it sounds like, a “hack attack” has nothing to do with computer security violations.hacker1.	an exceptionally skilled computer programmer.2.	a person who programs computers for recreation or as a hobby.3.	a person who “breaks into” computers without authorization, either for malicious reasons or just to prove it can be done; a CRACKER. See2600; COMPUTER SECURITY.hacker ethic the value system of computer enthusiasts who believe in help-ing each other advance technology by sharing knowledge without imme-diate concern for making money. See HACKER (definition 2). The hacker ethic has led to valuable cooperative projects such as GNU, LINUX, TEX,USENET, and the INTERNET.Usage note: The term hacker ethic is sometimes misappropriated by malicious individuals who believe they are somehow exempt from ordi-nary rules of ethics (see HACKER, definition 3). In its proper sense, the hacker ethic is an extension of ordinary ethics, not an exemption or loop-hole.Hacker Safe see SCANALTERT.hackish (slang) pertaining to the culture of HACKERs (definitions 1 and 2 and sometimes 3). For instance, using binary numbers on a birthday cake is a hackish thing to do.hafnium chemical element (atomic number 72) used as an insulator in inte-grated circuit transistors. hairline	224hairline a very thin line, usually about .003 inch wide.FIGURE 124. Hairline and other line widthsHAL1.	in Windows NT and its derivatives, the Hardware Abstraction Layer, the component of the operating system responsible for low-level inter-action with the CPU and closely related hardware.2.	the fictional computer in the movie 2001: A Space Odyssey (1968). Replace each letter of HAL with the next letter in the alphabet to see an amusing coincidence.half adder a logic circuit that adds two one-digit binary numbers, produc-ing two digits of output. See BINARY ADDITION.half duplex communication in two directions, but not at the same time. For instance, a two-way radio with a push-to-talk switch is a half duplex device; you cannot hear the other person while you are talking. ContrastFULL DUPLEX.halftone the reproduction of a continuous-tone image (containing shades of gray or colors) by converting it into a pattern of very small dots of vari-ous sizes. (For an example, look closely at a picture in a newspaper or magazine.) Laser printers and printing presses can print shades of gray only as halftones. See also GRAYSCALE; PHOTOGRAPH.FIGURE 125. Halftone image (enlarged)halting problem the problem of determining whether a particular computer program will terminate or will continue forever in an endless loop; a famous theoretical result in computer science.Consider a computer program A that analyzes other programs; call the analyzed program B. Suppose A can always determine, with complete SCROLL BAR225	hard-coded certainty, whether B will ever terminate. You could arrange for A to repeat endlessly if it finds that B terminates, and terminate if it finds that B repeats endlessly.You could then feed A a copy of itself (i.e., let B = A), and you’d have a program that terminates if and only if it does not terminate. This is a contradiction, proving that A, as described, cannot exist.hand tool a tool available in some graphical environments that looks like a human hand and allows you to move the picture around the screen. Ifthere is a hand tool provided, there will usually not be a	atthe side and bottom of the viewing window.handle1.	a nickname used in online communication.2.	in Windows systems programming, a POINTER to a window or other system resource.3.	(in programs with a graphical user interface) the little black boxes at the corners and midpoints of an object that has been selected for editing. As the name suggests, handles give you a place to “grab” onto an object with the mouse and manipulate it. Dragging a corner handle (any one of the four) will change the size of the object. Dragging a midpoint handle will stretch or shrink the object in one dimension.FIGURE 126. Handles (definition 3) on selected objecthandshaking the exchange of signals between two computers to indicate that data transmission is proceeding successfully.hang1.	to make a computer stop in its tracks because of a software bug or hardware failure.2.	(on a modem) to disconnect from the telephone line (hang up).hanging indent, hanging tab a new paragraph indicated by letting the first word extend to the left past the normal margin into the gutter. Also called OUTDENT. Each entry in this dictionary begins with a hanging indent.Happy Mac (pre-OS X only) the icon of a smiling Macintosh that greets you when you turn on your Macintosh and everything is well. ContrastSAD MAC.hard-coded written into a computer program; not easily alterable. For instance, the location of video memory was hard-coded into the BIOS of the original IBM PC. hard copy	226hard copy a printout on paper of computer output. Contrast SOFT COPY, which is a copy that is only viewable on the screen.hard disk a data storage medium using rigid aluminum disks coated with iron oxide. The read-write head travels across the disk on a thin cushion of air without ever actually touching the disk.Hard disks have much greater storage capacity than diskettes. In the early 1980s, 10 megabytes was a common size for hard disks. Today’s disks are measured in hundreds of gigabytes.See also DISK; DISKETTE; HARD DISK MANAGEMENT. On the interface between the hard disk and the computer, see ESATA; ESDI; IDE; PATA; SATA;SCSI.hard disk management a task faced by all computer users. Modern hard disks are so large that it is easy to lose track of what you have stored on them. Here are some tips:1.	Be systematic. Choose a place to put files whenever there isn’t a good reason to put them elsewhere. In Windows, the Documents folder serves this purpose.2.	Use meaningful filenames. For example, a paper about African violets should be named AfricanViolets rather than av9247 or MyPaper. Add “-old,” “-previous,” or the like when you need tokeep more than one version of the same file.3.	Group files by project, not by software. For example, if you are writing a magazine article that consists of a word processing doc-ument and three pictures, create a folder for the project and put all four files in it.4.	Make backups. Your disk drive will fail one day, probably when you least expect it.5.	Learn how to search for misplaced files. In Windows, go to the Start Menu and choose Search or Find.6.	Defragment your hard disk every few weeks or months.hard drive a HARD DISK.hard drive enclosure a box in which a hard disk drive can be mounted instead of mounting it in the computer’s case.hard edge in an image, an edge that is smooth and sharp, with no blending or blurring of the boundary. See Figure 127. Contrast SOFT EDGE. FIGURE 127. Hard edge, left; Soft edge on right VON NEUMANN ARCHI-227	Hayes compatibilityhard error a persistent, reproducible error (defect) on a data storage device. Contrast SOFT ERROR.hard hyphen see REQUIRED HYPHEN.hard page a forced page break; a place where the word processor must begin a new page whether or not the previous page was full. In many PC word processors, the way to type a hard page is to hold down the Ctrl key while pressing Enter.hardware the physical elements of a computer system; the computer equipment as opposed to the programs or information stored in the machine. Contrast SOFTWARE.hardware interrupt a CPU interrupt triggered by a hardware event, such as pressing a key. See INTERRUPT.hardware key a device that attaches to a computer to prove that it is licensed to run a particular piece of software; a dongle. See DONGLE.Harvard architecture a type of computer design in which the program andthe data are stored in separate memories. ContrastTECTURE. See COMPUTER ARCHITECTURE.hash function a function that converts a string of characters to a number or a shorter string. During data transmission, the value of an agreed-upon hash function can be transmitted along with the data so it can be verified if the data has been transmitted correctly. For example, see CHECKSUM. A one-way hash function is a hash function that is also a one-way function.See ENCRYPTION; ONE-WAY FUNCTION.hashing a storage mechanism where data items are stored at locations that are determined by a mathematical function of the data. For example, suppose you need to store a set of numbers in memory locations whose addresses run from 1 to 100. One example of a hashing function is to divide each number by 100 and use the remainder as the storage address. For example, the number 538 would be stored at memory location 38, and 1124 would be stored at location 24.The use of hashing makes it possible to store and retrieve the data items quickly, since it is not necessary to search through the list in order to find the item. However, there is one complication: A hashing function will sometimes assign more than one data item to the same address. For example, using the rule given, the number 638 would also be stored in location 38. To avoid that problem, a hashing system needs to be able to resolve collisions by storing the new data item in a separate place.hat1.	the character ˆ (circumflex).2.	See BLACK HAT; RED HAT; WHITE HAT.Hayes compatibility the ability of a telephone line modem to respond to the same set of autodialing commands as the Hayes Smartmodem (made HD DVD	228by Hayes Microcomputer Products, Inc., Norcross, Georgia). Almost all modems nowadays are Hayes compatible.Modems need not be Hayes compatible in order to communicate with each other. Hayes compatibility refers only to the commands used by the computer to control the modem. They are often called the AT COMMAND SET and begin with the letters AT. For example,ATDT706-555-2345tells the modem to dial 706-555-2345. See also MODEM; RS-232.HD DVD (high-density DVD) a high-density optical disc similar to, but not compatible with, BLU-RAY DISC, formerly marketed by Toshiba but dis-continued in 2008.HDD hard disk drive.HDMI (High-Definition Multimedia Interface) a standard interface and cable connector for carrying digitized audio and video. The connector is small and flat, like a USB connector, but is even smaller and contains 19 pins. Compare DVI (definition 1). For more information see www.hdmi.org.HDSL (High-bit-rate Digital Subscriber Line) a type of high-speed net-work connection provided through ordinary telephone lines. Unlike ordinary DSL (ADSL), HDSL is symmetric (i.e., the upload and down-load speeds are both 1.5 Mbps), but ordinary telephone service is not included. See DSL and cross-references there.HDTV high-definition television; television with a resolution of 1280  720 pixels or more and, normally, a widescreen format with an aspect ratio of 16:9. Although analog HDTV was attempted in the 1990s, HDTV is now broadcast digitally because digital data compression is essential to its success. Contrast SDTV.head1.	the part of a disk drive that reads and writes information magneti-cally. A double-sided disk drive or multilayered hard disk has a head for each side of each layer. (See DISK.)2.	the top of a page or printed piece (such as a newsletter).3.	short for headline.4.	tag used in HTML to indicate the beginning of the heading of a web page, which includes material such as the title. Contrast BODY. For an example, see HTML.headless term describing a computer that lacks a keyboard, screen, and a mouse.headset speakers (or rather earphones) and (usually) a microphone worn on a person’s head.heap a block of memory that belongs to a program but has not yet been 229	hertzgiven a specific use. For example, when a C# program creates character strings, it typically places them in space obtained from its heap.heat sink a device to carry heat away from an electronic component. The heat sink for a CPU is typically a chunk of metal with fins or fingers, accompanied by a cooling fan. A vital part of the system is a thin layer of thermally conductive paste that conducts heat from the CPU to the metal heat sink. If this paste deteriorates or is missing, the CPU will run hot no matter how good the heat sink. See also OVERCLOCKING.hecta- metric prefix meaning 100 (102). Hecta- is derived from the Greek word for “hundred.” See METRIC PREFIXES.Heisenbug (humorous) a bug (error) in a computer program that goes away or radically changes its behavior when attempts are made to investigate it.This is a common phenomenon. If the bug involves an UNINITIALIZED VARIABLE, the contents of the variable will be affected by other programs that have run recently. Thus, any attempt to investigate the bug will change its behavior. (From Heisenberg’s uncertainty principle in physics, which states that it is impossible to determine the exact position and the exact momentum of an object at the same time; any attempt to measure one will change the other.)See also BUG.Hejlsberg, Anders see C#; DELPHI.hello-world program a program that simply writes “Hello, world” on the most convenient output device, and terminates. Hello-world programs are the traditional way to verify that programming languages and output devices are working, at least to a minimal degree.help information provided by a computer program to assist the user. Many computer programs contain an on-screen help facility that a user can turn to when questions arise. For example, if you have forgotten how a par-ticular command works, you can consult a help facility (if one is avail-able) to refresh your memory.In Windows programs, one of the menu choices is usually Help. Windows has an elaborate help system accessible from the Start button.See also CONTEXT-SENSITIVE HELP; DOCUMENTATION.help desk, helpdesk a place where people who use computers can go for assistance; it may be a single desk or a whole department of a large orga-nization.Helvetica a popular sans-serif typeface designed around 1957 by M. Miedinger.hertz the number of times something is repeated per second; a unit of fre-quency, abbreviated Hz and named for Heinrich Hertz, discoverer of radio waves. See also CLOCK; GIGAHERTZ; MEGAHERTZ. heuristic	230FIGURE 128. Helvetica (normal weight)heuristic a method of solving problems that involves intelligent trial and error. By contrast, an algorithmic solution method is a clearly specified procedure that is guaranteed to give the correct answer. (See ALGO-RITHM.) For example, there is no known algorithm that tells how to play a perfect game of chess, so computer chess-playing programs must use a heuristic method of solution, using methods that are likely but not cer-tain to give good results in any particular case.Hewlett-Packard (HP) a leading manufacturer of computers and printers. Hewlett-Packard is headquartered in Palo Alto, California, and can be reached on the web at www.hp.com. The electronic test equipment divi-sion of HP is now a separate company known as Agilent.TABLE 7HEXADECIMAL EQUIVALENTS OF BINARY NUMBERSBinary	Hex		Binary	Hex											0000	0		1000	8	0001	1		1001	9	0010	2		1010	A	0011	3		1011	B	0100	4		1100	C	0101	5		1101	D	0110	6		1110	E	0111	7		1111	F						hexadecimal number a number written in base 16. Hexadecimal numbers use 16 possible digits, written 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, A (= 10), B (=11), C (=12), D (= 13), E (= 14), and F (= 15). For an example, the number A4C2 in hexadecimal means:10  163 + 4  162 + 12  161 +2  160 = 42,178Hexadecimal numbers provide a good shorthand way of representing binary numbers, since binary numbers can be converted to hexadecimal 231	hexadecimal number numbers by looking at only four digits at a time. For example, binary 1111 equals hexadecimal F, binary 0100 equals hexadecimal 4, and binary 11110100 equals hexadecimal F4:11110100__ F 4When converting a binary number to hexadecimal by this method, start by adding zeros at the left in order to make the number of digits a mul-tiple of 4.Table 7 shows the hexadecimal equivalents of the four-digit binary numbers, and Table 8 shows the decimal equivalents of the hexadecimal numbers from 00 to FF.TABLE 8HEXADECIMAL NUMBERS AND DECIMAL EQUIVALENTS (Part 1)00	=	0	20	=	32	40	=	64	60	=	9601	=	1	21	=	33	41	=	65	61	=	9702	=	2	22	=	34	42	=	66	62	=	9803	=	3	23	=	35	43	=	67	63	=	9904	=	4	24	=	36	44	=	68	64	=	10005	=	5	25	=	37	45	=	69	65	=	10106	=	6	26	=	38	46	=	70	66	=	10207	=	7	27	=	39	47	=	71	67	=	10308	=	8	28	=	40	48	=	72	68	=	10409	=	9	29	=	41	49	=	73	69	=	1050A	=	10	2A	=	42	4A	=	74	6A	=	1060B	=	11	2B	=	43	4B	=	75	6B	=	1070C	=	12	2C	=	44	4C	=	76	6C	=	1080D	=	13	2D	=	45	4D	=	77	6D	=	1090E	=	14	2E	=	46	4E	=	78	6E	=	1100F	=	15	2F	=	47	4F	=	79	6F	=	11110	=	16	30	=	48	50	=	80	70	=	11211	=	17	31	=	49	51	=	81	71	=	11312	=	18	32	=	50	52	=	82	72	=	11413	=	19	33	=	51	53	=	83	73	=	11514	=	20	34	=	52	54	=	84	74	=	11615	=	21	35	=	53	55	=	85	75	=	11716	=	22	36	=	54	56	=	86	76	=	11817	=	23	37	=	55	57	=	87	77	=	11918	=	24	38	=	56	58	=	88	78	=	12019	=	25	39	=	57	59	=	89	79	=	1211A	=	26	3A	=	58	5A	=	90	7A	=	1221B	=	27	3B	=	59	5B	=	91	7B	=	1231C	=	28	3C	=	60	5C	=	92	7C	=	1241D	=	29	3D	=	61	5D	=	93	7D	=	1251E	=	30	3E	=	62	5E	=	94	7E	=	1261F	=	31	3F	=	63	5F	=	95	7F	=	127											 HFS	232TABLE 8HEXADECIMAL NUMBERS AND DECIMAL EQUIVALENTS (Part 2)80	=	128	A0	=	160	C0	=	192	E0	=	22481	=	129	A1	=	161	C1	=	193	E1	=	22582	=	130	A2	=	162	C2	=	194	E2	=	22683	=	131	A3	=	163	C3	=	195	E3	=	22784	=	132	A4	=	164	C4	=	196	E4	=	22885	=	133	A5	=	165	C5	=	197	E5	=	22986	=	134	A6	=	166	C6	=	198	E6	=	23087	=	135	A7	=	167	C7	=	199	E7	=	23188	=	136	A8	=	168	C8	=	200	E8	=	23289	=	137	A9	=	169	C9	=	201	E9	=	2338A	=	138	AA	=	170	CA	=	202	EA	=	2348B	=	139	AB	=	171	CB	=	203	EB	=	2358C	=	140	AC	=	172	CC	=	204	EC	=	2368D	=	141	AD	=	173	CD	=	205	ED	=	2378E	=	142	AE	=	174	CE	=	206	EE	=	2388F	=	143	AF	=	175	CF	=	207	EF	=	23990	=	144	B0	=	176	D0	=	208	F0	=	24091	=	145	B1	=	177	D1	=	209	F1	=	24192	=	146	B2	=	178	D2	=	210	F2	=	24293	=	147	B3	=	179	D3	=	211	F3	=	24394	=	148	B4	=	180	D4	=	212	F4	=	24495	=	149	B5	=	181	D5	=	213	F5	=	24596	=	150	B6	=	182	D6	=	214	F6	=	24697	=	151	B7	=	183	D7	=	215	F7	=	24798	=	152	B8	=	184	D8	=	216	F8	=	24899	=	153	B9	=	185	D9	=	217	F9	=	2499A	=	154	BA	=	186	DA	=	218	FA	=	2509B	=	155	BB	=	187	DB	=	219	FB	=	2519C	=	156	BC	=	188	DC	=	220	FC	=	2529D	=	157	BD	=	189	DD	=	221	FD	=	2539E	=	158	BE	=	190	DE	=	222	FE	=	2549F	=	159	BF	=	191	DF	=	223	FF	=	255											HFS see HIERARCHICAL FILE SYSTEM (definition 2).hibernate to suspend the operation of a computer by copying the entire con-tents of memory to a disk file, so that the computer can be powered off, then turned on again to resume where it left off, without rebooting. While hibernating, a computer consumes no electric power. Contrast SUSPEND.hibernation file the data file on which the contents of memory are written when a computer hibernates. See HIBERNATE.hidden file a file whose presence is normally concealed from the user to keep it from being deleted or moved. Hidden files contain information used by the operating system. Under Windows, hidden files in a folder can be viewed by making the appropriate choice under “Tools, Folder Options” on the menu bar. 233	High Sierra formathierarchical arranged in such a way that some items are above or below others in a tree-like structure. Examples of hierarchies include the orga-nizational chart of a corporation, the arrangement of directories on a disk, and the arrangement of windows on a screen (because some of the windows are within others).The root of the hierarchy is the main item that is above all of the oth-ers, such as the CEO of a corporation, the root directory of a disk, or the window that comprises the whole screen.Many menu systems are arranged hierarchically, as menus within menus. For other examples of hierarchical data structures, see DIREC-TORY; OUTLINE; TREE.FIGURE 129. Hierarchical file systemhierarchical file system1.	a file system that allows subdirectories or folders to belong to a higher-level subdirectory or folder (Figure 129). See DIRECTORY.2.	(capitalized, Hierarchical File System, abbreviated HFS) the file sys-tem of the Macintosh. Besides disk drives, HFS is sometimes used on CDs, rendering them unreadable on computers other than the Macintosh.Compare JOLIET FILE SYSTEM; ROCK RIDGE.hierarchical menu a menu with other menus under it; a CASCADING MENU. For an example, see START BUTTON.high-level language a computer programming language designed to allow people to write programs without having to understand the inner work-ings of the computer. BASIC, C, Pascal, and Java are examples of high-level languages. By contrast, a machine language is at the lowest level, since machine language programming requires detailed knowledge of the computer’s inner workings. An assembly language is at a slightly higher level than a machine language, since it uses a notation more con-venient for the programmer.High Sierra format a standard format for recording files and directories on CD-ROMs, now superseded by ISO 9660. The two formats are closely highlight	234compatible, but some of the earliest CD-ROM software could read only High Sierra format disks.highlight1.	to make a menu item prominent (either lighter or darker than the oth-ers) to show that it is selected. In most graphical user interfaces, you can choose the highlighted item by pressing Enter.2.	the lightest part of an image. In artwork, highlights show texture, shape, and the direction of the source of light.HighMAT (High-performance Media Access Technology) a set of stan-dards co-developed by Microsoft and Matsushita (Panasonic) for CDs and DVDs that are created on personal computers but played back on consumer electronic devices such as CD players and DVD-equipped television sets. HighMAT provides a standard way for the user to orga-nize the files and select them for playback.hinting additional information encoded into a digital font to help the com-puter software correctly display and print the letters at different sizes and resolutions.HIPAA compliant meeting the standards set by the Health Insurance Portability and Accountability Act of 1996 for electronic data inter-change. See aspe.hhs.gov/admnsimp.hiragana see KANA.histogram a bar graph in which the bars represent how many times some-thing occurs (Figure 130). Histograms are often used in paint programs and scanner software to allow direct manipulation of the image charac-teristics. Each bar represents the total number of pixels of a particular shade of gray. By sliding the endpoints closer together, you decrease the image contrast; sliding the endpoints apart increases contrast. See alsoBRIGHTNESS; CONTRAST.FIGURE 130. Histogramhistory folder a folder that contains a list of the locations you have visited on the Internet.hit1. something found by a searching. For instance, if you search the Web for instances of the word “aardvark” and find 250 of them, you’ve found 250 hits. FOAF; PYRAMID SCHEME.235	hole2. on the World Wide Web, an instance of someone elsewhere calling up the web page and viewing it. The popularity of a web site is measured in hits per day.hive a major section of the Windows REGISTRY that is automatically backed up on an external file. Examples of hives include HKEY_CURRENT_USER,HKEY_CURRENT_CONFIG, and HKEY_LOCAL_MACHINE\System. Not all of the top-level branches of the registry are hives.hoax a piece of misinformation circulated as a deliberate prank; common on the Internet. (Compare MEME virus.) Hoaxes usually arrive in e-mail mes-sages that say “mail this to all your friends”—that is, the hoaxer does not want you to post the message in a public forum where knowledgeable people might debunk it. Some common Internet hoaxes are the following:1.	“A dying child (or maybe a charity) wants a gigantic number of postcards or a gigantic amount of e-mail.” Nobody wants a gigan-tic amount of e-mail; e-mail costs money to receive.One young cancer victim, Craig Shergold, did appeal for post-cards in 1989; his story is told in the Guinness Book of World Records, and his family is begging for the flood of postcards to stop. Unfortunately, his story is still circulating, often with altered names and addresses.2.	“Some branch of government, such as the FCC, is about to do something outlandish.”Sometimes these warnings come from well-meaning activists; more often they are pranks. In the 1970s a disgruntled license applicant started a rumor that the FCC was about to ban all reli-gious broadcasting; the story is still circulating and the FCC can’t afford to answer the flood of correspondence that it has generated. Newer hoaxes include a “modem tax” or bans on various uses of the Internet.3.	“If you get e-mail titled ‘Good times’ (or ‘Happy birthday’ or ‘Deeyenda’ or something else), it will erase your hard disk (or do other great harm).”Any file attached to e-mail could easily contain a virus or destructive program; do not open such files unless you are sure of their origin. However, the viruses described in these particular hoaxes apparently do not exist.Any piece of e-mail that is designed to spur you to immediate action is likely to be a hoax; before passing it on, you should check it out with your system administrator, your local computer security team, or another knowledgeable person. Better yet, do a web search to see what you can find out about it. The web site www.snopes.com specializes in debunk-ing hoaxes and revealing the real facts. See also Compare URBAN LEGEND.hole a place where an electron is missing from the crystal structure of a P-type semiconductor. A hole acts as a moving positive charge. See SEMI-CONDUCTOR. /home	236/home in UNIX, a directory that contains individual users’ home directories.home directory the main directory belonging to a particular user of a UNIX system or of a file server shared by multiple users.home page1.	the main WEB PAGE for a person or organization; the page that users are expected to read first in order to access other pages.2.	the WEB PAGE that a person sees first, immediately after starting up the BROWSER. Most browsers let you choose what web page this will be.See also HTML; WORLD WIDE WEB.honeypot a trap for people who tamper with computers maliciously through the Internet, just as a pot of honey traps flies.A honeypot is generally a computer that is rigged to look more vul-nerable than it really is, and to keep records of everything that happens to it. Honeypots serve several purposes: to catch individual crackers, to determine whether they can get into a network, and to observe how they carry out their attacks. See COMPUTER SECURITY; CRACKER.hook a provision, in a computer program, for interaction with other pro-grams that have yet to be written. For example, Adobe Photoshop and many web browsers provide hooks for plug-ins that add additional fea-tures. See PLUG-IN.Hopper, Grace (1906–1992), mathematician and U.S. naval officer (later admiral) who worked on the first electronic computers. She developed the first compiler and contributed to the development of COBOL. For many years, she was the highest-ranking woman in the U.S. Navy.horizontal side to side; across.host computer a computer that provides services to others that are linked to it by a network; generally, the more remote of two or more computers that a person is using at once. For example, when a user in Florida accesses a computer in New York, the New York computer is considered the host.hot list a list of bookmarks or favorites; a stored list of web addresses, file-names, or other data of immediate interest to the user. See BOOKMARK;FAVORITES.hot-pluggable able to be plugged in and unplugged while a computer is powered on and running.hot spot1.	(sometimes hotspot) a place where wireless access to the Internet is offered, such as for customers bringing their laptops into a coffee shop.2.	a place in a hypertext document where a user can click to call up fur-ther information. Hot spots are generally highlighted words or small pic-tures. Some large graphics can have multiple hot spots. See ANCHOR; 237	HTMLHYPERLINK; IMAGE MAP.3. the exact spot of a pointer (or any mouse cursor) that must touch an object in order to select it. The very tip of the arrow is the hot spot for the pointer.hot-swappable able to be replaced (swapped out) while a computer is pow-ered on and running.hot zone the area at the end of a line of type that triggers the computer to hyphenate words. If a word extends into the hot zone, it will be hyphen-ated to make it fit on the line. See HYPHENATION.hotfix a PATCH or rapidly distributed update for a piece of software. The term is used particularly with Microsoft Windows.hourglass (in Microsoft Windows XP and earlier) the shape of the mouse pointer while the computer is too busy to accept any input from the key-board or mouse. The pointer returns to its usual shape when the wait is over.FIGURE 131. Hourglasshover to leave the mouse cursor at a particular location more than momen-tarily, without clicking it. See ROLLOVER (definition 2).HR keyword used in HTML to indicate a horizontal rule. For an example, see HTML.HSB (hue, saturation, brightness) a way of describing colors by means of numbers in some computer programs. HSB descriptions are especially convenient for artists who are accustomed to mixing paint. The first number, hue, describes the color itself (red, green, blue, yellow, etc., along a continuum).The saturation is the vividness of the color, from maximum (extremely vivid) through paler colors all the way down to gray or white. For example, going from red to pale red to gray is a change of saturation.The brightness is the amount of light emitted from patches of the color on the screen, from maximum brightness down to black.HSDPA (High-Speed Downlink Packet Access) a system for wireless broadband. Information available at www.3gpp.org. Contrast EVDO.HTML (Hypertext Markup Language) a set of codes that can be inserted into text files to indicate special typefaces, inserted images, and links to other hypertext documents.The main use of HTML is to publish information on the Internet (see WORLD WIDE WEB). Here is a simple example of an HTML document. HTML	238 FIGURE 132. HTML example as displayed by browser<HTML><HEAD><TITLE>The University of Georgia</TITLE> </HEAD><BODY><P><IMG SRC=”ugaseal.gif”> <H1>University of Georgia</H1> <P>The University of Georgia is located in Athens, Georgia, 75 miles east of Atlanta. It was founded in 1785 and is the oldest state university.</BODY></HTML>Figure 132 shows how this looks when displayed by a web browser. HTML features are indicated by special codes, called tags. If there were an HTML tag called XXX, then the characters <XXX> would mark the beginning of this feature, and </XXX> would mark the end. For example, the keywords <TITLE> and </TITLE> mark the beginning and end of the title. <P> marks a paragraph break, and <IMG SRC=filename> embeds an image in the document; many image formats are supported but GIF is the most popular. Codes for special typefaces include the following: HTML; HYPERTEXT;239		HTTP<H1> ... </H1>	Heading, size 1 (largest)<H6> ... </H6>	Heading, size 6 (smallest)<B>	... </B>	Boldface<I>	... </I>	Italics<U>	... </U>	Underline<T>	... </T>	Typewriter type (like this)The tag <BR> inserts a line break; note that the line breaks on the dis-played web page will not necessarily match the line breaks on the origi-nal HTML text. The tag <HR> inserts a horizontal rule.An unnumbered list of items can be inserted as follows:<UL><LI> put first item in list here </LI> <LI> put second item here </LI><LI> put third item here </LI> </UL>An ordered (numbered) list is created in the same manner, except with <OL> used in place of <UL>.A link to another document looks like this:<A HREF=”XXXX.HTML”> Click here.</A>That means “Jump to file XXXX.HTML (another HTML document) if the user clicks on the words ‘Click here.’” A URL can appear in place of the filename. A link to another place in the same document looks like this:<A HREF=”#XXXX”>This is the text that will display the link</A>When the user clicks on this link, the browser will jump to the location in the current document marked with<A NAME=”XXXX”> This is the target of the link</A>Comments (to be ignored by the HTML system) look like this:<! This is a comment>Even with no special codes in it, a text file is still a valid HTML document.Although you can use almost any word processor or page layout pro-gram to produce HTML, it is much easier to use a program specially designed for the job (Adobe PageMill, Corel Xara, Microsoft FrontPage, and other “web publishers”). Many of the newer programs provide a WYSIWYG environment for designing web pages and then automati-cally produce the correct HTML codes.For other examples of HTML, see DYNAMIC HTML; FORM; FRAME; JAVA;JAVASCRIPT; REDIRECT; TABLE. See also CGI; WEB PAGE DESIGN.HTTP (Hypertext Transfer Protocol) a standard method of publishing information as hypertext in HTML format on the Internet. URLs(addresses) for web sites usually begin with http:. See INTERNET; URL; WORLD WIDE WEB. HTTPS	240HTTPS a variation of HTTP that uses SSL encryption for security.hub a device for joining multiple Ethernet cables by copying all the data packets onto all the cables. Hubs are only suitable for use with very lightly loaded networks. Compare SWITCH (definition 2); ROUTER.hue color (red vs. green vs. orange, etc.). See HSB.hunt and peck (slang) to type by gazing at the keyboard, hunting for the letters, and pressing them one by one with one finger, rather like a trained chicken.Hyper-Threading the ability of some Pentium microprocessors to follow two sequences of instructions at once. The central core of the micro-processor switches back and forth between the two threads, and some of the circuitry is duplicated in order to keep track of two tasks at once.Contrast DUAL-CORE PROCESSOR.hyperdocument see HYPERTEXT.hyperlink an item on a WEB PAGE which, when selected, transfers the user directly to another location in a hypertext document or to some other web page, perhaps on a different machine. Also simply called a LINK. For an example, see HTML.hypertext (hyperdocuments) electronic documents that present informa-tion that can be read by following many different connections, instead of just sequentially like reading a book. The World Wide Web is an exam-ple of hypertext, as are Microsoft Windows help files and CD-ROM encyclopedias.A hypertext document typically starts with a computer screen full of information (text, graphics, and/or sound). The user can then jump instantly to many other screens of information by clicking on words or pictures with a mouse or touchscreen.Encyclopedia information is especially suitable for hypertext presen-tation. Each entry can be a screen of information, and each cross-refer-ence can be a button that the user can click on in order to jump to that entry. Software help files are also a good application for hypertext because the user generally needs to find a particular piece of information as quickly as possible rather than reading through the whole document.There is a danger that the user might become lost in the middle of a hyperdocument. A good hyperdocument should include some form of navigational aid that allows the user to see an overview of the document. Also, it is helpful if the computer maintains a record of the path that has been followed, both so the user can go backward and so it is possible to retrace the same path at a future date if so desired. Often a hyperdocu-ment follows a particular sequence automatically if the user does not want to make all of the choices individually.A large hyperdocument (e.g., an encyclopedia) requires large amounts of storage such as provided by a CD-ROM. The World Wide Web is a DISCRETIONARY HYPHEN; REQUIRED HYPHEN.WORLD WIDE241	Hzway of publishing hypertext on the Internet, using many different com-puters as servers for different parts of the information. SeeWEB; HTML; BROWSER.hyphenation the practice of breaking words between syllables at the end of a line so that the lines will be more nearly the same length.Most desktop publishing software can automatically hyphenate text. The computer tries to put as many words as possible on one line. When it enters the last half inch of the line (the HOT ZONE), it calculates whether the next word will fit; if not, the word is looked up in the hyphenation dictionary, or hyphenated according to phonetic rules. The basic idea is that both parts of the word should be pronounceable. There is often more than one acceptable way to hyphenate a word, and dictionaries some-times disagree with each other. The most basic rule is, “Make both pieces pronounceable.”Fully justified type will always look better when hyphenated; other-wise loose lines (lines with big gaps between the words) become a prob-lem. If you are setting type flush left, ragged right, be aware that the size of the hot zone will affect how ragged the right margin is.Proofread carefully for unfortunate line breaks. The traditional exam-ple is the word “therapist”—don’t let the computer hyphenate it as “the-rapist.” Learn how to mark required hyphens so that hyphenated names and phone numbers won’t be broken across lines.See alsoHz see HERTZ. I-bar, I-beam	242II-bar, I-beam the shape of the mouse pointer in a text editing environment.See INSERTION POINT.FIGURE 133. I-bar (I-beam) cursor.I-triple-E see IEEE.I2 see INTERNET 2.IA-32 (Intel Architecture-32) the architecture of the Intel 80386, 80486, and Pentium microprocessors, used on the majority of the computers in existence today. Contrast IA-64.IA-64 (Intel Architecture-64) the architecture of the 64-bit Intel micro-processors, such as the ITANIUM, that are intended to be the successor to the Pentium family (IA-32). IA-64 microprocessors can switch into IA-32 mode for compatibility with older software.A feature of IA-64 is Explicitly Parallel Instruction Computing (EPIC), the ability to execute several instructions simultaneously when they have been grouped in parallel by the compiler. Compare HYPER-THREADING, which does not actually execute more than one instruction at once. See ITANIUM; PARALLEL PROCESSING. Contrast X64.IANA (Internet Assigned Numbers Authority) an organization based at the University of Southern California with the responsibility to make sure all IP addresses are unique. Their web address is www.iana.org. See IP ADDRESS; TLD.IBM (International Business Machines) manufacturer of computers and other office equipment. The company was formed in 1911 by the merger of three companies that made record-keeping equipment for businesses, one of which was the PUNCHED CARD company founded by Herman Hollerith.IBM started manufacturing mainframe computers in the 1950s. By the late 1960s, IBM controlled about 80 percent of the computer market with models such as the IBM System/360, and the name IBM was prac-tically synonymous with computing. Today, the company continues to make mainframe computers.In 1981 IBM introduced the IBM Personal Computer (PC), which quickly became one of the most popular microcomputers. Users felt that with IBM behind it, the personal computer had come of age as a practi- 243	IBM PCcal business machine, not just an experimental machine for laboratories or hobbyists. The IBM PC was designed in some haste, and very little of its design was patented; as a result, other companies (beginning with Compaq in 1983) were able to produce clones (compatible imitations) of it. The IBM PC became the most popular standard for microcomputers, even though most of the computers were produced by other companies, and Intel microprocessors and Microsoft operating system software became the defining elements of the standard. Today’s “PC” computers are distant descendents of the original IBM PC. In 2005 IBM sold its PC business to Lenovo. IBM is headquartered in Armonk, New York (web address: www.ibm.com). See also IBM PC.IBM PC popular lines of microcomputers manufactured by IBM. There are many variations of each; this article will mention only the most histori-cally important.The IBM Personal Computer (PC), introduced in 1981, was the first of a family of very popular microcomputers, including not only IBM products but also “clones” (imitations) made by other companies. The original IBM PC used very little proprietary technology. Thus, it was easy for competitors to build compatible machines without violating patents. See CLONE; PC COMPATIBILITY.IBM maintained a high level of upward compatibility within the PC and PS/2 line. This means that later-model machines would run virtually all software written for earlier models.FIGURE 134. IBM PC (1981)IBM’s two original machines, the PC and PC XT, are virtually iden-tical, featuring 4.77-MHz 8088 microprocessors with an 8-bit bus. The only difference was that the XT had a 10-megabyte hard disk and had eight expansion slots instead of five. The PC AT, introduced in 1984, was the first PC to use the 80286 microprocessor, enabling programs to run much faster. The PC AT had what is now known as the ISA (Industry Standard Architecture) bus; it accepted both 8-bit (XT-style) and 16-bit plug-in cards.The PS/2 machines were introduced in 1987 and discontinued in 1995. They were more compact than comparably configured PCs or ATs, and all but the lowest models used the Micro Channel bus, which made it possi-ble in some situations to use more than one CPU in a single machine. IBM RS/6000	244These computers use the ASCII character set (see ASCII). In addition, they define printed representations for all character codes from 0 to 255.Figure 135 shows the printable part of the special character set. At a Windows COMMAND PROMPT, these characters are typed by holding down the Alt key and typing the appropriate number on the numeric keypad at the right side of the keyboard. For example, to type a shaded block, hold down Alt, type 178, and then release Alt.(Note the arrangement: these are not in numerical order. 218 is the upper left corner box-drawing character; 192 is the lower left corner, and so on.)FIGURE 135. IBM PC special characters (also used in Windows console mode)IBM RS/6000 see WORKSTATION.IC1. abbreviation for “in character,” used in role playing games and the like to indicate a return to the imaginary situation after an out of charac-ter (OOC) conversation. When IC, all replies are from the user’s imagi- 245	IDE nary alter ego. Example: “Enough talk about iguanas. IC: This dragon is dangerous.” Contrast OOC; see example there.2. See INTEGRATED CIRCUIT.ICANN (Internet Corporation of Assigned Names and Numbers) a non-profit corporation established in 1998 to oversee the assignment of Internet domain names and addresses. See also IANA; NETWORK SOLU-TIONS, INC.; TLD; UDRP. Web address: www.icann.org.icon a small picture on a computer screen that represents a particular object, operation, or group of files. Icons are used extensively in graph-ical user interfaces such as Microsoft Windows and the Macintosh. SeeGRAPHICAL USER INTERFACE; MOUSE; WINDOW; WINDOWS (MICROSOFT).FIGURE 136. Icons representing system components, files, folders, programs, and Internet connectionsiconify to turn a window into an icon (same as MINIMIZE).ICQ a widely used Internet chat program distributed free of charge from www.icq.com, which also gives an index of users and topics. The name is short for “I seek you.” Compare AIM; IRC.ICRA (Internet Content Rating Association) an organization that promotes voluntary labeling of the content of web pages so that filter software can prevent access to objectionable sites. See www.icra.org. Webmasters who have rated their sites usually display an ICRA link or graphic.IDE1.	(integrated development environment) a software package for edit-ing and compiling programs and, often, designing the program’s win-dows graphically. Popular examples are Delphi, Visual Basic, and Visual Studio. See ECLIPSE.2.	(Integrated Device Electronics) a type of hard disk that has most of the controller circuitry built into it, to save space. See also ATA; ATAPI;SATA. Contrast SCSI. identifier	246identifier a symbolic name used in a program and defined by the program-mer. Most identifiers stand for variables (see VARIABLE); however, some languages allow the use of identifiers to represent constants, so that the value of a particular constant, wherever it occurs in the program, can be changed by changing the statement that defines the identifier. SeeCONSTANT.identity in Outlook Express or similar mail-reading programs, a setting that determines which of several individuals is using the computer. This enables several people to share a computer and keep their e-mail sepa-rate even though the operating system is not set up for multiple users.identity theft the crime of impersonation (i.e., pretending to be someone else), using forged documents of various kinds. Crucially, identity theft goes beyond the theft of a single credit card number or the forgery of a single document. It is an attempt to assume a person’s entire identity, including name and credit rating, in order to create new accounts under the victim’s name.Identity theft often involves electronic communications media by tak-ing advantage of the fact that online and mail-order merchants are often not very thorough in checking credit card users. See also COMPUTER ETHICS; EVIL TWIN; PHISHING.ideogram, ideograph a written symbol that represents an idea rather than the pronunciation of a word. Symbols such as &, $, numerals, and com-puter icons are ideograms.IDL (Interface Definition Language) see CORBA.IE abbreviation for INTERNET EXPLORER.IEC (International Electrotechnical Commission) an organization that sets numerous standards for the electronics industry. Web address: www.iec.ch.IEC power connector the type of connector commonly used to attach a PC’s power cord to the computer, using three prongs oriented in the same direction.FIGURE 137. IEC power connectorIEEE (Institute of Electrical and Electronics Engineers) the leading professional society for electrical and computer engineers in the United States. It publishes journals, holds conferences, and publishes many standards applicable to computer equipment. The IEEE is headquartered 247 ifin Piscataway, New Jersey, and can be reached on the Web at www.ieee.org.IEEE 1284 an IEEE standard for PC parallel ports, compatible with the designs used previously, but including provision for high-performance bidirectional communication with tape drives, disk drives, and other devices, not just printers. It is fully compatible with earlier EPP and ECP standards.The IEEE 1284 standard defines three connectors: the traditional 25-pin socket on the PC (IEEE 1284A), the 36-pin Centronics connector on the printer (IEEE 1284B), and a new miniature connector (IEEE 1284C).Parallel ports are IEEE 1284 compliant if they implement the whole IEEE 1284 standard; they are IEEE 1284 compatible if they implement one or more of the older standards of which IEEE 1284 is a superset.See CENTRONICS INTERFACE; PARALLEL PORT.IEEE 1394, 1394a, 1394b see FIREWIRE.IEEE 802.11 see 802.11.IETF (Internet Engineering Task Force) an international group of network professionals who work on advances in Internet architecture and work to ensure smooth operation of the Internet. The IETF is organized into sev-eral working groups on specific topics. Their web address is www.ietf.org.if a keyword in many programming languages that specifies that different actions are to be performed depending on the result of some test. Here is a simple example of an if statement in Java:if (price == 0){System.out.println(”Price can’t be equal to zero!”);}Here is a more practical example:if (hours<=40){pay=hours*wage;System.out.println(”No overtime hours”);}else{overtime = hours-40;pay= wage*40 + 1.5*wage*overtime; System.out.println(”Overtime paid”);}In C, C++, Java, and C#, a very common error is to write if (x=5) when you mean if (x==5) or the like. Here x=5 is a command to assign the value 5 to the variable x. Some compilers will object when you embed ignore list 248the command in an if statement, but in C, doing so is legal (and always comes out true if the value assigned is nonzero).ignore list a list of users you have blocked in a chat or instant messaging program. When another user is on your ignore list, your software will ignore messages from them. Synonyms: BLACKLIST; KILL FILE.IIOP (Internet Inter-ORB Protocol) a protocol that extends TCP/IP by adding CORBA defined messages for objects to connect to each other over the network. See CORBA; PROTOCOL.IIRC online abbreviation for “if I remember correctly.”IIS (Internet Information Services) the component of Microsoft Windows (professional and server editions only) that enables the computer to work as a web, FTP, and e-mail server.i.LINK see FIREWIRE.illegal operation an operation that a program is not permitted to perform, such as writing on a read-only disk or using memory allocated to some other program. Illegal operations are almost invariably the result of errors in programming.IM Instant Message or Messenger; see AIM; LIVE MESSENGER.iMac a line of Apple Macintosh computers with a distinctive streamlined design introduced in 1998. The current iMac computer is built entirely within its flat-screen monitor. See also APPLE; MACINTOSH.image1. a picture, represented in a computer’s memory. See GRAPHICS.2. a copy, on a disk, of the contents of a computer’s memory. A BOOT DISK may contain an image in this sense, i.e., a large block of data that can be copied directly into memory.image map a web page graphic to which multiple links have been assigned. It is possible to click on different parts of the picture and activate differ-ent links. For example, you might create a map of your community that appears on the web page; if the user clicks on the part of the map repre-senting one neighborhood, then the browser will jump to a link provid-ing information about that neighborhood. This gives the links a visual meaning that would not be there in a list of text-only links. Image maps can be created with some web-creation software programs, or they can be defined directly by hand-coding the proper HTML.image processing the use of a computer to modify pictures (usually bitmap images). Applications of image processing include computer vision, enhancement of photographs, and creation of works of art. Many image processing functions are built into PAINT PROGRAMs as FILTERs (definition 249 incremental compiler1). For examples, see ADD NOISE; BLEND; BLUR; BRIGHTNESS; COLOR/GRAY MAP; EDGE DETECT; EQUALIZE; HISTOGRAM; MOTION BLUR; PIXELATE, PIX-ELIZE; REMOVE SPOTS; SHARPEN; UNSHARP MASKING. See also COLOR; COM-PUTER; CONVOLUTION; DEBLURRING; RETOUCHING; VISION.imagesetter a high-quality output device. Imagesetters can deliver up to 2400 DPI (dots per inch) instead of the 300 to 1200 DPI of ordinary laser printers.IMAP (Internet Mail Access Protocol) a protocol for viewing e-mail on a personal computer while leaving it in place on the host system. Contrast POP, which delivers the mail and deletes it from the server. See PROTOCOL.IMG tag used in HTML to indicate an image file. For example, see HTML.IMHO online abbreviation for “in my humble opinion.”IMO online abbreviation for “in my opinion.”impact printer a printer that prints on paper by impacting a cloth ribbon coated with ink, thus transferring ink to the paper. Dot-matrix printers are the most common type of impact printer. Today, impact printers are used mainly for business forms that involve carbon paper or carbonless copy paper. See DOT-MATRIX PRINTER; LINE PRINTER; PICKUP ROLLER.impedance a measure of how easily an alternating current can pass through an electrical circuit. The impedance of a resistor is the same as its resis-tance. Capacitors and inductors also affect impedance, depending on the frequency of the current. The impedance of a capacitor decreases as the frequency increases; the impedance of an inductor increases as the fre-quency increases. The characteristic impedance of coaxial cable results from the interaction of its inductance and capacitance. It is not a resis-tance and cannot be measured with an ohmmeter. See COAXIAL CABLE.import to load a file from a format other than the application program’s native format. Many word processing and graphics programs have the ability to import text and graphics from several different file formats. Because importing is a type of file conversion, formatting and image detail may be lost in the process. See CONVERSION PROGRAM.in the loop (jargon) involved in decision-making or control, like an auto-matic control system that uses feedback. See LOOP (definition 2).incremental backup a backup operation that only copies files that have changed since the last backup. See BACKUP COPY.incremental compiler a compiler that compiles the lines of a program as they are typed into the computer, rather than compiling the whole pro-gram at once. The purpose is to keep the programmer from having to wait a long time for the complete program to be compiled when it is fin-ished. See COMPILER. ARTIFICIAL INTELLI-indecency 250indecency material that offends ordinary people but does not meet the legal criterion of obscenity. In the United States, indecent material is protected by the First Amendment (freedom of speech) and is legal in printed publications, cable TV, and the Internet, but not on radio and TV broadcasts that use publicly available channels. See also ICRA; OBSCEN-ITY; PORNOGRAPHY.indent to leave a space at the beginning of the first line of a paragraph. Indented margins are also used to set off long quotations. See also HANG-ING INDENT.index1. an alphabetical listing of important words and concepts found in a book and the pages on which these terms may be found. Many page lay-out and word processing programs have the ability to automatically gen-erate indexes from properly tagged terms (see DESKTOP PUBLISHING; TAG). Note that the process is not wholly automatic; the human still has to specify which words should be indexed.2. a pictograph of a pointing hand. Sometimes called a fist.3. the number that picks out an element of an array. For example, in A[2], 2 is the index. See ARRAY.FIGURE 138. Index (definition 2)indexed file a file in which the order of the items is recorded in a separate file called the index. For example, if the computer is looking for John Smith’s billing records, it first looks up “Smith, John” in the index, and then the index tells it where to look in the billing record file.index.htm, index.html on many web servers, the file name that is used for a WEB PAGE when no file name is specified in the URL. Thus, index.html is usually the main page of a web site (hence its name). CompareDEFAULT.ASP.Industry Standard Architecture see ISA.INF file a type of file, with extension .inf, that tells the Windows Setup program how to install a particular piece of hardware or software.inference engine the part of an EXPERT SYSTEM that draws conclusionsby reasoning logically from information. See alsoGENCE; PROLOG.inferior character a subscript; small letters and numbers set on or below the baseline like this. Used mainly in mathematical typesetting. See SUB-SCRIPT. Contrast SUPERIOR CHARACTER; SUPERSCRIPT. 251 INI file.info a suffix for a web or e-mail address. Unlike .biz, etc., .info domain names can be assigned to any individual or organization, in any country.Contrast .BIZ, .COM. See also ICANN; TLD.Infobahn German for INFORMATION SUPERHIGHWAY.informatics the study of information and computation; a European name for computer science. See COMPUTER SCIENCE.information extraction the act of extracting, by computer, recognizable information from documents written in a human language; an example would be reading weather reports in English and constructing a table of dates and temperatures. Compare INFORMATION RETRIEVAL.information hiding see STRUCTURED PROGRAMMING.information retrieval the act of identifying, by computer, documents writ-ten in a human language that are relevant to some specific question. This goes beyond simply searching for words and phrases and often includes statistical analysis of vocabulary (to determine subject matter) and a considerable amount of NATURAL LANGUAGE PROCESSING.Information Superhighway a network of electronic and digital communi-cation equipment that is quietly revolutionizing businesses and our pri-vate lives (see INTERNET).Not only is information being disseminated faster than ever, but worldwide electronic communication is creating a unique community— one that does not have a physical location, but rather exists in what is called CYBERSPACE. Some portions of cyberspace are rather lawless right now; an ill-considered message may bring down an avalanche of angry messages (called FLAMEs) upon your e-mail account. However, some standards of ethics and personal behavior are being worked out. SeeCOMPUTER ETHICS.The term information superhighway is also used to refer to a system of fiber-optic cables that may one day link every home to the Internet, replacing the current television and telephone cables.infrared a kind of radiation similar to visible light but at a slightly longer wavelength, used to transmit data through the air in TV remote controls, wireless mice, wireless keyboards, and the like, and occasionally for short-distance computer-to-computer links. Like light, infrared signals require a clear line of sight and cannot go through walls.Infrared radiation is abundant in ordinary sunlight and, at the levels used for communication, is not hazardous.inheritance the process by which one object is defined to be just like another except for some specified differences. The second type of object then “inherits” the properties of the first. See OBJECT-ORIENTED PROGRAMMING.INI file a file (with the extension .ini) that stores initialization information for Microsoft Windows or a specific piece of software. Windows INI files consist of editable text. initialize 252initialize1. to store a value in a variable for the first time. If a program tries to use a variable that has not been initialized, it will get a random value, and the results will be unpredictable.2. to prepare a tape for use, erasing any data that may already be on it.inkjet printer a printer that forms characters by shooting tiny droplets of ink at the paper. Advantages include speed, high resolution, and quiet operation. An ink jet printer is often an economical alternative to a laser printer; in black and white, the image quality is comparable, but printing is slower. Many color inkjet printers can print photo-quality pictures on special glossy paper; with ordinary paper, the picture quality is lower.Contrast LASER PRINTER. See also GICLÉE; SEPARATOR PAD.inlining the conversion of a FUNCTION or PROCEDURE into machine instruc-tions that perform the computation directly, without calling the function or procedure. In effect, the statement that calls the procedure is replaced by a copy of the procedure itself. This speeds up execution but requires slightly more memory. See COMPILER; FUNCTION; PROCEDURE.Inprise Corporation see BORLAND INTERNATIONAL.input information that is given to a computer; the act of giving information to a computer. (Note that the terms input and output are always used from the computer’s point of view.) The input data may be either num-bers or character strings (e.g., a list of names). The computer receives input through an input device, such as a keyboard, or from a storage device, such as a disk drive.insertion point the place, in a full-screen editor or drawing program, where characters will appear if you start typing. The insertion point, which is different from the mouse pointer, looks like a thin vertical bar or, in some contexts, a tall, thin letter “I”; it is relocated by clicking at the desired location. See CARET (definition 3); I-BAR.insertion sort an algorithm for placing the elements of an array in ascend-ing or descending order. This method is efficient if the list is already close to being in order.To perform an insertion sort, examine every item in the list except the first. Whenever you find an item that should come before the item that immediately precedes it, pick up the current item, shift its neighbor on the left one space to the right, and see whether you can put the current item in the space thus vacated. If not, shift that item to the right and try again.Consider for example the following list:2404 8653 1354 5781The steps of the insertion sort are the following:1. 2404 and 8653 are in the right order, so proceed to the next pair.2. 8653 and 1354 are in the wrong order, so pick up 1354: 2404 8653 .... 5781 253 insertion sort Shift 8653 one space to the right:2404 .... 8653 5781Can 1354 be put into the empty space? No; its neighbor on the left would be 2404, a larger number. So shift 2404 one space to the right:.... 2404 8653 5781Now you can put 1354 into the empty space:1354 2404 8653 57813. Now examine 8653 and 5781. They are in the wrong order, so pick up 8653 and shift it one space to the right, then put 5781 into the empty space:1354 2404 8653 ....1354 2404 .... 86531354 2404 5781 8653Now all the elements are in order, and the process is complete. Figure 139 shows a program that performs an insertion sort. See also SHELL SORT.class insertionsort{/* Java example of insertion sort *//* Array to be sorted, and number of items in it: */ static int a[] = {29,18,7,56,64,33,128,70,78,81,12,5}; static int num=12;public static void main(String args[]){/* Perform the insertion sort */ for (int i=1; i<=num-1; i++){int value = a[i]; int position = i;while ( (position > 0) && (a[position-1] > value) ){a[position] = a[position-1]; position--;}a[position]=value;}/* Display the results */ for (int i=0; i<=num-1; i++){System.out.println(a[i]);}}}FIGURE 139. Insertion sort instance variables 254instance variables the variables (fields) that contain data unique to each object in OBJECT-ORIENTED PROGRAMMING.instant messaging the sending of brief text messages instantly to other users of a network through software such as AOL Instant Messenger, Microsoft MSN messaging, ICQ, or (originally) the talk command in UNIX. Besides computers, instant messages can be sent to and from pagers and cellular telephones.instantiated created or initialized. Once a new instance of a particular object type is created, it is said to be instantiated. In Prolog, a variable is said to be instantiated when it is given a value. See INSTANCE VARIABLES;OBJECT-ORIENTED PROGRAMMING; PROLOG..int a suffix indicating that a web or e-mail address belongs to an interna-tional organization such as the United Nations. Along with .com, .edu,.gov, .net, .org, and .mil, this is one of the original set of Internet top-level domains. Contrast .COM. See also ICANN; TLD.int data type representing an integer in Java and other languages.Occupying 32 bits of memory, a Java variable of type int can hold val-ues from –231 = –2, 147, 483, 648 to 231 – 1 = 2, 147, 483, 647.integers whole numbers and negated whole numbers, such as 1, 2, 3, 0,–10, –26, 157, 567, and –2397. An integer does not contain a fractional part. Thus, 3.4 and 22⁄3 are not integers.integrated circuit (IC) an electronic device consisting of many miniature transistors and other circuit elements on a single silicon chip. The first integrated circuits were developed in the late 1950s, and since then there has been continued improvement. The number of components that can be placed on a single chip has been steadily rising. See Figure 79, page 141.The advantages of integrated circuits over discrete components include the facts that they are very small (most are less than 14 inch [6mm] square), their internal connections are more reliable, they consume much less power, they generate much less heat, and they cost less than similar circuits made with separate components.Integrated circuits are classified by their level of complexity. Small-scale integration describes circuits containing fewer than 10 logic gates; medium-scale integration, circuits containing 10 to 100 gates; and large-scale integration, circuits with more than 100 gates.An integrated circuit is made by adding impurities to a silicon crystal in specific places to create P-type and N-type regions, and adding metal conductive paths to serve as wires. (See SEMICONDUCTOR.) The whole process uses light-sensitive chemicals whose action is controlled by a tiny image projected through a device that is like a microscope working backward; thus, tiny regions on the IC can be created from a much larger picture of the desired layout. 255 interlaced GIFIntegrated circuits are mass produced by making many identical cir-cuits at the same time from a single wafer of silicon. Each circuit must be individually tested, because a single defect in the crystal can com-pletely ruin the circuit.The ultimate integrated circuit is the microprocessor, which is a single chip that contains the complete arithmetic and logic unit of a computer, and sometimes other parts of the computer as well. See MICROPROCESSOR.Intel the manufacturer of the microprocessors used in PC-compatible com-puters, although other companies also make compatible equivalents. Intel products include the 8088, 80286, 386, 486, Pentium and its descendents, and Celeron. See MICROPROCESSOR. Earlier, Intel developed the first microprocessors (4004, 8008) and the microprocessor for which CP/M was developed (the 8080, soon superseded by Zilog’s Z80). Intel Corporation is headquartered in Santa Clara, California. Its web address is www.intel.com.interactive system a computer system in which the user communicates with the computer through a keyboard and screen. The computer pre-sents the results almost immediately after an instruction has been entered, and the user can type in new instructions after seeing the results of the previous ones. Nowadays, almost all computing is interactive.Contrast BATCH PROCESSING.intercaps capitalized letters in the middle of a word. The word PostScript contains an example. See also CAMEL NOTATION; PASCAL NOTATION. The companies that have coined such trademarks contend that the unusual capitalization is part of the correct spelling.Not everybody accepts these unusual spellings. At one time there was a company that claimed the only correct spelling of its name was “en•vos.” Almost everyone printed it as Envos. The moral of the story is that names need not reproduce trademarks and logos. In fact, a company can be at a real disadvantage if it makes its name too hard to print—jour-nalists may simply ignore it.intercharacter spacing the spacing in between characters within words. Also called LETTERSPACING. See KERNING; TRACKING.interface the connection between two systems through which information is exchanged. For example, in computer hardware, an interface is an electrical connection of the proper type. In software, it is a standard for-mat for exchanging data. The USER INTERFACE of a piece of software is the way it interacts with the human being who is using it. See also DATA COMMUNICATION; USER INTERFACE.interlaced GIF a BITMAP file that has been optimized for downloading to a remote site. An interlaced GIF displays faster because it can be displayed in rough form before all the information has been received. The picture initially appears in coarse blocks, which refine themselves into finer detail as the complete file is downloaded. See GIF. Internet 256Internet a cooperative message-forwarding system linking computer net-works all over the world. Users of the Internet can view information on the World Wide Web, exchange electronic mail, participate in electronic discussion forums (newsgroups), send files from any computer to any other via FTP, or HTTP, and even use each other’s computers directly if they have appropriate passwords. See ELECTRONIC MAIL; FINGER; FTP;HTTP; NEWSGROUP; RLOGIN; TELNET; TLD; URL; WORLD WIDE WEB.Every machine on the Internet has an address. For example, the addressbeetle.ai.uga.edumeans:beetle machine (“beetle”)ai subnetwork (Artificial Intelligence Lab) uga site (University of Georgia)edu type of site (U.S. educational)Here beetle.ai.uga.edu is a domain address that gets translated into a numeric IP address, such as 128.192.12.9, by the network itself. See IP ADDRESS; TCP/IP.The cost of running the Internet is paid largely by the sites that receive messages, and the sites that pass them along, not by the sites that send messages out. This has important legal and ethical implications.Unsolicited advertising via e-mail or in newsgroups is almost always unwelcome, as is any self-serving misuse of electronic communications, because the sender of the material is not paying the cost of distributing it. For further ethical guidelines see COMPUTER ETHICS; SPAM; USENET.The Internet grew out of the ARPAnet (a U.S. Defense Department experimental network) as well as BITNET, Usenet, and other wide area networks. See USENET; WIDE-AREA NETWORK. Contrast INTRANET.Usage note: Many people confuse the Internet with the World Wide Web, which is only one of several forms of communication that take place on the Internet.Internet 2 a consortium of universities working with business and govern-ment to create a high-performance successor to the original INTERNET. For more information see www.internet2.org.Internet cafe a small business selling Internet and computer use by the minute or hour. Most Internet cafes also offer other services including everything from food to live musical performances.Internet casino see GAMBLING.Internet Content Rating Association see ICRA.Internet Explorer the World Wide Web BROWSER included in Microsoft Windows; a derivative of MOSAIC. Compare FIREFOX; OPERA. 257 interpreterInternet gambling see GAMBLING.Internet radio the transmission of sound from a radio station, or similar real-time audio programs, to computer users over the Internet. This makes hearing distant and specialized radio stations possible. However, it can clog up networks because a separate copy of each data packet has to be sent to each computer. (Real radios all pick up the same signal at the same time.) The BBC World Service can be heard by Internet radio at www.bbc.co.uk. See also IPTV.Internet service provider (ISP) (access provider) a company that provides its customers with access to the INTERNET, typically through DSL, a CABLE MODEM, or DIAL-UP NETWORKING. Major service providers in the United States include Microsoft, Comcast, Earthlink, America Online, and var-ious phone companies. Typically, the customer pays a monthly fee, and the Internet service provider supplies software that enables the customer to connect to the Internet. Some ISPs also provide file space for pages on the WORLD WIDE WEB and FTP file storage.Internet telephony the making of telephone calls by digitizing the sound and transmitting it through the Internet. This is often a much cheaper alternative to conventional long-distance or international calling, but there can be delays or loss of quality when the requisite parts of the Internet are congested with heavy traffic.Originally, Internet telephony was only possible when the people on both ends were using computers with soundcards, microphones, and spe-cial software. More recently, Internet telephony gateways have been established that allow calling from and to ordinary telephones. See alsoVOIP, SKYPE.interoperability the ability of machines or programs to work together. Two computers are interoperable if they can be used together in some useful way, working on the same files or sharing data through a network.interpolation1. in mathematics, the process of estimating an unknown value of a function in between two known values. For example, if it takes 18 min-utes to cook a 1-inch-thick steak and 40 minutes to cook a 2-inch steak, you can interpolate and find that it should take about 30 minutes to cook a 11⁄2-inch steak.2. in computer graphics, the process of smoothing the pixels in an image that has been enlarged by filling in intermediate colors or shades of gray, thus reducing the stairstep appearance that would otherwise result from enlarging a small bitmap; also known as resampling. See RESAMPLE. interpreter a program that executes a source program by reading it one line at a time and doing the specified operations immediately. Most Perl and Python systems are interpreters. Contrast COMPILER. interrupt 258FIGURE 140. Interpolation (resampling) to enlarge an imageinterrupt an instruction that tells a microprocessor to put aside what it is doing and call a specified routine. The processor resumes its original work when the interrupt service routine finishes. Interrupts are used for two main purposes:1. to deal with hardware events such as a key being pressed or a char-acter arriving through a serial port. These events cannot be ignored; the incoming data must be either processed immediately or stored in a buffer.2. to call subroutines that are provided by the hardware or operating system. On the PC, most DOS and BIOS services are called through interrupts rather than through the ordinary instruction for calling a subroutine. Windows services, however, are called as ordinary subroutines.These correspond to the two main ways of causing an interrupt: by receiving a signal from outside the microprocessor (a hardware interrupt) or by executing a machine instruction (a software interrupt).interrupt service routine see INTERRUPT.intersection the set of elements that are in both of two other sets. For exam-ple, the intersection of {a, b, c} with {c, b, r} is {b, c}.interword spacing the spacing between words, sometimes calledWORDSPACING.intranet the opposite of INTERNET; a network confined to a single organi-zation (but not necessarily a single site). Intranets often include web pages, so a web browser can be used to view the content. This makes the intranet appear just the same as part of the World Wide Web; the only difference is that it is not accessible to those outside the organization. Keeping it separate from the outside world is essential if it carries con-fidential data, such as internal business records. Contrast EXTRANET.Intuit a leading producer of personal financial software (the program Quicken). Web address: www.intuit.com.intuitive obvious without conscious thought. The operation of a piece of software is said to be intuitive if the operation fits the task so well that 259 IPthe user can guess how to perform common operations without consult-ing manuals or pausing to figure things out. Compare USER-FRIENDLY.invalid media disks or tapes that cannot be used because of physical defects or because they have been partly erased by a magnetic field. In the latter case the media can be formatted (erased) and used again. See also ABORT; FAIL; RETRY.invert1. to turn an image into a photographic negative of itself, substituting black for white and white for black, and changing colors to their com-plements. Compare REVERSE.2. (less commonly) to turn an image upside down. For other senses, seeINVERTER.FIGURE 141. Inverted photographinverter1. a NOT gate. See NOT GATE.2. a device that converts direct current to alternating current for power supply purposes (e.g., to power a computer from a car battery).invisible watermark a code secretly hidden in a picture to carry copyright information or other secret messages. An invisible watermark consists of a very slight change of contrast over large areas of the picture, invisible to the human eye, even fainter than the watermark on a piece of paper. Suitable software can recover the invisible watermark even if the image has been printed out, photographed, and scanned in again. SeeSTEGANOGRAPHY.Iomega manufacturer of Zip drives and other portable storage devices. Their corporate web site is at www.iomega.com.IOW online abbreviation for “in other words.”IP1. Internet Protocol. See IP ADDRESS; IPV4, IPV6; TCP/IP.2. intellectual property (i.e., patents, copyrights, and trademarks, espe-cially patented or copyrighted designs for components of equipment or software). DYNAMIC IPIP address, IP number 260IP address, IP number (Internet Protocol address) the numeric address of a machine, in the format used on the Internet (IPv4 or IPv6). For exam-ple, the IPv4 address of one of the University of Georgia’s computers is 128.192.76.80. Convert each of the four numbers into binary, and you get the true 32-bit binary address, which can also be written as an 8-digit hexadecimal number.Three blocks of IPv4 addresses are reserved for private networks and will never be officially assigned. Therefore, if you must make up an unofficial IP address, you should choose it from one of these blocks. They are 10.0.0.0–10.255.255.255, 172.16.0.0–172.16.255.255, and 192.168.0.0–192.168.255.255. The address 127.0.0.1 on any machine connects it back to itself.IPv6, a newer version of the protocol, uses 128-bit instead of 32-bit addresses, so that a much larger number of addresses is available. The packet format is different in a number of ways that make routing moreefficient.Contrast DOMAIN ADDRESS; MAC ADDRESS. See alsoADDRESS; INTERNET; STATIC IP ADDRESS.IP spoofing see SPOOFING.IP telephony see INTERNET TELEPHONY.iPhone popular portable phone, web browser, and music and video player from APPLE.IPO (Initial Public Offering) the first sale of a corporation’s stock to the public. Innovative computer companies have often begun as privately held corporations, motivating employees by offering stock options that become valuable after the IPO if the market price rises. Outside investors who buy stock soon after the IPO will profit from further increases in the stock price. However, this type of investment is very risky because there is no guarantee the stock price will rise.iPod a portable audio and video player introduced by APPLE in 2001; one of the many brands of portable music player. The iPod plays MP3 and many other file formats, including videos. The iPod Touch specifically can download applications such as calendars or even games, and is inte-grated with Apple’s iTunes store. See ITUNES; MP3 PLAYER.IPTV (Internet Protocol Television) a system for transmitting television signals on request. Unlike traditional broadcast signals, which send all channels to all customers, an IPTV system conserves bandwidth by only sending signals for channels that customers have requested. However, traditional Internet transmission sends a separate signal for each user, so broadcast television signals would consume excessive bandwidth if they were sent this way. Instead, IPTV signals for multiple users are com-bined into a multicast signal, which contains one copy of the broadcast signal as well as the addresses of those users to whom the signal is to be 261 iSCSIsent. As the signal gets closer to the users it is split into separate signals for each user.IPv4, IPv6 older and newer versions of the Internet Protocol, the protocol for routing traffic through large networks. IPv4 and IPv6 were intro-duced in 1981 and 1994 respectively. As of the date this book was writ-ten, IPv4 was still predominant. See IP ADDRESS.IPv6 version 6 of the Internet Protocol, which will provide for improve-ments in routing network traffic and will increase the number of avail-able network addresses. There will be a transition period when this version will gradually replace the current version (IPv4). See www.ipv6.org. See also PROTOCOL.IPX/SPX (Internetwork Packet Exchange/Sequenced Packet Exchange) a data transmission protocol developed by Novell and widely used in local-area networking. See PROTOCOL. Contrast ATM; NETBEUI; TCP/IP.<JohnBoy> Does anybody know where I can get some information about SCSI? <Gweep> Hello, John Boy!<JohnBoy> Hello, Gweep!<Hermes> There’s a newsgroup called comp.periphs.scsi. Look there. <JohnBoy> Thanks, Hermes.<Gweep> What’s a newsgroup?FIGURE 142. IRC sessionIRC (Internet Relay Chat) a multi-user conversation conducted over the Internet in real time. Figure 142 shows what a chat session looks like. Numerous CHANNELS (conversation forums) exist. Participants normally identify themselves by nicknames.In addition to typing remarks for transmission to the other partici-pants, the IRC user can type commands such as /list to see what chan-nels are available, /join #frogs to join a channel called frogs or create it if it doesn’t exist, and /bye to sign off. See also AIM; CHAT ROOM.IRL online abbreviation for “in real life,” meaning the mundane, real, physical world as opposed to the glamorous exciting life in CYBERSPACE.IRQ (Interrupt Request) a type of bus signal used on PC-compatible com-puters to allow input-output devices to interrupt the CPU.ISA (Industry-Standard Architecture) a term often used to describe the con-ventional IBM PC AT (16-bit) bus and the associated card edge connec-tor, as opposed to EISA. See BUS; EISA.ISA slot a slot in a computer where ISA accessories can be added.iSCSI (Internet SCSI) a method of communicating with a disk drive on a server using the same protocol as if it were attached to a SCSI port, but wrapping the SCSI data in Ethernet packets. Compare SCSI. ACCESS PROVIDER; INTERNET SERVICEISDN 262ISDN (Integrated Services Digital Network) a type of all-digital telephone service that can transmit computer data (e.g., for Internet connections) as well as voice, with a maximum speed of 128 kbps. ISDN service began in the United States in 1988 but has largely been superseded by DSL, which obtains higher communication speeds over conventional tele-phone lines. See DSL; T1 LINE; T3 LINE.ISO1. International Organization for Standardization (abbreviated ISO in all languages), an organization that sets standards for many industries. For example, there are ISO standards for the Pascal and Prolog programming languages and for sizes of printer paper. See also ANSI; PAPER SIZES (ISO).2. online abbreviation for “in search of.”3. a system for rating the sensitivity (speed) of photographic film or CCDs, based on an ISO standard, equivalent to the older ASA system. An ISO 100 film or CCD will make a correctly exposed picture of a sun-lit landscape when exposed for 1/100 second at f/16.ISO 9000 an ISO standard specifying various ways of ensuring the quality of manufactured products. It does not denote any specific computer technology.ISO 9660 the basic standard format for recording computer data on com-pact discs (CD-ROMs). Unlike most disk formats, CD-ROMs are not tied to the operating system of a particular computer; any computer can read the data files from any standard CD-ROM. (Software recorded on the CD-ROM may of course require a specific computer in order to run.) ISO 9660 format supersedes the earlier High Sierra format, with which it is closely compatible. See JOLIET FILE SYSTEM; ROCK RIDGE.ISO paper sizes see PAPER SIZES (ISO).ISP (Internet service provider) a company that provides accounts allowingcustomers to access the Internet. SeePROVIDER.ISV (independent software vendor) a company that writes software inde-pendently of the company selling the hardware.IT (information technology) computers and electronic communication.italics letters slanted to the right and designed with a more calligraphic feel than their roman counterparts. This sentence is set in italics. Italics are used for emphasis, for setting the titles of books and articles, and for for-eign words. Italic type corresponds to underlining on a typewriter.Itanium a family of Intel 64-bit microprocessors implementing the IA-64 architecture, introduced in 2001. See IA-64.iteration the process of repeating a particular action. For examples see FOR; WHILE. Contrast RECURSION, in which instead of merely being repeated, the action creates another action of the same type within itself. 263 iWorkITU (International Telecommunication Union) an organization headquar-tered in Geneva, Switzerland, that sets standards for electronic commu-nication technology. For more information see the web page www.itu.int.ITU-T the telecommunication section of the ITU, formerly known as the CCITT.iTunes an online store integrated with iPods to allow for easy purchase of music and audiobooks, as well as movies, television shows, and appli-cations. See APPLE; IPOD.iWork a set of software applications (word processing, spreadsheets, and presentations) provided by APPLE. J# 264JJ# (pronounced “J sharp”) a programming language very similar to JAVA but implemented in the Microsoft .NET Framework; essentially a combina-tion of Java and C#. Most Java 1.1 programs will run unaltered in J#, and in addition, the full functionality of the .NET Framework is available.See C#; .NET FRAMEWORK.jack a connector into which a plug can be inserted.FIGURE 143. Jaggies (stairsteps) in an improperly-sized bitmapjaggies the property of an improperly sized bitmap that shows the image bro-ken into blocky squares. See ANTIALIASING; BITMAP; PIXELATE; RESOLUTION.Japanese writing see KANA.JAR file (Java Archive) a file containing a collection of Java class files that can be downloaded more efficiently than would be possible if each file were downloaded separately.Jargon File a glossary of computer terms begun at Stanford University in 1975, extensively enlarged at MIT, and eventually published, in highly revised form, as The New Hacker’s Dictionary, by Eric S. Raymond (MIT Press, 3rd edition, 1996). Versions are also available on the World Wide Web. The Jargon File is important because it is one of the first instances of computer enthusiasts functioning as a cultural movement.Java a programming language developed at Sun Microsystems in the mid-1990s to enable networked computers to transmit computations to each other, not just data. For example, an Internet user can connect to a Java APPLET (program) on the World Wide Web, download it, and run it, all at the click of a mouse, using a Java-compatible WEB BROWSER. Applets can include features such as animation. Figure 144 shows an example of a Java program.Java is designed for OBJECT-ORIENTED PROGRAMMING. The language is extensible: classes are defined in terms of other more general classes, HTML; WORLD WIDE WEB.265 Java and they inherit their properties. It is closely based on C++ but is funda-mentally different in a number of ways that make it easier to use. Its spe-cial features include:• architecture neutrality (a Java program can, in principle, run under any windowed operating system);• garbage collection (memory is cleared automatically when objects no longer need it, eliminating one of the major headaches of C++ programming);• security (Java applets downloaded from the web are restricted in their access to the machine’s files and operating system; this is an essential feature of programs loaded through the Internet from a possibly untrustworthy source);• uniform support for windowing environments (programs produce similar screen displays no matter where they are run);• support for multiple threads of program action (concurrency). Various Java development environments are available. Java softwarecan be downloaded free of charge from www.java.com. The current ver-sion number is 6.0 (which jumped from 1.4.2 for the previous version.)To compile a program, type a command such asjavac program1.javawhere program1.java is the program to be compiled. This command will create the file program1.class.Java programs are compiled not into machine code, which would not be portable, but into a concise code known as Java bytecode. How to run the bytecode depends on the type of Java program, since there are two kinds, applications (stand-alone programs) and applets. If the program is an application, it can be executed with this command:java program1If the program is an applet, it can be included in a web page through HTML statements such as<applet code=”program1.class” width=200 height=200> </applet>The applet will be executed by the browser’s Java virtual machine when-ever anyone views the web page. The user does not need to have the Java compiler, only the run-time support for Java that is provided by most web browsers.Java is important not only because it allows computations to be down-loaded through web pages, but also because it is the first portable pro-gramming language for windowed operating systems. It does for windowing what BASIC did for the keyboard and screen, making it pos-sible to write programs that run the same way regardless of the kind ofmachine. See also Java 266/* File showball.java *//* Displays red or blue circles when user clicks mouse */import java.awt.*; import java.applet.*;public class showball extends Applet { Graphics g; int rc,gc,bc; ball ballobject;public void init() { setLayout(null);Button bluebutton = new Button(“Blue”); add(bluebutton); bluebutton.reshape(0,0,100,20);Button redbutton = new Button(“Red”); add(redbutton); redbutton.reshape(110,0,100,20); ballobject = new ball();}public boolean action(Event e, Object o) { if (”Red”.equals(o)){rc=255; gc=0; bc=0;} else if (”Blue”.equals(o)){rc=0; gc=0; bc=255;} return false;}public boolean mouseDown(Event e, int x, int y) { ballobject.xc = x; ballobject.yc = y; ballobject.redcode = rc; ballobject.greencode = gc; ballobject.bluecode = bc;repaint(); return false;}public void paint(Graphics g) { ballobject.drawball(g);}}/* File ball.java */import java.awt.*; import java.applet.*;public class ball {int xc,yc,redcode,greencode,bluecode; public void drawball(Graphics g2) {Color rgbcolor = new Color (redcode, greencode, bluecode);g2.setColor(rgbcolor);g2.fillArc(xc,yc,25,25,0,360);/* fillArc is included with the class applet*/}} FIGURE 144. Java program (comprising 2 files) 267 JCL JavaScript a language that allows a web page to include commands to be executed by the web browser. For example, you may wish your web page to have dialog boxes that appear when the user clicks on certain places. Or, if your web page will send information back to the server, you can use JavaScript to catch some common data entry errors before it is sent. This will save work for the server.JavaScript was originally developed by Netscape. The name comes from the fact that JavaScript shares some syntax and object-oriented fea-tures with Java, but they are actually quite different languages. JavaScript commands are interpreted by the web browser (see INTER-PRETER), rather than being compiled into bytecode as Java programs are. JavaScript is easier to learn than Java, particularly if you already know HTML, but, unlike Java, it does not have the features of a complete pro-gramming language.Figure 145 is an example of how JavaScript can be used together with an HTML form to receive information from a customer. In this example, the user enters the item number, price, and quantity for two different items into the form. Two different JavaScript functions are defined. The function addup is called by the event handler onChange that is located in the HTML code for the form. Whenever one of these fields is changed, JavaScript checks for negative values and then multiplies the price times the quantity and adds up the total dollar amount.Figure 146 shows the appearance of the screen when this web page is loaded.For another example of programming in JavaScript, see ROLLOVER.JBOD (just a bunch of disks) DISK SHARING in which the disks are treated separately, not organized into a RAID system. See RAID.JCL (Job Control Language) the command language used in batch jobs to tell a computer what to do. (See BATCH PROCESSING.) The acronym JCL usually refers to the job control language used on large IBM computers, but sometimes designates very different languages used for the same purpose on other computers.The following is an example of JCL for an IBM 360; the language is the same for all IBM mainframe computers running operating systems derived from OS/360, such as OS/VS2, and MVS://JONES JOB 123456,TIME=5 // EXEC PLIXCG//PLI.SYSIN DD DSN=JONES.SAMPL.PLI,DISP=SHR //GO.SYSIN DD *1 2 3 4 /*//The first statement is the JOB card, which gives the job a name, specifies the user’s account number, and establishes a CPU time limit of 5 min-utes for the whole job. The EXEC PLIXCG statement calls up the procedure to compile and execute a PL/1 program; its operation happens to consist of two steps, PLI (compile) and GO (execute). JCL 268<HTML><HEAD><TITLE> JavaScript Example</TITLE><SCRIPT LANGUAGE=”JavaScript”>function multiply(a,b) {if ((a>=0)&(b>=0)) {return a*b;}else {window.alert(”Value can’t be negative”); return ”invalid”;}}function addup() {var totalamount=0; var amount=0; for (i=1; i<=2; i++) {amount= multiply(document.orderform.elements[i*4-2].value, document.orderform.elements[i*4-1].value); document.orderform.elements[i*4].value=amount;totalamount+=amount;}document.orderform.total.value=totalamount;}</SCRIPT></HEAD><BODY><H1> Order Form Example </H1><FORM NAME=”orderform”><ACTION ”mailto:youraddress@xyz.com”>Your Name: <INPUT TYPE= ”text” NAME=”customername” SIZE=20><br>Item Number:<INPUT TYPE=”text” NAME=”item1” SIZE=5> Price:<INPUT TYPE=”text” NAME=”price1”SIZE=5 onChange=”addup();”> Quantity:<INPUT TYPE=”text” NAME=”quant1”SIZE=5 onChange=”addup();”>Value:$<INPUT TYPE=”text” NAME=”value1” SIZE=8 onChange=”addup();”><BR>Item Number:<INPUT TYPE=”text” NAME=”item2” SIZE=5>Price:<INPUT TYPE=”text” NAME=”price2”SIZE=5 onChange=”addup();”>Quantity:<INPUT TYPE=”text” NAME=”quant2”SIZE=5 onChange=”addup();”>Value: $<INPUT TYPE=”text” NAME=”value2”SIZE=8 onChange=”addup();”><BR>Total Value:$<INPUT TYPE=”text” NAME=”total”SIZE=8 onChange=”addup();”><BR><INPUT TYPE=”SUBMIT” NAME=”submit” VALUE=”E-Mail your order” SIZE=40></FORM></BODY></HTML> FIGURE 145. JavaScript example 269	joystickFIGURE 146. JavaScript in actionThe DD (data definition) statements define files. SYSIN (standard system input) for the PLI step is defined as a cataloged disk data set named JONES.SAMPL.PLI; SYSIN for the GO step is given in the job itself, beginning after DD * and ending with /*. The // card marks the end of the job.JDK (Java Development Kit) a system provided free by Sun Microsystems that can be used to write Java programs. It comes with a Java compiler, standard class library, and an applet viewer.jewel case a rigid clear-plastic case that protects compact discs (CDs). The paper insert for the face of a jewel case is 120 mm (4.7 inches) square.JIT compiler (just-in-time compiler) a compiler that converts intermediate code into native machine language the first time it is encountered; this allows subsequent execution of that code to occur faster.job see BATCH PROCESSING.join see RELATIONAL DATABASE.Joliet file system an extension to the ISO 9660 format for recording CD-ROMs, created by Microsoft in order to support long filenames and long directory names. It is the usual format for CD-ROMs to be used under Windows 95 and later. See also ROCK RIDGE.joo LEETSPEAK for “you.”joule a unit for measuring amounts of energy. 1 joule equals 1 kilogram-meter2/second2. One watt is equivalent to one joule per second. SeeVOLT; WATT; WATT-HOUR.joystick a computer input device especially helpful when playing computer games. The joystick consists of a handle that can be pointed in different directions. Because the computer can sense in which direction the joy-stick is pointed, the joystick can be used to control the movements of objects displayed on the computer screen. JPEG	270JPEG (Joint Photographic Experts Group) a file format for storing bitmap images, including lossy compression (i.e., a file can be compressed to a very small size if some blurring of detail is tolerable). JPEG file format is often used for high-quality photographic images. Filenames usually end in .jpeg or .jpg.JScript a version of JAVASCRIPT developed by Microsoft.jukebox a device for automatically selecting disks (usually CD-ROMs) from a library and inserting the desired one into the disk drive.Julian calendar the calendar introduced in 46 B.C. by Julius Caesar, with a leap year every four years. See GREGORIAN CALENDAR; LEAP YEAR.Julian date1.	the number of days elapsed since January 1, 4713 B.C., counting that date as day 1. The Julian date is now used primarily by astronomers. It is unrelated to the Julian calendar. The Julian date was introduced in 1582 A.D. by J. J. Scaliger, who named it for his father Julius. Its origi-nal purpose was to convert dates between various ancient calendars without using negative numbers. The Julian date of January 1, 2000 is 2,451,544.2.	more loosely, any date expressed as a day count rather than as year, month, and day.jump list a WEB PAGE consisting mostly of links to other web pages. SeeHTML; INTERNET; LINK (definition 3); WORLD WIDE WEB.JumpDrive Lexar’s trademark for a brand of USB FLASH DRIVE. See Figure 285, page 507.jumper a removable electrical connector that joins two pins on a circuit board. Many of the internal settings in computers are made by moving or removing jumpers.junction the part of a diode or transistor where two opposite types of semi-conductor material meet. See DIODE; TRANSISTOR.junk e-mail unsolicited electronic mail, usually containing advertisements.See SPAM.junk fax an unsolicited advertisement transmitted by fax. In 1991, sending junk faxes was made illegal in the United States because it imposes an expense on the recipient (who has to pay for the paper) and ties up machines that are needed for more important messages. This law (47 USC 227) was overturned by a district court in 2002 (Missouri v. American Blast Fax), but the law was reinstated by an appeals court in 2003.junkware unwanted demonstration software added to a new computer by the seller. 271	Jythonjustification the insertion of extra space between words in lines of type so that the left and right margins are even and smooth. Most of the type in this book is justified.Most word processors and desktop publishing programs can automat-ically do the computations necessary to justify type. Problems generally arise only when the column width is too narrow, or too large a HOT ZONE has been specified. Then you will get rivers of white space running down the column (not too attractive). To cure this, make sure hyphenation is enabled, shrink the hot zone, go to a smaller type size, or increase the column width. See RIVER.JVM (Java Virtual Machine) software that executes Java bytecode. A pro-gram written in Java is first compiled into class files, written in byte-code. To execute these files, the computer needs to use the JVM to interpret the code. The JVM is built into web browsers that are capable of executing Java applets. The bytecode is the same for all platforms, but the JVM will be different on different platforms because it needs to exe-cute using the native code of the machine it is running on.Jython an implementation of PYTHON based on JAVA. See www.jython.org. X WINDOW SYSTEM GNOME.k, K	272Kk, K abbreviation for KILOBYTE (or, less commonly, kilohm or other metric units). By convention, a capital K stands for a factor of 1,024, and a low-ercase k stands for a factor of 1000, as in the metric system (kilograms, kilometers, etc.). See also BYTE; MEGABYTE; MEMORY.K56flex a standard formerly used by Rockwell and other modem manu-facturers for transmitting data on telephone lines at speeds up to 56,600 bits per second.K6 chip microprocessor introduced by AMD in 1997 as a competitor to the Intel Pentium.kana the Japanese phonetic writing system. There are two styles, hiragana and katakana. Kana contrasts with kanji, the Chinese-derived symbols for whole words. Written Japanese uses a mixture of kana and kanji.kanji see KANA.katakana see KANA.KB abbreviation for KILOBYTE.kBps kiloBytes per second. See also BAUD; KILOBYTE.kbps kilobits per second. See BIT.KDE (KDesktop Environment) a widely used graphical desktop environ-ment for Linux and UNIX systems, based on the	andoriginated by Matthias Ettrich in 1996. CompareKerberos an authentication protocol that allows users and computers to identify each other without risk of impersonation and to communicate securely by encrypting their data. A Kerberos system uses a central authentication server to issue tickets, which are temporary authorizations to communicate. Each ticket is valid only for a specific user and for a limited length of time. Thus, an intercepted or stolen ticket is of little use. Because of the encryption used, forged tickets are virtually impos-sible to produce.Kerberos was developed at the Massachusetts Institute of Technology, which distributes an implementation of it free of charge (web.mit.edu/kerberos/www). Kerberos has many commercial imple-mentations. See PROTOCOL. In Greek mythology, Kerberos (in Latin, Cerberus) is the dog that guards the gate of Hades.Kermit a protocol for transferring files from one computer to another by modem without going through the Internet. Kermit is also the name of a program (distributed free by Columbia University) that implements this protocol. 273	keyboardingKermit makes an exact copy of the original file even when transmit-ting over a noisy line. All data packets are error-checked, and erroneous packets are retransmitted. See PROTOCOL.kernel the central part of an OPERATING SYSTEM. In many operating systems, only the kernel can access hardware directly.Usage note: For obscure reasons this term is often spelled kernal. This may be nothing more than a typing error that appeared in an influ-ential manual and caught on.kerning adjustment of the amount of space between certain combinations of letters in proportional-pitch type. If the combination “To” is typeset with the same letter spacing as “Th,” the letters seem to be too widely spaced. “To” looks better if the top of the “T” is allowed to overhang the “o” slightly. See Figure 147. Compare TRACKING.To	Without kerningTo	With exaggerated kerningFigure 147. Kerningkey1.	a button on a computer keyboard.2.	the item by which a data file is sorted or searched. For instance, if a file of names and addresses is sorted by zip codes, then the zip code is the key.3.	the password or other secret information needed to decode an encrypted message. See ENCRYPTION.keyboard the primary computer input device for alphanumeric data. There are many different types of keyboard layouts; for the most part the alpha-bet and numbers are consistently placed, but there is considerable varia-tion in the placement of the auxiliary characters, editing keys, and function keys. Most keyboards have a numeric keypad (for typing dig-its) at the right; if you use a mouse a lot, but don’t type many numbers, you may prefer a narrower keyboard that omits the keypad and lets you put your mouse closer to where you sit. Some keyboards have a mouse-like pointing device built in; these are generally fine for menu selection but not precise enough for drawing.When buying a new computer, be sure to evaluate the keyboard care-fully. A keyboard that feels “dead” can be tiring to use. Practice typing on several different models to find one that feels good to you. Spending a few extra dollars for a good keyboard can be a wise investment; after all, it is the part of your computer that you are in contact with constantly.keyboard shortcut see SHORTCUT.keyboarding entering data through the keyboard; typing. keying	274keying1.	typing; inputting information into the computer by means of the key-board.2.	the process of digitally combining video images by using a subtrac-tive background. See CHROMA-KEYING.keylogging see KEYSTROKE LOGGING.keystroke logging the act of recording the keys that a person presses as he or she uses a computer. Keystroke logging is often done surreptitiously by malicious SPYWARE in an attempt to capture passwords.keyword1.	a word that has a special meaning in a particular programming lan-guage. For example, for is a keyword in C, and BEGIN is a keyword in Pascal.2.	words or phrases that, when included in the META tag of an HTML document, help SEARCH ENGINEs catalog the contents of that web page.kibi- proposed metric prefix meaning 1024 (210), the binary counterpart of kilo-. See METRIC PREFIXES.kibibyte 1024 bytes.kill file a list of people whose incoming e-mail messages or newsgroup postings are automatically deleted or hidden from view. Many mail and news reading programs allow you to set up a kill file so that obnoxious messages from known senders can be avoided. It is hard to avoid SPAM this way because spammers constantly change their names. CompareBLACKLIST; IGNORE LIST.killer app (slang) a software application that becomes so desirable that it is the reason people purchase a computer, computer peripheral, or oper-ating system. For example, Visicalc was a killer app for the Apple II, and Lotus 1-2-3 was a killer app for the original IBM PC. Some operating systems, such as OS/2, have failed to become popular because of the lack of a killer app.kilo- metric prefix meaning 1000 (103) or, in rating computer memories and disks, 1024. Kilo- is derived from the Greek word for “thousand.”See METRIC PREFIXES.kilobyte a unit of computer memory capacity equal to 1024 characters. The number 1024 is significant because 210 = 1024.By convention, a capital K stands for a factor of 1024, and a lower-case k stands for a factor of 1000, as in the metric system (kilograms, kilometers, etc.). See also BYTE; MEGABYTE; MEMORY.kilowatt-hour a unit of electrical energy consumption equal to 1000 watt-hours. See WATT-HOUR. 275	Kylix Kindle a portable reading device introduced by Amazon in 2007, includ-ing a screen with an appearance similar to paper. Books and periodicals can be downloaded to the Kindle wirelessly.kiosk a small stand containing a computer that people can walk up to and use to retrieve information. Kiosks often display current information about local events. They are used in museums, airports, and other public places.kluge (pronounced “klooge”) an improvised, jury-rigged, and poorly thought-out solution to a problem, usually intended only for temporary use. The word kluge may be derived from German klug, which means “clever.” In Britain it is sometimes spelled kludge and pronounced to rhyme with “sludge.”knife (drawing program) a tool that cuts an object into pieces, defining a new outline along the cut edge and thus preserving the fill attributes of the original object.knockout an area where an underlying color has been cut out so that the overprinting color can remain pure. Some DESKTOP PUBLISHING software automatically creates knockouts and TRAPs when preparing files for duplication on a printing press (see PREPRESS).(Knockout applies to mass production printing with a printing press, not to inkjet or laser printers attached directly to computers.)knowledge base a collection of knowledge that is used as the basis for solving problems or making recommendations. See EXPERT SYSTEM.Koch snowflake see FRACTAL.KVM switch (Keyboard-Video-Mouse switch) a device that allows sev-eral computers to share a single keyboard, screen, and mouse. By press-ing a button on the KVM switch or by typing special keystrokes, the user can connect the keyboard, screen, and mouse to any of the com-puters.Kylix a development environment for C++ and Delphi programming under Linux, produced by Borland International. See DELPHI. L1 cache	276LL1 cache (level-1 cache) the memory cache that is closest to the CPU or included within it.L2 cache (level-2 cache) a memory cache outside the CPU. Contrast L1 CACHE.L33T, L33TSPEAK see LEETSPEAK.L@@K comical way of writing look (i.e., “look at this”) in online adver-tisements.label1.	an identifying name or number attached to a particular statement in a computer program.2.	a block of information recorded on a tape to identify it.3.	an identifying name recorded on a disk and displayed by the dir com-mand in Windows and similar operating systems.lag the delay in transmitting data over a network. In online video, lag may be experienced as choppy movement or images freezing for several sec-onds. In online games, lag can cause a significant problem for players who cannot see an attack in time to respond to it.lambda calculus the use of LAMBDA EXPRESSIONS to define functions.lambda expression a formula that defines a function, originally using the Greek letter lambda (λ) to mark arguments.Lambda expressions were introduced into formal logic by Alonzo Church in the 1930s. The key idea is that when a mathematician says, “Let f(x) = x + 2,” this is really a definition of f for any argument, not just x. To make this explicit, one can say, “Let f = (λx) x + 2,” where (λx) indicates that x is not part of the function, but merely stands for an argument value.In C#, the same lambda expression is written (x) =>x+ 2 and can be used in place of a DELEGATE (function pointer).LAN see LOCAL-AREA NETWORK.landscape a way of orienting paper so that it is wider than it is high, like a landscape painting. That is, the paper is positioned sideways compared to the way it would otherwise be used (“portrait orientation”). Laser printers typically offer a choice of portrait or landscape orientation. FIGURE 148. Landscape vs. portrait orientation 277	last known goodlaptop a small, lightweight computer (under 8 pounds) with a flip-up screen. Such a computer is powered by rechargeable batteries and is easily portable. Laptops are especially valuable for people who travel frequently and need to be able to work on a computer while on the road. See also DOCKING STATION; PCMCIA. Because of their portability, laptops need special precautions against theft. See COMPUTER SECURITY. Compare NOTEBOOK.Usage note: The distinction between “laptop” and “notebook” has become blurred; some vendors call all their portable computers note-books.large-scale integration the construction of integrated circuits that contain more than 100 logic gates. See INTEGRATED CIRCUIT.laser (light amplification by stimulated emission of radiation) an electronic device that produces rays of light that are exactly matched in wavelength and phase. Laser beams can be used to detect microscopically tiny detail (such as the pattern on a CD-ROM) and to concentrate energy in a small, precisely located space (as in a laser printer).laser printer a computer printer that generates an image by scanning a photoconductive drum with a laser beam and then transferring the image to paper by means of electrostatic toner. Laser printers provide high-quality output of text and graphics; they are quiet and run fast. ContrastDOT-MATRIX PRINTER; INKJET PRINTER. See  also  CORONA WIRE; DRUM;FUSER; PICKUP ROLLER; SEPARATOR PAD; TRANSFER ROLLER.lasso a selection tool commonly found in PAINT PROGRAMs; it looks like a rope lariat, and you use it to define an area that you wish to work with. Crucially, the area need not be rectangular; it can be any shape. After selecting the lasso icon, you drag the mouse freehand around the desired area. Once the area is defined, you can scale, move, rotate, change color, apply filters, or perform any operation that is available.Because the lasso is a freehand tool, it is dependent upon your skill as a mouse operator. Knowing how difficult it is to draw accurately with a mouse, you may want to see if your paint program has other selection tools (such as a MAGIC WAND) that would suit your needs better. See SELECT.FIGURE 149. Lasso tool last known good describing the configuration of a computer the last time it was used successfully, before changes. If you disrupt Microsoft Windows last mile	278while installing patches or drivers, you can, in most cases, use the System Restore feature to boot from the last known good configuration.last mile the connection of individual homes or businesses to a communi-cation network. For examples see DSL; WIMAX.LATEX (pronounced “la-tekh” or “lay-tekh”; alternatively written LATEX) a typesetting system designed by Leslie Lamport and implemented as a set of macros for Donald Knuth’s TEX (see TEX). There are two versions in wide use, LATEX 2.09 and LATEX 2ε; LATEX 3 is under development.The key idea of LATEX is to separate the job of the author from that of the publication designer. The author uses commands such as \chapter{...} and \section{...} to mark chapter and section titles, figures, quotations, and the like. (Figure 150 shows an example.) Separately, a file called a style sheet specifies how these things should be printed and keeps them consistent. So, while other word processing programs work like a computerized typewriter, LATEX does the job of an expert typist and layout artist. See LOGICAL DESIGN.LATEX is especially popular for typesetting scientific and mathemati-cal books because the full power of the TEX mathematical typesetting system is available. Many scholarly journals are typeset with LATEX, as are most of the books published by several major publishers. LATEX is also popular with graduate students writing theses and dissertations because it is easy to conform to standard formats—just use your univer-sity’s official style sheet.Implementations of LATEX are available for a wide range of comput-ers. The text of this book is written using LATEX.Latin the language of the ancient Romans; the Roman alphabet (including j, v, and w, which were added to it in modern times), as opposed to the Greek or Russian alphabet.launch1.	to advertise and release a new product.2.	to start a computer program, especially in a multitasking operating system.LCD (Liquid Crystal Display) the type of display used on most digital watches, calculators, and laptop computers, and in flat-panel computer displays.LCDs use liquid crystals, which are chemicals whose response to polarized light can be controlled by an electric field. A polarizing filter is built into the LCD; through this filter, the liquid crystal compound looks light or dark depending on its electrical state.lead a metal that forms poisonous compounds and should be kept out of landfills and public water supplies. Lead is found in electronic equip-ment in lead-acid batteries and formerly in SOLDER. See ROHS.leader a line of dots that connects one side of the page with another, often used in tables of contents, like .......................... this. 279	leading \documentclass{article}\title{An Example} \author{Michael A. Covington} \date{June 27, 2008}\begin{document}\maketitle\section{Overview}This is a sample of a short paper typed with \LaTeX. Notice the commands I use to get \emph{italics} and \textbf{boldface}. I can also typeset mathematical formulas such as $\sum_{x=1}^{5} A_x$.Notice that I skip lines between paragraphs in the input. This is the second paragraph.\section{Another section}This is the second section. It is very short.\end{document}An ExampleMichael A. CovingtonJune 27, 20081. OverviewThis is a sample of a short paper typed with LATEX. Notice the commands I use to get italics and boldface. I can also typeset math-ematical formulas such as ∑5x 1  Ax.Notice that I skip lines between paragraphs in the input. This is the second paragraph.2. Another sectionThis is the second section. It is very short.FIGURE 150. LATEX input and outputleading (pronounced “ledding”) the insertion of extra space between lines of type. On old printing presses, this was originally done by inserting strips of lead between rows of type cast in lead. See TYPEFACE.These linesare typeset with extra leading.In some cases, it is actually beneficial to use negative leading—for instance, when setting type in all caps, it is not necessary to allow space for descenders. Such headlines usually look best with negative leading.THESE LINES ARE TYPESETWITH NEGATIVE LEADING. leak	280leak an error in a program that makes it fail to release memory or other sys-tem resources when it terminates. Thus, the available memory, disk space, or other resources are gradually eaten up until the computer is rebooted. Memory leaks are a common error in Windows programs.Java and .NET programs avoid this problem because of the garbage collection process, which automatically releases memory locations that are no longer being referenced by the program.leap year a year in which an extra day, February 29, is added to keep the calendar year in step with the earth’s revolution around the sun. If there were no leap years, the calendar would get out of step with the earth’s motion, so that after many centuries, January 1 would occur in the sum-mer instead of the winter.The rule for identifying leap years is as follows:•	Years divisible by 400 are always leap years. Thus, 2000 and 2400 are leap years.•	Years divisible by 100, but not by 400, are not leap years. Thus, 1900 and 2100 are not leap years.•	Otherwise, years divisible by 4 are leap years. Thus, 2004, 2008, and so on, are leap years.Leap years were introduced by Julius Caesar. See GREGORIAN CALENDAR; JULIAN CALENDAR.learning curve a graph representing mastery of a skill plotted against the time spent on learning it. If something is hard to learn to use, it is some-times described as having a steep learning curve (although, logically, a steep curve should indicate rapid learning).The term originated in behaviorist psychology but is now used very imprecisely.lease the right to use an IP ADDRESS temporarily assigned by DHCP. If a lease runs out while the computer is still connected to the network, DHCP automatically renews it or assigns a new address.LED (light-emitting diode) a semiconductor device that emits light when an electric current passes through it. The bright red display on some dig-ital clocks is made of LEDs. Other available LED wavelengths are infrared, orange, yellow, green, and blue.White LEDs consists of a blue-light emitter plus a fluorescent phos-phor. They are a very efficient source of illumination because nearly all the electricity is turned into light, unlike a traditional incandescent light bulb, which turns most of its incoming energy into heat.leetspeak (from “elite speak”) written slang that modifies words by replac-ing letters with symbols or digits to make them look more computerish, using phonetic spellings, and adopting common typing errors as conventions.A familiar example is the spelling of elite as leet or |33t. It is com-mon to see 3 for E, 1 for I, and @ or 4 for A. There is almost infinite PLUG AND PLAY.281	letter sizevariation in the ways that ASCII characters can be used creatively to form letters, and so there is no official dictionary of leetspeak; people make it up as they go.Although leetspeak was originally used to exclude the uninitiated and to bypass chat room filters that block dirty words, it is now most fre-quently seen in jokes.left-click to CLICK with the left-hand mouse button (or the right-hand but-ton if the mouse is set up for a left-handed person).legacy anything left over from a previous version of the hardware or soft-ware. For example, legacy applications are applications from earlier ver-sions of DOS or Windows; legacy hardware is hardware that does notsupportlegacy-free not burdened by the need for compatibility with substantially older equipment or software. Microsoft is promoting legacy-free PC design as a way to make PCs more reliable and easier to upgrade.Since 1984, the IBM PC AT architecture has reserved a number of port addresses, interrupt request (IRQ) numbers, and memory addresses for various purposes. These were assigned long before sound cards, video capture devices, and other modern computer peripherals were invented. Because of this, the person installing a sound card into a modern PC often has to choose memory addresses and IRQ numbers for it—a job that should be left to the designer of the machine. Legacy-free PCs break free of these requirements by allowing the operating system to control the hardware directly. Legacy-reduced PCs are intermediates between legacy-free and conventional PCs. See also IRQ; PC 2001; PLUG AND PLAY.legal size the size of paper used for legal documents in the UnitedStates, 81⁄ 2  14 inches. Contrast EXECUTIVE SIZE; LETTER SIZE. See also PAPER SIZES.Lenovo company that bought IBM’s personal computer division in 2005 (web address: www.lenovo.com/us/en).letter size the size of paper used for business letters in the United States, 81⁄ 2  11 inches. Elsewhere, ISO size A4 is the nearest equivalent.Contrast EXECUTIVE SIZE; LEGAL SIZE. See also PAPER SIZES. FIGURE 151. Legal- and letter-sized paper letterspacing	282letterspacing the space between letters (characters). See Figure 152. Look for the letterspacing controls with other FRAME attribute commands (let-terspacing is sometimes called TRACKING).FIGURE 152. LetterspacingLF (line feed) the character code that tells a printer or terminal to advance to the next line; ASCII code 10. UNIX uses LF to indicate the end of a line in a text file. The Macintosh uses CR; Windows uses CRLF. See CR; CRLF. Contrast FF (form feed).Li-ion (Lithium-ion) a type of rechargeable battery widely used in portable computers. They have higher capacity than NiCd and NiMH batteries but are more expensive and require different charging circuitry. SeeNICAD, NICD; NIMH.library1.	a collection of files, computer programs, or subroutines. A loader library is a file containing subroutines that can be linked into a machine language program.2.	a collection of reference materials and software tools, such as clip art, prerecorded sounds, and predefined objects.license permission to use patented or copyrighted material. See PER COM-PUTER; PER SEAT; PER USER; SHRINKWRAP LICENSE; SOFTWARE LICENSE.LIFO (last-in-first-out) a STACK (definition 1); a data structure or memory device from which items are retrieved in the opposite of the order in which they were stored. Contrast FIFO.ligature a printed character representing a combination of two or three let-ters (Figure 153). Some of the most sophisticated word processing pro-grams, such as TeX, change pairs of letters into ligatures automatically.FIGURE 153. Ligatureslight1.	visible electromagnetic radiation.2.	type that is designed and drawn with very fine strokes; the opposite of BOLD. 283	limits of computer power3. (in 3D and animation software) virtual device that mimics the effect of real light upon the computer-generated scene. Computer lights can be adjusted in many of the same ways as their real-life counterparts: inten-sity, position, direction, and color. See also AMBIENT LIGHTING.light-emitting diode see LED.light pen a pen-like light-sensitive device that can be used like a mouse to communicate with a computer. The operator holds the pen up to the screen, and the computer can sense what point on the screen the pen is touching. Light pens were popular in the 1970s but have largely been replaced by mice.limitcheck PostScript error that occurs when a drawing is too complex to be printed.limits of computer power things that computers cannot do, which is a sub-ject of continuing theoretical study.Computers can perform only tasks that can be reduced to mechanical procedures (algorithms). They are therefore inapplicable to tasks that can-not or should not be reduced to mechanical form, such as judging the great-ness of a work of art or administering psychotherapy. Rather surprisingly, however, there are some tasks that are mathematically precise but that pre-sent-day computers cannot perform. These fall into two major types: (1) problems with no known algorithmic solution, and (2) problems whose best known algorithmic solutions require unreasonable amounts of time.An example of a problem of the first type (one with no presently known algorithmic solution) is how to get a computer to recognize the structures of sentences in a human language such as English. Obviously, this is something computers will have to be able to do if we are ever to be able to communicate with them in English, and there is no reason to think it impossible. The difficulty is simply that English (and all other human languages) are so complicated that complete algorithms for pro-cessing them have not yet been discovered.A good example of the second type of problem, one that takes an unrea-sonable amount of time to solve, is the so-called traveling salesman prob-lem. The task is to find the shortest route by which a salesman can visit a particular set of cities (in any order). The only known way to solve this problem is to try all possible routes. A few shortcuts are possible—for instance, the testing of each route can be abandoned as soon as its length exceeds the shortest length already found, without pursuing it to the end— but the number of steps is never substantially fewer than N factorial, where N is the number of cities (see FACTORIAL). Suppose the fastest imaginable computer could perform one step in this algorithm by moving an electric charge a distance of 1 millimeter at the speed of light. This would mean that it could perform 3  1011 steps per second. Then the times required to solve the traveling salesman problem (in N! steps) would work out as follows: Lindows, LindowsOS		284						Number of	Number of	Time		Cities	Steps	Required				5	120	0.36 picosecond10	3,628,800	12 microseconds15	1.3  1012	4.4 seconds20	2.4  1018	94 days25	1.6  1025	1.6 million years30	2.7  1032	2.8  1013 yearsAnd this is with a computer millions of times faster than any that presently exist. Obviously, it will never be feasible to solve the traveling salesman problem for more than a few cities unless a much better algo-rithm is found.Another interesting class of computational problems, known as NP-complete problems, has been proved to be equivalent to the traveling salesman problem; if a better algorithm is found for any NP-complete problem, it will be applicable to all of them.See also CHURCH’S THESIS; COMPLEXITY THEORY.Lindows, LindowsOS the original name of LINSPIRE; it was changed to avoid infringing Microsoft’s trademark rights to the name Windows.line1.	in geometry, the shortest path connecting two points. A geometric line is always perfectly straight and has no width.2.	in graphics, a visible representation of a geometric line. A line in this sense has a definite color and width (normally at least 0.5-point for good visibility on paper; see HAIRLINE) and may be continuous, dashed, or dotted.3.	a printed line of type. Text is most readable with a line length of about 65 characters. See also LINESPACING; WORD WRAP.4.	an electronic communication path, such as a telephone line. See T1LINE; T3 LINE.line cap the end of a drawn line. In most DRAW PROGRAMs, you can choose square or rounded ends, or even arrowheads.line drawing an illustration that can be represented as a series of hard-edged black lines and black areas on a white background. Line drawings are easily converted to vector images by tracing them. ContrastGRAYSCALE and PHOTOGRAPH.line feed the character code that tells a printer or terminal to advance to the next line; ASCII code 10. UNIX uses LF to indicate the end of a line in a text file. The Macintosh uses CR; Windows uses CRLF. See CR; CRLF.line in (on a sound card) line-level audio input. See LINE-LEVEL.line-level (describing an audio signal) a signal level of about 0.1 to 1 volt, designed to connect to the input of another amplifier. Some line-level outputs can drive headphones; others cannot. DEAD LINK.RADIAL FILL.285	linked listSpeaker-level audio is a slightly higher voltage, but the main differ-ence is that speaker-level outputs can deliver much greater current (amperage) in order to drive speakers. Microphone-level audio is a much lower level, about 0.001 volt.line out (on a sound card) line-level audio output. See LINE-LEVEL.line printer a type of IMPACT PRINTER that prints an entire line of type at once, formerly used on mainframe computers.line spacing the spacing in between lines of type. Also called LEADING.linear fill a way of filling an object with color so that it makes a smooth tran-sition from one color at one side of the object to another color at the otherside. You can specify the angle of the linear fill. ContrastFIGURE 154. Linear filllink1.	any kind of communication path between two computers.2.	an entry in one directory or menu that points directly to something in some other directory or menu; a SHORTCUT. Links can be used to make the same file accessible from more than one directory or to put the same program on more than one menu.3.	an item on a WEB PAGE which, when selected, transfers the user directly to some other web page, perhaps on a different machine. Also called aHYPERLINK. For example see HTML.4.	in Windows, an OLE communication path between programs. See OLE.5.	to combine the machine instructions for a program with the machineinstructions for any predefined procedures that it uses. For example, a pro-gram that does trigonometric calculation might use predefined procedures to find sines, cosines, and tangents. Some compilers perform linking auto-matically; others require you to execute a linker as a separate command.6. a pointer in a linked list or tree. See LINKED LIST; TREE.FIGURE 155. Linked list link, dead seelinked list a way of organizing data items in a computer so that they are retrievable in a particular order that is not necessarily the same order as linked list	286 the physical locations in which they are stored. Each data item consists of two parts: the data itself, and a number giving the location of the next item. Figure 155 shows how this is usually diagrammed. To read the items in order, you need only know which item is in the beginning (the head) of the list; having located it, you can go next to the item whose address was stored with it; and so on.FIGURE 156. Linked list: inserting elementsLinked lists allow items to be added or removed without requiring that other items be moved to make room. For instance, the list A–D–E of Figure 155 can be changed into A–B–C–D–E by adding two items. As Figure 156 shows, the newly added items B and C can be placed in the unused area after the E, and inserted into the list by changing the address associated with item A.Figure 157 shows that an item can be deleted by changing the addresses so that there is no longer a path to that item. In either case, using linked lists can eliminate the need to move hundreds or thousands of data items whenever an insertion or deletion takes place.FIGURE 157. Linked list: deleting an elementFigure 158 shows a way to construct a linked list in an ordinary two-dimensional array; this can be done in practically any programming lan-guage. Each row of the array contains a data item and an integer indicating which row the next item is on (or zero, to indicate that there are more items). In the example, it is assumed that the first item in the list will always be in row 1; if you wish to be able to delete the first item, you can use a separate integer, outside the array, to keep track of where the list starts. See also DATA STRUCTURES.Item		Data		Address of						No.		item		next item						1		A		4	2		D		3	3		E		0	4		B		5	5		C		2	 FIGURE 158. Linked list stored in an array 287	LispLinkedIn a social networking site (www.linkedin.com) designed for busi-ness professionals to use for work contacts and communication.linker a program that puts separately compiled routines together to make a complete, working program. See LINK (definition 5).Linspire a commercial distribution of Linux, based on Debian and later Ubuntu, that was marketed by Linspire, Inc. (www.linspire.com). In 2008, the company’s name changed to Digital Cornerstone. See DEBIAN;LINUX; UBUNTU.Linux (usually understood as “Linus’ UNIX”) a freely distributed UNIX-compatible operating system for PCs and a number of other processors.Linux was developed by Linus Torvalds and others and is distributed under terms similar to those of Gnu’s “copyleft” (see GNU). Copies can be given away free provided they are complete and intact, but most users prefer to purchase commercially produced CD-ROMs containing Linux together with application software.Linux is quite reliable and highly compatible with UNIX; as a result, it is very popular with universities, Internet service providers, and small businesses that need multi-user computing at minimum cost. More infor-mation can be found on the World Wide Web at www.linux.org. See alsoDEBIAN; LINSPIRE; RED HAT; UBUNTU.Linux box (slang) a small computer running Linux.Lion common misspelling of LI-ION.liquid crystal display see LCD.Lisp (List Processor) a programming language developed in the late 1950s at MIT under the direction of John McCarthy. Because of the ease with which it can handle complex data structures, Lisp is used for artificial intelligence research and for writing programs whose complexity would render them unmanageable in other languages.A Lisp program is easy to recognize because of the accumulation of closing parentheses at the end of the program. All Lisp statements and most Lisp data structures are linked lists, written as lists of elements in parentheses (see LINKED LIST). Programmers build a complete program by defining their own statements (actually functions) in terms of those previ-ously defined. For example, a function that computes the factorial of X is:(DEFUN FACTORIAL (X) (IF (= X0)1(* FACTORIAL(-X 1))))Translating into English: “This is the definition of a function called FACTORIAL whose parameter is X. If X is zero, its factorial is 1; otherwise, its factorial is equal to X times the factorial of X – 1.” The IF keyword works like a Pascal if-then-else statement. This function calls itself recursively; recursion is a normal way of expressing repetition in Lisp. list	288list1.	a set of data items that are to be accessed in a particular order; for instance, a list of the students in a class might be accessed in alphabetical order. Lists are stored in arrays or linked lists. See ARRAY; LINKED LIST.2.	to display a program line by line (especially in BASIC).3.	a MAILING LIST to which messages are distributed by e-mail.list administrator the person responsible for maintaining a MAILING LIST.list box an area in a dialog box where the user can choose among a list of items, such as files, directories, printers, or the like. For an illustration, see DIALOG BOX.list processing1.	the manipulation of linked lists. See LINKED LIST; LISP.2.	the processing of mailing lists and similar data. See DATABASE MAN-AGEMENT.LISTSERV a commercial software package for operating e-mail mailing lists and discussion groups, produced by L-Soft International (www.lsoft.com). LISTSERV runs on a server, which can be a mainframe or microcomputer. The first version of LISTSERV was implemented by Eric Thomas on BITNET in 1986. The current version includes the abil-ity to filter out spam and viruses. Compare MAJORDOMO.Usage note: “LISTSERV” does not mean “e-mail list.” Not all e-mail mailing lists use LISTSERV software.literal in a programming language, a written representation that always represents the same value. For example, the literal 2.5 always stands for the number 2.5, and the literal “abc” always stands for the character string abc. Names defined by the programmer, such as variable and function names, are not literals.little-endian a system of memory addressing in which numbers that occupy more than one byte in memory are stored “little-end-first,” with the lowest 8 bits at the lowest address.For example, the 16-digit binary number 1010111010110110 occu-pies two 8-bit bytes in memory. On a little-endian computer such as the IBM PC, the lower byte, 10110110, is stored at the first address and the upper byte, 10101110, is stored at the next higher address. On a big-endian machine, the order is reversed. Contrast BIG-ENDIAN.The terms “big-endian” and “little-endian” are from Gulliver’s Travels; they originally referred to the parties in a dispute over which end of a boiled egg should be broken first.Live Microsoft’s collection of online services (e-mail, photo sharing, etc.; web address: www.windowslive.com/Home).Live Messenger an Instant Messaging application provided by Microsoft. A competing service is AOL’s AIM (AOL Instant Messenger). A key fea- 289	local variableture of IM programs is their ability to display your status, whether “Online” or “Away,” to your regular contacts. IM programs also allow you to designate “Friends” and provide the ability to block communica-tion with unwanted persons. IM messages are typically brief and heav-ily abbreviated. Icons are sometimes used to express emotions.Messaging programs are also becoming popular with businesses, especially when members of a tight-knit work group are traveling.LiveJournal a web site (www.livejournal.com) providing a popular web log service (see BLOG) that also provides basic SOCIAL NETWORKING fea-tures. Users can control whether their posts are public or only visible to defined FRIENDs. Compare BLOGGER; WORDPRESS; XANGA.ln the function, in several programming languages, that calculates the nat-ural (base e) logarithm of its argument. For example, ln(X) finds the nat-ural logarithm of X. See LOGARITHM; E.LN abbreviation for “like new” (describing items for sale).LN– abbreviation for “like new minus” (i.e., almost new, almost unused, showing only slight wear). Contrast EX+.LNIB abbreviation for “like new, in box” (i.e., slightly used but supplied with original packaging). Compare NOS (definition 2).load to transfer information from a disk or other outside device into the memory of a computer. Contrast SAVE. See also LOADER.loader a computer program whose function is to load another program into memory and transfer control to it. All operating systems include loaders. For example, in Windows, if you have a program named myfile.exe and you type the commandC:\> myfileyou are telling the loader to find myfile.exe and load it.local located at the user’s computer or site. Contrast REMOTE.local-area network (LAN) a network that connects several computers that are located nearby (in the same room or building), allowing them to share files and devices such as printers. See ETHERNET. Contrast WIDE-AREA NETWORK.local bus a separate bus in a computer, designed to provide extra-fast access to the CPU for specific devices, such as video cards. It contrasts with the main bus, which connects to most other parts of the computer. For examples see PCI, AGP.local variable a variable that has meaning only within a particular function, subroutine, or other program unit. The name of a local variable can be used in another subroutine elsewhere in the program, where it will refer localization	290to an entirely different variable. Local variables contrast with global variables, which are recognized throughout the program.The advantage of using local variables is not obvious in short pro-grams. However, it is a good idea when writing a long program to make as many variables as possible local, because then there will be no prob-lem if you wish to use the same name to mean something else elsewhere in the program. This rule is even more important if several different peo-ple are writing subroutines that will be combined into one main program.See also SIDE EFFECT.localization the process of adapting software to run in a particular part of the world. Localization might involve translating screen displays into French or German, adapting to a foreign-language keyboard, printing the date differently (e.g., 2009 Oct 21 in Japan vs. 21 Oct 2009 in Britain and Oct. 21, 2009 in the United States), setting the clock for daylight saving time on different dates, or even writing numbers differently (3,000.95 vs. 3 000.95 or even 3.000,95).lock1.	(on the Macintosh) to mark a file or disk as “Do not change” by click-ing on the “Locked” box in the “Get Info” window.2.	(in various operating systems) to mark a file as in use so that other programs running concurrently will not change it.log the function, in many programming languages, that calculates the nat-ural (base-e) logarithm of its argument. However, in some languages and spreadsheets, log(x) is the common (base-10) logarithm and ln(x) is the natural logarithm. See LOGARITHM.log in see LOG ON.log on, log in to identify yourself as an authorized user of a computer or a network at the beginning of a work session.logarithm the power to which a number must be raised in order to give another number.If y = ax, then x is the logarithm of y to the base a (written as x = loga y). The most commonly used bases for logarithm functions are 10 and e (approximately 2.718). Base-10 logarithms are called common loga-rithms; base-e logarithms, natural logarithms (because integrals and derivatives are simpler with base e than with any other base). For exam-ple, the common logarithm of 10,000 is 4 (log10 10,000 = 4) because 104 = 10,000.If no base is specified in the expression log a, then usually base 10 is meant; the natural logarithm of a is written loge a or ln a.logged drive see CURRENT DRIVE.logic circuits electronic circuits that accept binary digits (bits) as inputs and produce an output bit according to a specified rule. For examples see 291	logic circuits AND GATE; OR GATE; NAND GATE; NOR GATE; NOT GATE; FLIP-FLOP. For infor-mation on how logic circuits are used, see BINARY ADDITION; COMPUTERARCHITECTURE; DECODER; XOR GATE.A typical computer represents 1 (logic “true”) as +5 volts and 0 as 0 volts. More precisely, 1 is represented by a connection to the +5-volt power supply (directly or through a resistance), and 0 is represented by a connection to ground. Note that 0 is not merely the absence of a volt-age; logic circuits differ as to how they handle an unconnected input.Basically, logic circuits are switching circuits. Figure 159(A) shows a NOT gate implemented as a switch. The output is +5 volts (binary 1, logic “true”) whenever the switch is not closed. (When the switch is closed, the resistor dissipates the voltage and the output is connected to ground.) That is, the output is the negation of the state of the switch.FIGURE 159. NOT gate built with a switch (A) and a transistor (B)For this to be usable in a computer, the switching has to be controlled by an electrical signal. Figure 159(B) shows what happens when the switch is replaced by a switching transistor. The transistor conducts when its base is at least 0.6 volts above ground (i.e., when its input is binary 1). When the transistor is conducting, the effect is the same as the closed switch, and the output is 0. Thus, the output is the negation of the input, and the NOT gate works correctly.Figure 160 shows how to build a NAND gate out of two diodes, two resistors, and a transistor. This circuit is very similar to what is used inside TTL integrated circuits. The output is 0 (“false”) if and only if both of the inputs are binary 1 (+5 volts). In that situation, the diodes do not conduct, the base of the transistor receives current through the resis-tor, and the transistor conducts. But if even one of the inputs is binary 0 (connected to ground), the base of the transistor is held low and the tran-sistor does not conduct, so the output is binary 1. To understand this cir-cuit, it is very important to remember that binary 0 is represented by a logic circuits	292 connection to ground, not merely the absence of a voltage. Like real TTL ICs, this circuit happens to treat disconnected inputs as binary 1.NAND gates are important because all the other gates can be built from them (Figure 161). A NOT gate is simply a NAND gate with only one input, or with all its inputs tied together; an AND gate is a NAND gate followed by a NOT gate; and so on. In a similar way, all the types of gates can be built from NOR gates.FIGURE 160. NAND gate built with transistors and diodes FIGURE 161. Logic circuits made from NAND gates ELECTRONIC CIRCUIT293	LOGOInstead of TTL circuits, newer ICs use CMOS (complementary metal-oxide semiconductor) switching transistors, which come in pairs that respond to opposite polarities, so that one switches off whenever the other switches on. This makes it easy to connect the output either to +5 volts or to ground depending on the input. However, the circuits inside practical CMOS gates are too complicated to diagram here.logic diagram an electronic circuit diagram that shows gates and other components that affect logic signals, but does not show the power sup-ply or other non-digital electronic subsystems. SeeDIAGRAM SYMBOLS.logic programming a method of writing computer programs based on the mathematical study of logical reasoning. Logic programming is used in the computer modeling of human thinking. For examples, see PROLOG.logical1.	possessing or pertaining to logic (in any of various senses).2.	described from the viewpoint of software. For example, if a single disk drive is divided into two partitions which the computer handles sep-arately, it can be said to comprise two logical disk drives.logical design1.	the design of an electronic circuit using logic gates. See GATE and cross-references there.2.	the design of the logic of a computer program (as opposed to its user interface or data files).3.	the practice of designing a document by using tags to indicate the function rather than the appearance of each element. For example, chap-ters are labeled as such rather than just being indicated by words typed in a particular arrangement on the page.Logical design is the approach followed by LATEX, SGML, and XML; it is not followed by WYSIWYG word processors. Logical design is gener-ally superior for complicated documents because decisions about the appearance of elements of the document can be made independently of the text. If you want to change the appearance of chapter headings, for instance, you need to make the change in only one place because all chapter headings are recognized as instances of the same unit. In a WYSIWYG system, you would need to change each heading individu-ally because the computer does not know that they are alike. Documents with tags specifying the logical design are also easier to handle effec-tively in computer databases.logical drive one of several divisions of a single partition on a hard disk. Logical drives are treated as separate disk drives.logo a trademark or printed emblem; short for logotype.LOGO a programming language developed by Seymour Papert of MIT for use in teaching programming to children. Papert’s fundamental insight lol, LOL	294was that computer-aided instruction is of little use unless the pupil can control the computer, rather than the other way around. To experiment with this idea, he designed a language that is markedly easier to use than BASIC and does not share BASIC’s preoccupation with numerical calculation.Although LOGO offers a full range of computer functions, most ele-mentary LOGO exercises revolve around the “turtle,” originally a robot that rolled around on a sheet of paper making marks with a pen. (The present-day turtle is a triangle that moves around the screen, drawing a line if told to do so.) Drawing shapes with the turtle appeals to children who would not be attracted to mathematical calculation or verbal input-output; at the same time, it serves as a good medium for teaching geom-etry and logical problem solving.LOGO is an extensible language; that is, programs are constructed by defining statements in terms of previously defined statements. For exam-ple, the following procedure draws a square:TO SQUARECLEARSCREENFORWARD 50RIGHT 90FORWARD 50RIGHT 90FORWARD 50RIGHT 90FORWARD 50ENDThat is: “Clear the screen (and put the turtle in the center), go forward (up) 50 units, do a 90-degree right turn, go forward 50 units, do a 90-degree right turn,” and so forth. Since LOGO procedures can call them-selves recursively, complicated snowflake-like patterns are relatively easy to generate. See also KOCH SNOWFLAKE.lol, LOL online abbreviation for “laugh out loud.”long an integer with more bits stored than the normal-length integer. For example, in Java, a variable of type int fills 32 bits; a variable of typelong fills 64 bits (allowing 264 different values, ranging from –263 to 263 – 1 (approximately ± 9  1018).long cross the character †, a symbol used to mark footnotes. See also FOOT-NOTE. Also called a DAGGER or OBELISK.Longhorn internal code name used for Windows Vista (Windows 6.0) before its release. See WINDOWS (MICROSOFT). Compare BLACKCOMB; CAIRO; CHICAGO; MEMPHIS; WHISTLER.look and feel the overall visual appearance and USER INTERFACE of a com-puter program. See COPYRIGHT. 295	lurkloop1.	a series of statements in a computer program that are to be executed repeatedly. For examples see FOR and WHILE.2.	anything that receives electrical energy from a POWER SUPPLY.3.	(more fully, feedback loop) a control system in which one thing affects another, and its effect is sensed in order to make control deci-sions. For example, a heater, the air temperature, and a thermostat form a feedback loop. See also FEEDBACK; IN THE LOOP.loose letterspacing that has been adjusted to increase the space between the letters. Contrast TIGHT. See LETTERSPACING for an illustration.lost cluster a group of disk sectors that are not marked as free but are not allocated to a file. Lost clusters result when the operation of creating a file is interrupted. They waste space and should be cleaned up periodi-cally; under Windows this is done with the SCANDISK tool.Lotus 1-2-3 a popular SPREADSHEET program, widely used on IBM PCs since its introduction in 1983 by Lotus Development Corporation. Lotus is now part of IBM (web address: www-306.ibm.com/software/lotus). The original Lotus 1-2-3 contained significant innovations in graphing and data handling ability.lowercase the “small” letters a, b, c, d, and so on, as opposed to uppercase or capital letters, A, B, C, D, and so on. The term lowercase goes back to the early days of letterpress printing. The metal type was kept in divided drawers called cases; the capital letters were traditionally kept in the upper case, and the small letters in the lower.LPI (lines per inch) a measure of the resolution of a halftone screen (see HALFTONE). Most newspaper screens are 85 LPI; good quality magazines use 150 LPI. 300-DPI screened output is roughly equivalent to a 50-LPI screen (draft quality). 600 DPI on a plain paper typesetter should be acceptable for most work; it can produce the equivalent of a 100-LPI halftone. When higher resolutions are needed, the file should be output to a 2400-DPI imagesetter.LPT1 the filename by which Windows refers to the first parallel printer port. Additional parallel ports are known as LPT2 and LPT3.luminosity brightness; the property of glowing with light. Some 3D pro-grams can render objects that seem to be emitting light by setting a high luminosity level.lurk (slang) to read an online forum regularly without contributing any messages of your own. It’s advisable to lurk for a while before posting any messages in order to make sure you understand the purpose and nature of the discussion. Most forums have more lurkers than the partic-ipants realize. Mac	296MMac nickname for MACINTOSH.MAC address (Media Access Control address) a built-in number that uniquely and permanently identifies a network adapter, such as the Ethernet card in a PC. It consists of 12 hexadecimal digits, which may be written with or without hyphens, such as 13-24-6C-2D-FF-3A or 13246C2DFF3A.Under Windows 2000 and later, the MAC address can be displayed by typing ipconfig /all at a COMMAND PROMPT.Contrast IP ADDRESS, which is assigned manually or automatically by network administrators. See also GUID.Mac OS the operating system for Macintosh computers; the latest version is version X (ten). It is based, in part, on the UNIX operating system. As a matter of fact, OS X runs BSD UNIX in its command line window. The foundation of Mac OS X is very reliable and solid; its user interface is still uncluttered, consistent, and easy to use. The elegantly simple idea of choosing an object and then telling the computer what to do with it has been carried through all versions of the Mac operating system, desk-top accessories, and third-party applications. The idea was even adopted by the PC world (see WINDOWS). Today, a person familiar with one oper-ating environment can pretty much sit down at any computer and, within minutes, be doing productive work.The minor updates to OS X have been given the names of members of the big cat family:Jaguar	v10.2Panther	v10.3Tiger	v10.4Leopard	v10.5Mac OS X Snow Leopard, the next upgrade, is scheduled to ship in 2009.See APPLE MENU; CLASSIC MODE; DOCK; FINDER.machine-dependent program a program that works on only one particu-lar type of computer.machine-independent program a program that can be used on many dif-ferent types of computers. The usual way to make a program machine-independent is to write it in a widely used programming language, such as C or C++, and compile it separately for each machine. A Java program is machine-independent because it is compiled to a standard bytecode, which can be run using the Java virtual machine (JVM) available for each specific machine. 297	Macintoshmachine language instructions that a computer can execute directly. Machine language statements are written in a binary code, and each statement corresponds to one machine action.The difference between machine language and assembly language is that each assembly-language statement corresponds to one machine-lan-guage statement, but the statements themselves are written in a symbolic code that is easier for people to read. (See ASSEMBLY LANGUAGE.) A sin-gle statement in a high-level language such as C may contain many machine instructions.Macintosh a family of personal computers introduced by Apple in 1984; the first widely used computers with a graphical user interface, windowing, and a mouse. The Macintosh user interface was derived from that of Xerox workstations; it has been imitated by a number of other operating systems, including Microsoft Windows and OS/2 Presentation Manager.The mechanisms for using windows, icons, and mouse menus are pro-vided by the operating system, which means they look virtually the same in all programs. Thus, anyone who knows how to use any Macintosh software package will also know how to perform similar operations in any other software package. Macintosh hardware is simple to set up because of Apple’s early commitment to widely recognized standards such as PostScript, PDF, and SCSI.There have been three generations of Macintosh hardware. The orig-inal Macintosh used the Motorola 68000 family of microprocessors. In 1994, Apple changed to the PowerPC microprocessor, and in 2006, to the Intel Pentium.Macintoshes have always been on the forefront of practical computer graphics and related technology (for example, TrueType scalable fonts and QuickTime video), thus making them the preferred platform for the commercial arts.The Macintosh uses BSD UNIX as its command-line mode and can run UNIX software without modification. This makes the Macintosh popular with scientists and programmers, which complements Apple’s original markets of graphic designers and office workers.Although the selection of available software is smaller than with PCs, Apple maintains a loyal and vocal following for the Macintosh. The computers perform well and the Mac user community is close-knit. This may be one reason Macintosh computers are not as plagued with viruses as the Windows community.Apple currently offers a variety of Macintosh computers, giving their customers a wide variety of solutions for their computing needs. There are two basic lines of laptops: iBooks and Powerbooks (student versus professional models, respectively). As for desktop-style computers, Apple offers a traditional high-end line of computers called Power Macs, but also offers the iMac, eMac, and Mac mini. The iMac is a very ele-gantly designed multi-use computer. eMacs were designed as a lower-cost option to the iMac, but are adequate for most computing tasks. The macosx	298Mac mini is very small and easily transported. It can quickly hook up to any available monitor, mouse, and keyboard. The Mac mini is ideal for PC-users who also need access to a Macintosh computer.The current version of the Macintosh operating system is MAC OS X(read “ten,” not “x”). See also MICROPROCESSOR; POSTSCRIPT; POWERPC;QUICKTIME; SCSI; TRUETYPE FONT.macosx [sic] incorrectly spaced and capitalized abbreviation for Mac OS X, the tenth version of the Apple Macintosh’s operating system. SeeMAC OS.macro a user-defined sequence of instructions for a computer.In assembly language and in programming languages such as C, macros are user-defined abbreviations for sequences of program state-ments. When the program is compiled, each occurrence of the macro is replaced by the instructions for which it stands. This contrasts with a FUNCTION, PROCEDURE, or METHOD, which is stored in one place in mem-ory and called by the main program every time it is needed.In application programs, macros are user-defined sequences of oper-ations, which can be assigned to specific keys, placed on menus, or com-bined with pre-existing operations such as Open and Close. In Microsoft Office applications, you can use the Macro Recorder to save a sequence of keystrokes or mouseclicks as a macro, or you can program a macro inVISUAL BASIC.macro assembler any program that translates assembly language programs into machine code (see ASSEMBLY LANGUAGE) and allows the program-mer to define macro instructions (see MACRO).macro virus a virus written using the macro language of a particular appli-cation. For example, if a Microsoft Word document contains a macro virus that is designed to execute when the file is opened, an unsuspect-ing user who downloads the file and then opens it with Word will suffer the consequences of whatever the virus is programmed to do. Macro viruses are particularly dangerous because they can hide in word pro-cessing documents. Formerly, viruses could only be placed in executable code. Contrast TROJAN HORSE; VIRUS.Macromedia producer of software to enhance the audiovisual content of web pages, including Dreamweaver, Flash, and Freehand. Macromedia was acquired by Adobe in 2005.MAE (Metropolitan Area Exchange) a major connecting point where Internet service providers connect to the Internet. There are several MAEs in the U.S., divided into regions (MAE East, MAE Central, and MAE West). Web address: www.mae.net.magenta a purplish-red color that is one of the standard printing ink colors.See CMYK. 299	mainframe computer magic number (slang) an important number (such as an interest rate or a file size limit) buried deep within a computer program where those revising the program are likely to overlook it.This is a bad programming practice; instead, important numbers should be defined prominently near the beginning of the program.magic wand an editing tool that selects an entire area of a particular color, regardless of its shape; magic wands are found in many photo editing programs (Adobe Photoshop, Aldus Photostyler, Corel PhotoPaint). You use the magic wand to select an area for editing. Its power lies in its abil-ity to do a lot of tedious work for you. When you click on a pixel, the magic wand selects an area of that particular color, no matter how jagged the edges. You can then copy, delete, move, rotate, flip, shrink, stretch, or apply filters to this area as if it were a single object. See alsoBITMAP; PAINT PROGRAM; SELECT; SELECTION TOOLS.FIGURE 162. Magic wand selection toolmail see ELECTRONIC MAIL.mail bombing the practice of trying to flood an obnoxious person with gigantic amounts of e-mail. This is a very bad idea for several reasons. It clogs up facilities needed by other people, not just the intended recip-ient. More importantly, people who act obnoxiously on the Internet gen-erally falsify their addresses, thereby bringing down floods of wrath upon innocent victims. See DENIAL-OF-SERVICE ATTACK.mail merge see MERGE.mailing list an online discussion conducted by relaying copies of all mes-sages to all the participants by ELECTRONIC MAIL. Mailing lists are prefer-able to NEWSGROUPs when the group of interested people is relatively small or the discussion would be heckled if it were open to the general public. See LISTSERV; MAJORDOMO. See also NETIQUETTE.mainframe computer a large computer occupying a specially air-condi-tioned room and supporting hundreds of users at one time. The IBM 370 and IBM 3090 are examples of mainframe computers. Contrast MINI-COMPUTER; PERSONAL COMPUTER. Majordomo	300Majordomo a free, open-source software package for operating e-mail mail-ing lists and discussion groups, distributed from www.greatcircle.com. Majordomo is written in Perl and runs primarily on UNIX systems. (The major-domo is the head servant in an aristocratic household.) CompareLISTSERV.make a command, in UNIX and similar operating systems, that manages the steps of creating a machine-language program or some other com-plex product of computation.Typically, a large machine-language program is made by compiling sev-eral different source files, producing a group of object files, and then link-ing the object files together. (See SOURCE CODE; OBJECT CODE.) The make command manages this process. It looks at a makefile (Figure 163) that tells it how to create each of the files needed to generate the complete pro-gram. Then it looks at the date on which each file was last modified. If any file is newer than the other files made from it, make will do whatever is needed to update those files (typically compiling or linking). By using make, the programmer avoids recompiling anything that has not been changed.The make command can actually manage any process in which files are made from other files. All it needs is a makefile containing the appro-priate commands.#	Each entry consists of:#	A file#	A list of other files that file depends on#	A command to generate it from them#	Each indented line must actually begin with#	the Tab character (ASCII 9), not spaces.#myprog:	myprog1.o myprog2.occ myprog1.o myprog2.o -o myprog#myprog1.o:	myprog1.ccc -c myprog1.c#myprog2.o:	myprog2.ccc -c myprog2.cFIGURE 163. Makefilemakefile a file that controls the operation of the MAKE command. Under UNIX, by default, it is named makefile or Makefile and resides in the current directory.malware malicious software. For examples see ADWARE; SPYWARE; VIRUS.man pages (manual pages) the online documentation built into UNIX and accessed by the commandman command 301	Map Network Drivewhere command is the command or system function you want to know about. A selling point of UNIX since the earliest days has been that its manuals are online. See UNIX.management information systems (MIS) a field of study that deals with effective systems for the development and use of information in an orga-nization. The complete information system includes not just the com-puters but also the people. Any effective information system must determine:1.	what the goals of the organization are;2.	what information is needed to accomplish those goals;3.	how that information is originated;4.	how the information needs to be stored and transferred to accom-plish those goals.Mandelbrot set a famous fractal (i.e., a shape containing an infinite amount of fine detail). It was discovered by Benoit Mandelbrot. The Mandelbrot set is the set of values of c for which the series zn+1 =(zn)2 + c converges, where z and c are complex numbers and z is initially (0, 0).See COMPLEX NUMBER.The detail in the Mandelbrot set fascinates mathematicians. In Figure 164, the x and y axes are the real and imaginary parts of c. The Mandelbrot set is the black bulbous object in the middle; elsewhere, the stripes indicate the number of iterations needed to make |z| exceed 2.FIGURE 164. Mandelbrot setmanifest a list of the contents of a shipment; a list of files transmitted as a group.Map Network Drive the operation, in Windows, that makes a directory on another computer act as if it were a local disk drive. To map a network drive, right-click on the Computer icon and select the Map Network Drive menu item. You will need to specify the server name, directory to map, and the drive letter to use. See also UNC. GUI; MOUSE; SELECT.Mapquest	302Mapquest a web site (www.mapquest.com) that allows users to create cus-tomized maps or find directions to specific addresses. Mapquest is now a subsidiary of AOL.marching ants (slang) the moving dashed lines that indicate the borders of a selected object in a paint or draw program (see MARQUEE SELECT). Some programs allow you to hide the ants if they distract you.markup language any language that provides ways to indicate underlin-ing, italics, paragraph breaks, section headings, and so on, in text. For examples see HTML; SGML; TEX.marquee select a method of selecting more than one object at a time in a graphical user interface (GUI). It gets its name from the animated effect of the dashed line of the bounding box—it resembles a theater marquee.To marquee-select items, sight along the top and the left edge of the group of items you wish to select. Position the mouse cursor there. While holding down the mouse button, pull diagonally down and to the right. When the marquee encloses all the items, release the mouse button. SeealsoFIGURE 165. Marquee selectmashup a combination of two or more works to create an original deriva-tive work. They are most commonly seen in music, where a famous example is DJ Danger Mouse’s Grey Album, a mashup of the Beatles’ “White Album” and Jay-Z’s “Black Album.”mask1.	(in draw programs) to create an object with a hole in it, so that the view of an underlying object is controlled.2.	(in paint programs) to mark an area of the drawing as protected from the drawing tools. The mask can be removed as the drawing progresses. This is analogous to the masking used in watercolor painting.3.	(in programming) to isolate part of a binary number by ANDing it with another binary number. For example, the first four bits of any byte can be isolated by ANDing the byte with 11110000. See AND GATE; SUB- NET MASK. 303	mediaFIGURE 166. Maskmaster1.	the controlling unit in a pair of linked machines. Contrast SLAVE (def-inition 1).2.	one of a pair of IDE hard disks or other devices connected to the same IDE cable. Generally, jumpers have to be set on IDE devices to identify them as master and slave. Contrast SLAVE (definition 2).master browser see BROWSE MASTER.master page a design template that defines the overall appearance of every page of a printed document. See also GRID SYSTEM.MathML (Mathematics Markup Language) an extension of HTML designed to facilitate the display of mathematical expressions. (See the W3C web site at www.w3.org.)matrix see ARRAY.maximize to make a window take over the whole screen or become as large as possible. To do this, click the mouse on the maximize button (see pic-ture at WINDOW). See also MINIMIZE; RESTORE. Maximize is also an option under the CONTROL MENU.On a Macintosh, use the ZOOM box (at the far right side of the win-dow’s title bar) to enlarge a window.MB abbreviation for MEGABYTE.MBps megabytes per second. See also BAUD; MEGABYTE.Mbps megabits per second. See BIT.MCSE (Microsoft Certified Software Engineer) a credential for computer professionals who pass a series of proficiency exams from Microsoft.MDI (multiple document interface) the ability to edit more than one file or drawing with a single copy of a Windows program.meatware (slang, humorous) computer users (the parts of a computer sys-tem that are made of meat). Compare PEBKAC.mebi- proposed metric prefix meaning 1,048,576 (220), the binary coun-terpart of mega-. See METRIC PREFIXES.mebibyte 1,048,576 bytes.media plural of MEDIUM. WINDOWS (MICROSOFT).Media Center Edition	304Media Center Edition seemedia error a defect in the surface of a disk or tape, sometimes curable by formatting the disk or tape again.media, invalid see INVALID MEDIA.medium (plural media)1.	material used for storage of information. Magnetic disks, tapes, and optical disks are examples of storage media.2.	a way of presenting information to the computer user. Vision is one medium; sound is another. Multimedia computing uses visible displays of several types together with sound.3.	a means of mass communication, such as television.medium-scale integration the construction of integrated circuits that con-tain from 10 to 100 logic gates. See INTEGRATED CIRCUIT.meg short for MEGABYTE.mega- metric prefix meaning 1,000,000 or, in rating computer memories and disks, 1,048,576 (= 1024  1024). Mega- is derived from the Greek word for “big.” See MEGABYTE; METRIC PREFIXES.megabyte (MB) an amount of computer memory equal to 220 = 1,048,576 bytes = 1,024 kilobytes. One megabyte can store more than one million characters. See MEMORY.In measuring disk capacity, megabyte sometimes means the same as when measuring memory. Sometimes, however, megabyte is used to mean 1 million bytes, or even 1,024,000 bytes (1024  1000). These dif-ferent systems of measurement result in different numbers being used to describe the capacity of the same disk. See also GIGABYTE.megahertz (MHz) million hertz or million cycles per second, a measure of the clock speed of a computer or the frequency of a radio signal. SeeCLOCK; MICROPROCESSOR.megapixel one million PIXELs, a measure of the size of a graphical image. For example, a 1024  1024-pixel image is often referred to as a megapixel. Table 9 shows how many megapixels are needed for various levels of image quality. See also DIGITAL CAMERA; EFFECTIVE MEGAPIXELS;GROSS MEGAPIXELS; PAINT PROGRAM.meme an idea or belief that spreads quickly from person to person, like an infection, whether or not it is true.The term meme was introduced by evolutionary biologist Richard Dawkins in 1976 to denote important ideas, skills, or habits that are passed along from person to person almost like genes. On the Internet, however, the term is usually applied to sweeping fads. 305	menuTABLE 9MEGAPIXELS AND IMAGE QUALITYMegapixels	Typical use		0.2–0.5	Picture on a web page0.5–1.5	Snapshot or small photograph1.5–3.0	Sharp full-page photographic print3.0–8.0	Image sharp enough that portions of it will	remain sharp when extracted and enlarged16	Equal to the very sharpest photographs		memory (formerly called core) the space within a computer where infor-mation is stored while being actively worked on. Most microcomputers have a small amount of read-only memory (ROM), containing the built-in programs that start the operation of the computer when it is turned on, and a large amount of random-access memory (RAM) for user’s pro-grams and data. Except for ROM, memory goes blank when the com-puter is turned off; any data in it must be copied to the hard disk, a CD, or a USB flash drive in order to be saved.See also DRAM; EDO; RDRAM; SDRAM; SIMM.memory leak see LEAK.MemoryStick a type of flash-memory non-volatile storage device similar to CompactFlash but physically long and thin, developed by Sony Corporation. Compare COMPACTFLASH; FLASH MEMORY CARD; MULTI-MEDIACARD; SECURE DIGITAL CARD; SMARTMEDIA.Memphis Microsoft’s internal code name for the Windows 98 development project. Compare BLACKCOMB; CAIRO; CHICAGO; LONGHORN; WHISTLER.menu a list of choices that appears on the screen in response to your actions. Most windows have a MENU BAR just under the title bar. When you click on an item in the menu bar, its corresponding menu will appear. You select the command you want by moving the mouse pointer to it. Commands with ellipsis dots (. . . ) after them will pop up a dialog box for you to give the computer further instructions before executing the command. If there are keyboard shortcuts for any command, they will often be listed to the right of the command. FIGURE 167. Menu menu bar	306menu bar a horizontal menu across the top of the screen or window. Depending on the software, the items in the menu bar are chosen by clicking on them with a mouse, or by typing the first letter of each item, or possibly by typing the first letter while holding down Alt. Usually each item is a further menu. For an illustration, see MENU.merge to insert data (e.g., names and addresses) from one file into a docu-ment that resides in another file (e.g., the text of a form letter).merge sort an algorithm for sorting data (see SORT). Merge sort takes advantage of the fact that it is easy to combine two lists that are already sorted; just keep looking at the first element of each list and taking whichever element comes first. For example, to combine the listsAdams	BushBuchanan	ClintonLincoln	Kennedydo the following: Compare Adams to Bush; take Adams. Then compare Buchanan to Bush and take Buchanan. Then compare Lincoln to Bush and take Bush, and so on. This will give you a list of all six names in alphabetical order.To perform a complete merge sort, first divide your data into several small sorted lists. These can be sorted with some other sorting algorithm; or they can be two-element lists which are sorted by swapping the two elements where needed; or they can even be one-element lists, which do not need sorting. Then combine these lists, two at a time, until they all have been put together into a single sorted list.A big advantage of merge sort is that you never need to see more than the first element of any list. Thus, merge sort can take its data from tapes or from linked lists, which cannot easily be sorted by any other algo-rithm. See LINKED LIST; SEQUENTIAL-ACCESS DEVICE.mesh network a network with multiple paths connecting nodes. Devices on the network cooperate to determine routing of messages. It can func-tion when a node fails by rerouting messages along paths that bypass the failed node. The Internet is one example.message board a web page, NEWSGROUP, or dial-up computer system where users can post messages and reply to messages posted by others. Messages are usually public and visible to all users. Most bulletin boards associate replies with the original messages, creating threads. See alsoFLAME; NEWSGROUP; POST; THREAD.message box a small window that appears to present information to the user (Figure 168). When the user acknowledges reading the message by pressing a mouse button, the message box disappears.message sending one way of describing object-oriented programming: objects receive “messages,” which are calls to procedures associated with them. See OBJECT-ORIENTED PROGRAMMING. 307	metadataFIGURE 168. Message boxmeta tag a piece of information added to a WEB PAGE for the benefit of indexers, SEARCH ENGINEs, web page creation software, or other pur-poses. Meta tags are not displayed when the page is viewed.Most meta tags consist of a name and a content field. Some names are widely recognized. Here are some examples:<meta name=”author” content=”Catherine Anne Covington”> <meta name=”keywords” content=”oranges, apples, lemons”> <meta name=”robots” content=”noindex, nofollow”>The first identifies the author as Catherine Anne Covington; the second asks search engines to index the page under oranges, apples, and lemons; and the third asks search engines not to index the page at all, nor follow its links.It is entirely up to a search engine to decide whether or not to comply with these requests. Also, meta tags are often used deceptively to bring in additional visitors to a site by making it come up when they search for something else. In a few extreme cases, pornography sites have been known to put an entire dictionary of the English language in the key-words meta tag.Another type of meta tag adds information to the header sent with the HTTP file; this information helps control the browser. For example,<meta http-equiv=”charset” content=”iso-8859-1”>tells the browser to use the ISO-8859-1 character set, and<meta http-equiv=”Refresh” content=”15”> <meta http-equiv=”Refresh” content=”15;URL=http://www.xyz.org”>say, respectively, to refresh (reload) the web page after 15 seconds and to jump to www.xyz.org after 15 seconds.metacrawler a program that submits search queries to the major Internet indexes (such as Google, Yahoo, or Lycos) and creates a summary of the results, thus giving the searcher the benefit of using all the SEARCH ENGINEs simultaneously.metadata information about information, such as information about the origin, format, or ownership of a data file. metafile	308metafile a file format that provides a common ground between two or more proprietary formats, and thus a translation path from one piece of soft-ware to another. For example, a Windows Metafile (.wmf) is a vector-based drawing format that is recognized by most drawing, page-layout, and word-processing programs. See also CGM.metal see BARE METAL.meta language any language used to describe another language. For exam-ple, Backus-Naur Form can be used as a metalanguage to describe the syntax of programming languages. Lisp and Prolog have the interesting property that the programs can read and modify themselves, so these programming languages can be put to practical use as metalanguages for themselves. See BACKUS-NAUR FORM; LISP; PROLOG.method (in object-oriented programming) a procedure associated with an object type. See OBJECT-ORIENTED PROGRAMMING.		TABLE 10		METRIC PREFIXES		Prefix	Abbreviation	Meaning		yotta-		1,000,000,000,000,000,000,000,000zetta-		1,000,000,000,000,000,000,000exa-		1,000,000,000,000,000,000peta-		1,000,000,000,000,000tera-	T	1,000,000,000,000 (or 1,099,511,627,776)giga-	G	1,000,000,000 (or 1,073,741,824)mega-	M (not m)	1,000,000 (or 1,048,576)kilo-	k	1000 (or 1024)hecta-		100deca-		10deci-	d	÷10centi-	c	÷100milli-	m (not M)	÷1000micro-	µ	÷1,000,000 (unofficially abbreviated u)nano-	n	÷1,000,000,000pico-	p	÷1,000,000,000,000femto-	f	÷1,000,000,000,000,000atto-	a	÷1,000,000,000,000,000,000		metric prefixes the prefixes used in the metric system to denote multipli-cation of units, such as kilo- meaning “thousand.” The complete set is shown in Table 10.When measuring computer memory or disk capacity, kilo-, mega-, giga-, and tera- often stand for powers of 1024 (= 210) rather than pow-ers of 1000; those are the alternative values shown in the table. Even more confusingly, mega- occasionally means 1000  1024 = 1,024,000. 309	microprocessorIn 1998, the International Electrotechnical Commission (IEC) pro-posed the “binary” metric prefixes kibi-, mebi-, gibi-, and tebi- to be used in place of kilo-, mega-, giga-, and tera- where powers of 1024 rather than powers of 1000 are meant.In Greek, kilo-, mega-, giga-, and tera- mean, respectively, “thou-sand,” “big,” “giant,” and “monster.”mezzanine bus a special bus that connects the CPU to some of the faster peripherals, such as memory modules, and is separate from the slower bus used for slower peripherals. See BUS.MFP (Multi Function Printer) a printer that also performs other functions such as faxing and scanning.MHz see MEGAHERTZ.MIB (describing items for sale) mint in box (i.e., new and never unpacked). See also EBAY; NRFB.micro- metric prefix meaning ÷1,000,000 (one millionth). For example, a microsecond is one millionth of a second (0.000001 second), and a microfarad is one millionth of a farad. Micro- is derived from the Greek word for “small.” See METRIC PREFIXES.microblogging publishing short descriptions of your current activity to a blog or web site designed for microblogging, such as Twitter, often from a phone or other mobile device. Status and mood updates on web sites such as Facebook and LiveJournal can be considered a form of microblogging.microbrowser a web browser using less memory than a conventional browser so it can be used on a small device such as a palmtop.microcomputer a computer whose CPU consists of a single integrated cir-cuit known as a microprocessor. Commonly, a microcomputer is used by only one person at a time. All home computers are microcomputers. SeeINTEGRATED CIRCUIT; MICROPROCESSOR.microcontroller a microprocessor designed specifically for controlling equipment. Microcontrollers usually contain some memory and input-out-put circuitry on the same chip with the microprocessor. This enables the microcontroller to work as a self-contained unit. Microcontrollers are used in consumer products such as telephones, automobiles, and microwave ovens, as well as in industrial equipment. See also EMBEDDED SYSTEM.Microdrive see COMPACTFLASH.microprocessor an integrated circuit containing the entire CPU of a com-puter, all on one silicon chip, so that only the memory and input-output devices need to be added. The first popular microprocessor, the Intel 8080, came out in 1973 and cost approximately $400.Microprocessors are commonly described as 16-bit, 32-bit, or the like. The number can refer either to the number of bits in each internal Microsoft 310data register, or to the number of bits on the data bus (see BUS); these two numbers are usually the same. Other things being equal, larger registers and a larger bus enable the processor to do its work faster.Clock speed is also important. The clock is the oscillator that causes the microprocessor to proceed from one step to the next in executing instructions. (Each machine instruction takes several clock cycles.) Clock speed is measured in megahertz (MHz) or gigahertz (GHz): 1 MHz is 1 million cycles per second, and 1 GHz = 1000 MHz. Higher clock speeds result in faster computation, but only if exactly the same machine instruc-tions are being executed; it is misleading to compare the clock speeds of processors of different types. It is even possible for two microprocessors with the same instruction set and clock speed to perform computations at different rates because of differences in internal design. A 100-MHz Pentium II, for example, is faster than a 100-MHz Pentium.Microsoft the world’s leading software-producing company, headquartered in Redmond, Washington, and founded by Bill Gates and Paul Allen in 1975 when they wrote a version of BASIC for an early hobbyist micro-computer, the Altair. In the late 1970s the company grew as it sold ver-sions of BASIC to other computer makers, but it was still fairly small when it was approached by IBM to design the operating system for the IBM PC (released in 1981).This operating system (known as PC-DOS, MS-DOS, or simply DOS) became a huge seller, since almost all PC and PC clones used it. In 1990 Microsoft released version 3.0 of Windows, providing a graph-ical user interface for the PC. Updated versions of Windows continue to be the most common operating system on the PC (see WINDOWS).Microsoft is also a big seller of applications software for both PCs and Macintoshes. The suite Microsoft Office includes the word process-ing program Microsoft Word, the spreadsheet Excel, the database pro-gram Access, and the presentation program Powerpoint. In 1991 Microsoft introduced Visual Basic, an advanced version of BASIC that allows the programmer to take advantage of the graphical environment of Windows. It now provides development environments for other lan-guages in the Visual Studio series. Other products include the Microsoft Network Online service, the web browser Internet Explorer, and X-box game machines.For further information about Microsoft and its products, see www.microsoft.com.Microsoft antitrust charges a series of official accusations that Microsoft has violated U.S. antitrust laws (laws that prohibit businesses from monopolizing a market or conspiring to stifle competition).In 1994, while under investigation by the U.S. Department of Justice, Microsoft accepted a consent decree in which it promised to change some of its licensing practices. For example, prior to 1994, Microsoft gave computer manufacturers a discount on Windows if the manufactur-ers paid Microsoft a fee for every computer made, regardless of the oper- 311 MIDIating system installed on it. In effect, this amounted to requiring them to license a Microsoft operating system for every computer.In 1998, the U.S. Department of Justice filed a broad antitrust case against Microsoft. Among other accusations, they charged that Microsoft violated antitrust laws by including its Internet Explorer browser free with Windows, because this killed off the market for other browsers (e.g., Netscape) that were available commercially. In 2000, Judge Thomas Penfield Jackson ruled that Microsoft had violated the Sherman Antitrust Act, and ordered that the company be broken up. In June 2001, an appeals court affirmed that Microsoft was guilty of monopolizing, but they vacated the order that the company be broken up and instead sent the case back to a different federal judge. The company and the U.S. Justice Department reached a negotiated settlement in November 2001. In the settlement, Microsoft agreed not to retaliate against hardware makers that worked with other software makers, and Microsoft’s behavior would be subject to monitoring. Some states felt that the settlement with the Justice Department was too lenient to Microsoft, but in November 2002 the court approved the settlement with minor modifications.Microsoft has also settled private antitrust suits with some of its com-petitors.The case has raised concerns because the main U.S. antitrust statute (Section 2 of the Sherman Antitrust Act) is so vague that it does not spec-ify exactly which behavior is illegal. Some feel that Microsoft is being punished for technological innovation and financial success.Microsoft has also been fined by the European Union for antitrust violations and it is still under investigation for further antitrust charges in Europe.Microsoft Windows see WINDOWS.middleware1. in a three-tier system, the system that is between the user interface and the database access software. See THREE-TIER ARCHITECTURE.2. software that occupies a middle position between application soft-ware and the operating system, so that programs written for the compat-ible middleware can run under different operating systems without change. Examples of middleware include web browsers and well-stan-dardized programming languages (such as FORTRAN and Java). Because of middleware, programs and web pages can be written so that they run identically on completely different computers.MIDI (Musical Instrument Digital Interface) a standard way for commu-nicating information about music between different electronic devices, such as computers and sound synthesizers.MIDI comprises two things: an electrical connection between musi-cal instruments and computers, and a file format for representing musi-cal sounds. MIDI files are more like musical scores than digitized audio; they represent notes and instrumentation, not sound waves, and the com- .mil 312puter must “play” them like a musician. Thus, MIDI files are much more compact than WAVE FILEs..mil a suffix indicating that a web or e-mail address belongs to a military site in the United States. Along with .com, .edu, .gov, .int, .net, and .org, this is one of the original set of Internet top-level domains. Contrast.COM. See also ICANN; TLD.millennium bug a software defect reflecting the Year 2000 Problem. SeeYEAR 2000 PROBLEM.milli- metric prefix meaning ÷1000 (one thousandth). For example, one millisecond is one thousandth of a second (0.001 second), and a mil-limeter is one thousandth of a meter. Milli- is derived from the Latin word for “thousandth.” See METRIC PREFIXES.MIME (Multipurpose Internet Mail Extensions) a standard proposed by N. Borenstein and N. Freed for including material other than ASCII text in e-mail messages. MIME messages can be recognized by the “Content-Type” declaration in the header, and the range of content types is poten-tially unlimited. Most e-mail software now supports MIME encoding.See also CGI.mind virus see MEME.mini-CD, mini-DVD a CD or DVD that is 8 cm in diameter (about 3 inches) instead of the usual 12 cm (43⁄4 inches).minicomputer a computer intermediate in size between a mainframe com-puter and a microcomputer; two classic examples were the Digital Equipment Corporation VAX and the IBM AS/400. A minicomputer typi-cally occupied a large area within a room and supported 10 to 100 users at a time. In recent years minicomputers have been replaced by networks of microcomputers. Contrast MAINFRAME COMPUTER; PERSONAL COMPUTER.minimize to make a window as small as possible; usually this means it becomes an icon rather than a window. In Windows minimized icons go to the taskbar at the bottom of the screen.To minimize a window, click on the minimize button (see WINDOW). This is a handy way to get one piece of software out of the way tem-porarily while you turn your attention to something else. You can then RESTORE the window when you want to resume working with it.Caution: the minimized program is still taking up memory and you’ll find that graphics intensive programs (such as paint programs) need all the memory you can give them. You should CLOSE all minimized pro-grams before launching a program that needs lots of memory. Some util-ities can run minimized in the background (e.g., print spoolers). See alsoLAUNCH; MAXIMIZE; RESTORE.mint (describing items for sale) perfectly preserved, in new condition. 313 miterminty (describing items for sale) almost, but not quite, MINT—a vague term that encourages optimism but makes no precise claims.MIPS (Million Instructions Per Second) a measurement of the speed with which computer programs are run. Because different instructions take different amounts of time, speed measured in MIPS depends on the exact program that the computer is running. For this reason, speed tests are usually done with standard programs such as Whetstone and Dhrystone.Another problem is that equivalent programs take different numbers of instructions on different CPUs. To compare different computers meaningfully, it is common practice to calculate MIPS using the number of instructions that a program would require on a VAX rather than on the computer actually being tested. This way, equivalent programs are always viewed as having the same number of instructions and the speed of the computer under test is the only variable.mirror1. to flip an image so that the resulting image is a mirror image of the original.2. to reproduce the entire contents of an FTP or WEB SITE so that the same files are available from more than one location.3. to maintain an extra copy of a disk drive on a second disk drive auto-matically. See RAID.FIGURE 169. Mirror, definition 1mirroring the practice of maintaining a copy of data that is automatically kept up to date. For an example see RAID.MIS see MANAGEMENT INFORMATION SYSTEMS.misfeature (slang) an ill-conceived feature; a feature with unforeseen and unfortunate effects. For example, the choice of / as the option character in DOS 1.0 was a misfeature because it made it impossible to use / as the directory separator when DOS 2.0 introduced directories. As a result, DOS and Windows use \ where UNIX uses /, to the annoyance of pro-grammers everywhere.miter to cut at an angle. A way of specifying how lines should intersect; mitered joints come to neat points. If intersecting lines are not properly mitered, there are ugly gaps at the intersection, or the square endpoints of the lines overlap.Most drawing programs let you set the miter limit; the threshold at which the computer bevels a sharp angle when two lines have a narrow angle of intersection. This prevents the pointed joint from extending way past the end of the line. VECTOR PROCESSOR.mixed case 314FIGURE 170. Mitered vs. unmitered jointmixed case type set with normal capitalization, neither all caps nor all low-ercase.mixed-signal handling both ANALOG and DIGITAL signals. For example, a sound card is a mixed-signal device.mixer a software control that determines the relative loudness of various kinds of sound produced by a SOUND CARD, such as MIDI music, wave file playback, and synthesized speech.MMO (massive multiplayer online) a term describing game software that lets hundreds of users participate simultaneously. MMO is usually pre-fixed to another abbreviation to describe the type of game more pre-cisely. For example, a MMORPG is a massive multiplayer online roleplaying game, such as World of Warcraft. See ROLE-PLAYING GAME.MMORPG abbreviation for massive multiplayer online role-playing game. See MMO.MMX (multimedia extensions) a set of additional instructions added to the later models of the Pentium microprocessor and its successors to support high-speed processing of animation and sound. The MMX instructionssupport a limited form of vector processing. Seemnemonic a symbol or expression that helps you remember something. For example, the expression “Spring forward, fall back” helps you remember which way to adjust your clocks in the spring and fall for day-light saving time.A mnemonic variable name is a variable name that helps the pro-grammer remember what the variable means. For example, in a payroll program the variable to represent the hours worked could be named X312W17HK, but it would be much better to give it a mnemonic name such as HOURS.MO (mega-octet) French abbreviation for MEGABYTE.mobo (slang) motherboard.mod1. abbreviation for modulo, used to refer to the remainder in integer division. For example, in Pascal, the expression 24 mod 7 has the value 3, since 3 is the remainder when 24 is divided by 7. In C, C++, C#, and Java, this operation is symbolized by %.2. abbreviation for modification (a change to a piece of hardware or software). 315 monitormodal dialog box a dialog box that requires an immediate response from the user; other windows cannot be used until the modal dialog box has been dealt with. Modal dialog boxes generally warn of problems such as run-ning out of printer paper or losing a network connection. See DIALOG BOX.mode the state that a piece of hardware or software is in, defining the way it can be used.modem (modulator-demodulator) a device that encodes data for transmis-sion over a particular medium, such as telephone lines, coaxial cables, fiber optics, or microwaves.modifier key a key that changes or extends the meaning of a keyboard key. Examples of modifier keys are Shift, CTRL, and ALT.Modula-2 a programming language developed by Niklaus Wirth in the late 1970s as a replacement for Pascal, which Wirth had developed some 10 years earlier. As its name suggests, Modula-2 is designed to encourage modularity (see STRUCTURED PROGRAMMING). Modula-2 is very similar to the extended versions of Pascal that most compilers now implement.module a part of a larger system. A module in a computer program is a part of the program that is written and tested separately and then is combined with other modules to form the complete program. See TOP-DOWN PRO-GRAMMING.moiré an unintended and distracting pattern that occurs when two or more halftone screens are overprinted at the wrong angle. See Figure 171.FIGURE 171. Moirémonadic operation an operation on one piece of data. For example, nega-tion (finding the negative of a number) is an operation that requires only one operand and is therefore monadic. Addition is not monadic because it requires two numbers to be added. Contrast DYADIC OPERATION.monetize the process of converting a web page or other digital content so that it earns revenue. For example, see the Amazon Associates program affiliate-program.amazon.com/gp/associates/join.monitor1. a computer program that supervises the activity of other programs.2. a device similar to a television set that accepts video signals from a com-puter and displays information on its screen. The monitor itself does no computing at all. See also COMPUTER; CRT; EYEGLASSES; LCD; SVGA; VGA. monospace 316monospace a typeface design that gives each letter the same width, like this. See Figure 104, page 193. See also COURIER; FIXED-PITCH TYPE; TYPEFACE.Monte Carlo engine a computer or program used for MONTE CARLO SIMULATION.Monte Carlo simulation a simulation method that uses random numbers to estimate complex probabilities.Suppose that you know the probability that a particular event will happen, but it is too difficult to calculate the probability that a compli-cated combination of events will occur. In the Monte Carlo method you use a random number generator to calculate a random number between 0 and 1, and then compare that number with the probability of the event. For example, if the probability of the event is .62, and the random num-ber generated is .58, then the program will simulate that the event has occurred. You can simulate thousands of such events and look at how the combinations of them add up. The name Monte Carlo comes from the fact that this method is a bit like a game of chance.MOO (MUD, Object Oriented) a type of MUD (Internet game or interac-tion environment). See MUD.Moore’s Law the prediction that the number of transistors that can be placed in an INTEGRATED CIRCUIT of any given size will double every two years. That is why microprocessors and other integrated circuits become cheaper and more efficient year by year.Moore’s Law was first expressed in 1967 by Gordon Moore, co-founder of Intel, and has proved accurate so far, although there is spec-ulation that true physical limits will be reached within a few more years.morph to transform one image gradually into another. A moving morph is an animation of the morphing process; a still morph is a single image of the transition in progress. See also ANIMATION; COMPOSITING.FIGURE 172. MorphMosaic one of the first graphical BROWSERs for the World Wide Web, dis-tributed free by the National Center for Supercomputer Applications (www.ncsa.uiuc.edu) and later incorporated into Microsoft Internet Explorer and other products.motherboard the main circuit board of a computer, containing the CPU and memorymotion blur (paint, 3-d programs) a filter that blurs the image along a spec-ified axis to give the effect of motion. 317 mousetrapFIGURE 173. Motion blurMotorola a major manufacturer of electronic equipment and parts, head-quartered in Schaumburg, Illinois. Motorola makes a number of micro-processors, including the 68000 series used in the original Apple Macintosh, the PowerPC, and the 6800, used in some early microcom-puters in the 1970s.In 2004, Motorola spun off its microprocessor division as Freescale Semiconductor, Inc., headquartered in Austin, Texas. Web address: www.freescale.com.mount to put a disk or tape into a computer and make it known to the oper-ating system. Under UNIX, a list of all currently mounted file systems can be displayed by typing the command mount. Windows shows all mounted drives under COMPUTER; Macintosh computers display mounted drives on the DESKTOP.mount point a directory that is actually a separate disk drive. For example, if a UNIX system has two disks, one of them will likely be mounted as / (the root directory) and the other as /home. Then the second disk drive will function as if it were a subdirectory of the first one.mouse a computer input device that is used by moving it around on your desk and pressing one or more buttons. Moving the mouse moves a pointer on the screen (see MOUSE POINTER). Graphical user interfaces such as Microsoft Windows and the Apple Macintosh operating system are built around the mouse. So are paint and draw programs. See DRAW PROGRAM; GRAPHICAL USER INTERFACE; MACINTOSH; PAINT PROGRAM;TRACKBALL; WINDOWS (MICROSOFT).mouse pointer a small symbol on the screen (usually an arrow) that indi-cates what the mouse is pointing to, and moves whenever the mouse is moved. Also called mouse cursor.mouse potato (slang) a computer user who is addicted to web-surfing and other computer-related activities. Obviously adapted from couch potato, a person who incessantly views television.mouseover see ROLLOVER (definition 2).mousetrap a web page that is programmed, using JavaScript or another scripting language, so that the “back” button on the browser no longer works as intended. That is, once you go to such a web page, you cannot back out of it. In 2001 the U.S. Federal Trade Commission took action against an advertiser who used mousetraps on thousands of web sites to compel viewers to view a barrage of obnoxious advertisements. POWER LINE PRO-MOV 318MOV1. (metal oxide varistor) an electronic component used to protect elec-tronic equipment from momentary voltage spikes. SeeTECTION; SURGE PROTECTOR.2. abbreviation for “move” in most ASSEMBLY LANGUAGES. The direction of the arguments depends on the language. On Intel processors, MOV 7,8 means move the value 8 into location 7; on Motorola processors it means move the value 7 into location 8.3. file format used by QUICKTIME for movies and other media types.Mozilla an open-source web browser established by Netscape; also the name of a lizard used as Mozilla’s mascot. Their web site is www.mozilla.org. See also FIREFOX.MP3 a file compression format for music that allows users to download music over the web. MP3 is short for MPEG, layer 3 (layer 1 and layer 2 refer to previous, less-advanced compression formats) and is promoted by the Motion Picture Experts Group (see MPEG).Sound waves can be represented as numbers indicating the amplitude of the wave at each moment in time. High-fidelity sound requires stor-ing a 16-bit number 44,100 times per second (a sampling rate of 44.1 kilohertz). This means that one minute of stereo music requires over 10 megabytes of disk space. This is generally too large to be practical, but MP3 compression reduces the file size to about 1 megabyte per minute. The compression method was developed after carefully studying human auditory perception and then designing the compression algorithm so that the information lost is imperceptible.MP3 format now allows many artists to make samples of their work available to the general public over the web, but the ease of copying music in MP3 format raises concerns about piracy. See also COPYRIGHT;DRM; SOUND CARD; WAVE FILE. Compare OGG VORBIS.MP3 player1. a small portable device that stores and plays music in the form of MP3 files. Compare IPOD.2. a computer program that plays music from MP3 files.MPC (multimedia personal computer) a personal computer that meets requirements specified by the MPC Marketing Council for compact disc, sound, and graphics capabilities. These requirements are revised frequently.MPEG (Motion Picture Experts Group) an ISO working group that sets standards for digital sound and video and the associated data compres-sion requirements. Web address: www.mpeg.org.MRU most recently used.MRU list a list of most-recently-used files or web addresses. Many word processors, web browsers, and so on, maintain MRU lists so the user can quickly return to a document that was used recently. Windows maintains an MRU list on the Start button. 319 multiple inheritancems (millisecond) one thousandth of a second. See ACCESS TIME.MS a common abbreviation for Microsoft. See MICROSOFT; MS-DOS.MS-DOS Microsoft Disk Operating System, the original operating system for IBM PC-compatible computers. Early versions were also marketed by IBM as PC-DOS; later MS-DOS and PC-DOS became separate prod-ucts derived from the same original code.Virtually all the commands of DOS are still usable at the COMMAND PROMPT in Windows. See also BAT FILE; COM; EXE FILE; EXTENSION;MICROSOFT; OS/2; PATH; WINDOWS.MS-DOS Prompt the COMMAND PROMPT in Windows Me and earlier.MSIE Microsoft INTERNET EXPLORER.MSN (Microsoft Network) an online network established by Microsoft which provides content as well as a connection to the Internet.MSRP manufacturer’s suggested retail price.MTBF (mean time between failures) a measure of the reliability of equip-ment. For example, equipment with an MTBF of 25,000 hours can be expected to run, on the average, 25,000 hours without failing. Some disk drives have an MTBF as high as 800,000 hours (90 years). However, the MTBF is only an average; there is always a risk that any particular piece of equipment will fail sooner.MUD (multi-user domain or multi-user dimension, formerly multi-user dungeon) a type of real-time Internet conference in which users not only talk to each other, but also move around and manipulate objects in an imaginary world.Originally conceived as multi-user ADVENTURE GAMEs, MUDs have developed into a promising format for collaboration and education through the Internet. Compare IRC.multicast see IPTV.multimedia the combination of sound and visual information presented either to inform or to entertain.Multimedia PC see MPC.MultiMediaCard a type of flash-memory non-volatile storage device sim-ilar to CompactFlash but physically smaller, the size of a postage stamp, and often used in digital music players. Compare COMPACTFLASH; MEM-ORY STICK; SECURE DIGITAL CARD; SMARTMEDIA.multiple inheritance a technique in object-oriented programming whereby an object type is defined to be a combination of two or more pre-exist-ing types. Some programming languages, such as C++, permit this, and others, such as Java, do not. See OBJECT-ORIENTED PROGRAMMING. multiprocessing 320multiprocessing the use of more than one CPU in a single computer system.multiprocessor free see FREE.multisession CD a compact disc (CD-ROM) that was not recorded all at once; rather, some files were recorded on it at one time and more files were added later. The directory of a multisession CD occupies more than one block of disc space, and some of the earliest CD-ROM software could not read multisession CDs. See CD-ROM and references there.multitasking the execution of more than one program apparently at the same time on the same computer. In reality, the CPU rapidly switches its attention among the various programs (see TIMESHARING). Multitasking makes it possible to print one document while editing another or to per-form lengthy computations “in the background” while working on some-thing else on the screen.The programs that run concurrently are called processes or tasks. An important concern is to keep tasks from interfering with each other. For example, two tasks cannot use the same area of memory or the same input-output device, such as a printer, at the same time.If tasks communicate with each other, it is important to prevent dead-locks, in which two tasks are each waiting for the other to do something, so that neither one can make any progress. See STARVATION.Multitasking can be either preemptive or cooperative. In preemptive multitasking (as in Windows 95 and later versions, and UNIX) the oper-ating system forces the CPU to switch regularly from each process to the next. In cooperative multitasking (as in Windows 3.0 and 3.1), each process has to voluntarily give up the CPU periodically so that other processes can run.A more primitive form of multitasking allows you to switch from one program to another, but only one program at a time actually runs; the others are frozen until you return to them. This kind of multitasking was implemented in versions of Microsoft Windows before 3.0 and in early versions of the Macintosh operating system..museum a suffix indicating that a web or e-mail address belongs to a museum (in any country). Contrast .COM. See also ICANN; TLD.MVS (Multiple Virtual Storage) an operating system for IBM mainframe computers; from the user’s point of view, it is almost completely com-patible with OS/360. See JCL; OS/360; TSO: Z/OS.mwahahahaha typewritten representation of an evil laugh.My Computer a FOLDER on the DESKTOP of Microsoft Windows that con-tains all the disk drives, the Control Panel, and other information about the system. 321 MySQLOrdinarily, folders are directories. The root directory of a disk drive is also a folder. My Computer is a special folder that gives you access to the entire machine.In Windows Vista, My Computer is called simply Computer.My Documents a FOLDER on the DESKTOP of Microsoft Windows in which the user is invited to store his or her files in the absence of a more elabo-rate file system. The My Documents folder was created to discourage beginners from storing files in the same folder as the software that created them. In Windows Vista, My Documents is called simply Documents.My Network Places the FOLDER on the Windows DESKTOP that enables the user to BROWSE (examine) the computer resources available through the network, if any. In Windows Vista, My Network Places is called simply Network.MySpace the most popular SOCIAL NETWORKING SITE (www.myspace.com). MySpace lets users customize their profile pages to a high degree. Because of its popularity, MySpace is often at the center of concerns that affect all web sites, particularly regarding the safety and privacy of its younger users.MySQL popular open-source database software. See SQL; www.mysql.com/. LOGIC CIRCUITS.IP ADDRESSES\n 322N\n in C and related languages, the symbol, within a character string, that indicates starting a new line. Thus, “hello\nworld” is a string which prints out as:hello worldnagware shareware that always opens with a message begging for remu-neration. Some nagware is exceptionally persistent and interrupts your work sessions with messages reminding you to register (and pay!). SeeFREE SOFTWARE; SHAREWARE..name a suffix indicating that a web or e-mail domain address belongs to an individual person (in any country). Contrast .COM. See also ICANN; TLD.nameserver a computer whose job is to translate names into for other computers.Most computers on the Internet do not contain their own directories of the whole network. Instead, they rely on nameservers to interpret names for them. Nameservers, in turn, obtain information from other nameservers. See DNS.namespace1. the set of names available for naming things such as files, variables in a program, or computers in a network. If two parts of a program have different namespaces, the same name can be used in both places for dif-ferent purposes without conflict. See also LOCAL VARIABLE.2. in C#, a section of a program that has its own namespace (definition 1).NAND gate (Figure 174) a logic gate whose output is 0 if both of the inputs are 1, and is 1 otherwise, thus:Inputs Output0 0 10 1 11 0 11 1 0A NAND gate is equivalent to an AND gate followed by a NOT gate. NAND gates are important because all the other types of logic circuitscan be built out of them. See FIGURE 174. NAND gate (logic symbol) 323 native file formatnano- metric prefix meaning ÷1,000,000,000. For example, 1 nanosecond is one billionth of a second. Nano- is derived from the Latin word for “dwarf.” See METRIC PREFIXES.nanometer (abbreviated nm) a distance of 10–9 meter, or a millionth of a millimeter. The wavelength of visible light is measured in nanometers, from violet (350 nm) to red (700 nm). The size of the parts of an INTE-GRATED CIRCUIT is measured in nanometers. For example, a “45-nanome-ter process” is a manufacturing process that can reproduce details as small as 45 nanometers across.nanosecond a unit of time equal to 10–9 second, that is to say, 1/1,000,000,000 second. During one nanosecond, a light wave, electri-cal field, or radio wave travels about one foot (30.5 cm). The access time of many computer memories is about 60 to 70 nanoseconds.NAP (network access point) a location where an Internet service provider or other network connects to a BACKBONE to access the Internet. See also MAE.Napster a company founded in 1999 to provide person-to-person sharing of MP3 music files. Instead of storing files in a central repository, Napster allowed any user to share music files anonymously with anyone else who wanted them. The resulting network traffic created temporary but serious overloads at some colleges and universities.Napster’s file sharing system was shut down by court order in 2001 (A&M Records v. Napster). Napster now provides a service where users can obtain unlimited legal access to over one million songs in return for a monthly subscription fee. Web address: www.napster.com. See alsoDIGITAL MUSIC; GROKSTER; ITUNES; MP3; P2P.NAS see NETWORK ATTACHED STORAGE.NAT see NETWORK ADDRESS TRANSLATION.national characters the characters on a computer are those whose appear-ance varies from country to country. For example, on IBM mainframes, the characters$, #, and @ are called national characters because they may be replaced by other characters such as £ or ¥ for use outside the United States.native1.	designed for a specific hardware or software environment (rather than for compatibility with something else).2.	consisting of CPU machine instructions rather than instruction codes to be interpreted by a program.native file format the file format proprietary to an application program; the format in which it normally saves documents or drawings. Most pro-grams can, with the IMPORT command, convert similar file types to their own format. If you want to convert a native file to a more generic file type, use the SAVE AS. . . or EXPORT commands. Contrast METAFILE. native method	324native method a computer program compiled in the machine language of the specific computer on which it is being run. For example, a Java pro-gram is normally compiled to Java bytecode, but in some cases it might link to a native method that was written in a language such as C++ and compiled into machine language.native resolution the RESOLUTION of a monitor determined by the available pixels. The sharpest image will occur if the number of pixels generated by the video processor matches the monitor’s available pixels. Other res-olutions may be available, but the image may be less sharp as interpola-tion is required to determine the color for intermediate pixels.natural language processing the use of computers to process information expressed in human (natural) languages.Getting computers to understand English, French, or other human languages is a difficult, largely unsolved problem. It includes SPEECH RECOGNITION, syntactic PARSING to determine sentence structure, seman-tic analysis to determine meaning, and knowledge representation to encode the meaning into a computer. The challenge of natural language understanding is that human language is far more complicated, and more poorly understood, than early computer scientists realized.It is much easier to process natural-language texts in a way that falls short of full understanding, but still allows some of the meaning to be extracted. In recent years, natural language technology has turned towardINFORMATION EXTRACTION and INFORMATION RETRIEVAL to help manage the huge quantity of natural-language documents now stored in computers.See also ARTIFICIAL INTELLIGENCE; ELIZA.natural logarithm logarithm to the base e (about 2.718). See LOGARITHM.FIGURE 175. Natural media paint programnatural media actual artists’ materials (paint, canvas, etc.) realistically simulated by a computer program (Figure 175). In a natural-media paint program, you can specify the kind of paper or canvas you are working on. The tools available behave very much as their real-world counter- 325	navigation parts would act—the chalk smears, the watercolors spread, and markers bleed. The main difference is the ability to combine unlikely media (try to paint watercolors over chalk in real life), and you don’t have to wait for anything to dry.Natural-media programs are very demanding of your hardware; they create huge files and require lots of RAM, fast video boards, and fast CPUs.navigation finding your way around a complex system of menus, help files, or the WORLD WIDE WEB. This can be a real challenge, but there are a few tricks to help you.•	Menu navigation. Learning how to navigate menus requires an adventurous spirit. Make yourself a map (if there’s not one already in the manual), as any good explorer would do. Sometimes the logic of grouping certain commands together will not be apparent to you and you’ll have to learn some rather arbitrary distinctions. The best defense is to be familiar with your software. If you know that there is a command to do whirligigs, but can’t remember whether it’s under File or Arrange, it’s only a matter of a fraction of a second to look under both categories.Menus can nest like wooden Russian dolls. One will lead to another in a rather infuriating way. Just remember to take one thing at a time. After making your decisions at each level, click OK. If you’ve gotten lost in the menus, you can back out at any time by choosing Cancel. Note: if you cancel out, the changes you made will not take place. Be aware that menus can interconnect at lower levels. This means that there can be more than one way into the same DIALOG BOX.•	Help and hypertext files. Programs for viewing HYPERTEXT files usually have a command called Back that allows you to backtrack to the previous screens. This is similar to Tom Sawyer using a rope to find his way around caves. A frequent frustration is to have a vague memory of a subject you read about yesterday, but can’t remember how to get there. Some programs have bookmarks to mark important sections; use them. Also, familiarize yourself with the search capabilities of the hypertext system; it can save you a lot of time. As always, a good index is worth its weight in gold. If the index is too general to be useful, write a complaint to the software vendor. (If enough users complain, something might be done.) In the meantime, you may want to make a few notes on an index card and slip it into the manual.•	World Wide Web. The links that make up the World Wide Web can lead you literally in thousands of different directions from any starting point. When searching for information on a particular topic, use one of the standard web search engines such as Yahoo!, Lycos, Google, or Excite (try www.search.com to access the major indexes). See SEARCH ENGINE to get specific search tips.When browsing the WWW for pleasure, you may want to explore a JUMP LIST; most service providers have one. Usually, a Navigator	326web site will contain a page of new links to follow. (This will take care of all the rest of your free time.)When you find a web page you think you’ll want to return to, bookmark it, or add it to your Favorites folder. During the same web-surfing session, you can also use the Back and Forward but-tons on your browser. Back returns you to the previous web page; after backing up, you can use Forward to retrace your steps. Your browser maintains a list of where you’ve been (the HISTORY FOLDER). The most recently visited sites are a mouse-click away under the Go menu.Navigator web browser developed by Netscape (and often simply called Netscape). See NETSCAPE NAVIGATOR.negative a photographically reversed image; black becomes white, white becomes black, and colors become their complements.In desktop publishing, white letters on a black background are usually called a REVERSE. A negative is the physical film that is the intermediate step between camera-ready copy and a printing plate. See INVERT.FIGURE 176. Negative imagenerd (slang) a person who is intensely interested in computers to the exclu-sion of other human activities (and even basic life skills). Unlike geek, nerd is usually at least mildly insulting, though among some sets of peo-ple it can be a compliment. Compare GEEK.nest to put a structure inside another structure of the same kind. For exam-ple, in BASIC, three nested FOR loops look like this:FOR I=1 TO 100 FOR J=1 TO 100FOR K=1 TO 100...statements to be repeated go here...NEXT K NEXT JNEXT I Note that indenting is used so that human readers can see how the loops are nested. 327	NetBEUI.net suffix indicating that an E-MAIL address or WEB SITE is located at a net-work with a particular name. Names ending in .net were intended to des-ignate sites that provide Internet connectivity to other networks, but in recent years .net has been used indiscriminately as an alternative to .com..NET see .NET FRAMEWORK..NET Framework an application program interface (API) for Microsoft Windows, introduced in 2001 as a downloadable add-on to Windows 2000 and XP and included in subsequent versions of Windows.The purpose of the .NET Framework is to reduce the amount of work programmers have to do, while at the same time increasing reliability and introducing Java-like portability. Despite the name, networking is not its main purpose, although networking is included.The .NET Framework is fully object-oriented. Programmers commu-nicate with the operating system with a rich system of data types rather than with pointers. For example, if a particular operation requires a list of strings, the programmer can give it a list of strings, rather than a pointer to a place where a carefully constructed list of strings has been stored. The called procedure can verify that a list of strings is indeed what it received. Thus, a very tedious and error-prone task is eliminated.The .NET Framework manages the memory used by each program. Uninitialized variables and uninitialized pointers—a common source of erratic behavior in earlier software—are generally ruled out. So are “resource leaks” (memory or other system resources allocated to a pro-gram and never released when the program ends). The software compo-nents used by each program are tightly tied together so that none of them will be replaced accidentally (see ASSEMBLY; contrast DLL HELL).Like Java, the .NET Framework normally compiles programs into bytecode, a concise notation that is converted to machine language when the program is ready to run.net neutrality a proposed regulation that would require all ISPs (Internet service providers) to treat all Internet traffic the same. This proposal would prevent ISPs from offering higher speed, higher cost service for certain high-volume traffic such as video downloads. Supporters of this proposal argue that the Internet has traditionally been open equally to all types of traffic. Opponents argue that the proposal would prevent ISPs from investing in enough Internet capacity because they would not be able to raise additional revenue from heavy users.Comcast had started reducing the transfer speed for some large files downloaded over the Internet, but the FCC ordered them to stop doing this in 2008. The debate continues as this book is published.net surfing see SURFING.Net, the a colloquial name for the INTERNET.NetBEUI (NetBIOS Extended User Interface) a data transmission protocol developed by IBM and Microsoft and widely used in local-area net-working. It is usually the preferred protocol for networking Windows NetBIOS	328systems but does not support routing. See PROTOCOL; ROUTER. ContrastATM; IPX/SPX; TCP/IP.NetBIOS (Network Basic Input-Output System) an operating system extension designed by IBM to allow software to access a network. NetBIOS includes a network protocol that was later extended to form NetBEUI. See NETBEUI; PROTOCOL.netcafe an INTERNET CAFE.netcam a camera attached to a computer, used to send images over a net-work. Compare WEBCAM. Physically, webcams and netcams are alike; the difference is in how they are used.netiquette (network etiquette) the conventional practices that make the INTERNET usable. More than just politeness, netiquette involves funda-mental respect for the rights of other users who are helping pay the cost of running the network.For example, it is unacceptable to post off-topic material in NEWS-GROUPS, be rude during chats, ask people to do your homework for you, or bother them with commercial solicitations. See also ACCEPTABLE-USE POLICY; COMPUTER ETHICS; NEWSGROUPS.netizen (Internet citizen) a person who is part of the Internet community inCYBERSPACE.Netscape Navigator a pioneering WEB BROWSER, produced by Netscape Communications Corporation (see their web site at netscape.aol.com). In 1998 Netscape was acquired by AOL.network a set of computers connected together. See INTERNET; INTRANET; LOCAL-AREA NETWORK; WIDE-AREA NETWORK.network address translation (NAT) the automatic changing of IP addresses by a router or gateway so that several computers can share a single IP address visible to the outside world. This is commonly done in home networks, where the DSL or cable modem has a single IP address. This is a solution to the problem that only about 3 billion IPv4 addresses exist, which is not enough for all the computers in the world.The router changes the IP addresses on the data packets as they come in and go out so that each computer can communicate with the Internet. By varying port numbers, it ensures that incoming packets can be matched up with the computers for which they are intended. See also PIX.network attached storage file storage that resides on a separate computer, which owns and manages the files. See FILE SHARING. Contrast DISK SHARING; STORAGE AREA NETWORK.Network Solutions, Inc. the organization that, until 1998, had sole respon-sibility for maintaining the registry of top-level domain names (TLDs) ending in .com, .net, and .org, under a contract with the United States Government. Its web address is www.networksolutions.com. 329	neural networkSince 1998, Network Solutions is one of several competing domain name registrars. This process is supervised by ICANN. See also DOMAIN NAME HOARDING; DOMAIN NAME POACHING.neural network a computer program that models the way nerve cells (neu-rons) are connected together in the human brain. Neural networks enable a computer to train itself to recognize patterns in a strikingly human-like way. Like the human brain, neural networks give only approximate results, but they can do things that no other kind of computer program can do efficiently.Figure 177 shows how a neural network is set up. Each neuron has several inputs but only one output. Some of the inputs excite (activate) the neuron while others inhibit it, each with a particular strength. The idea is that each output neuron will be activated when one particular kind of pattern is present at the input. In the computer, the neurons and con-nections are simulated by arrays of numbers.FIGURE 177. Neural networkTraining a neural network is like training an animal. Patterns are applied to the input, and a simple algorithm adjusts the weights of the connections to try to get the desired output. After many training runs in which many different patterns are utilized, the neural network “learns” to recognize patterns of a certain kind. Even the programmer need not know exactly what these patterns have in common, because the patterns are analyzed by the neural network itself.Neural networks are good at recognizing inputs that are vague, ill-defined, or likely to contain scattered variation. For example, a neural OBJECT-ORIENTEDnew	330network can recognize images of human faces, or patterns of weather data, or trends in stock market behavior. However, a neural network is never 100% reliable, and even simple calculations can be quite slow.new the command in C++, C#, and Java that calls the CONSTRUCTOR for aclass  to  create  a  new  object  of  that  class.  SeePROGRAMMING.new media the means of communication that are displacing newspapers and television at the beginning of the 21st century. Chief among them is the World Wide Web or, more generally, the Internet. Some differences between the Web and earlier media include the following:•	Lack of central control. Almost anyone can publish almost anything without an editor’s or publisher’s approval.•	Audience participation. Readers of new media often have the abil-ity to communicate with the producers and often post comments for other readers to see.•	Very low cost of production. You do not have to own a TV station or newspaper company to express yourself; everyone has a voice. In the 1500s, the printing press had a similar impact. It allowed any educated and reasonably prosperous person to print handbills and give them out. Later, newspapers and magazines arose as means of mass communication through printing.•	Computer-assisted access. Computers can help you find material you are interested in and filter out things you do not want to see. (See SEARCH ENGINE.)•	Multimedia. New media can combine the effects of print, painting, photography, music, motion pictures, and animation while adding new capabilities of their own, such as hypertext. See HYPERTEXT.•	Volatility. The contents of a web page can be changed at any time; it is possible to rewrite history and deny what you published a few weeks earlier. Libraries need to address this by archiving the World Wide Web for the public good. See WAYBACK MACHINE.One thing everyone agrees on is that the new media are in their infancy and their most common uses fifty years hence will probably involve techniques that have not yet been invented or foreseen.newbie (slang) newcomer (to the Internet, a newsgroup, etc.).newsfeed a link on a web page that is automatically updated with current information. See RSS.newsgroup a public forum or discussion area on a computer network. All users of the network can post messages, and every user can read all the messages that have been posted. The most famous newsgroups are those distributed worldwide by the Usenet system, covering thousands of top-ics. See USENET.newspaper columns a word processor mode that specifies a newspaper-like format with text flowing from one column into the next. ContrastPARALLEL COLUMNS. 331	non-breaking spaceFIGURE 178. Newspaper columnsnewsreader a piece of software that enables the user to read Usenet news-groups. See NEWSGROUP.NFS (Network File System) a FILE SHARING protocol originally developed by Sun Microsystems in the mid-1980s and now widely used on UNIX-based systems. Compare CIFS.NIB (describing items for sale) “new, in box.”nibble a group of 4 bits, or half of 1 byte.NIC (Network Interface Card) the circuit board inside a computer that con-nects it to a local-area network.nicad, NiCd (nickel-cadmium) a type of rechargeable battery formerly used in laptop computers. Nickel-cadmium batteries are toxic and should not be discarded in ordinary trash. See also LI-ION; NIMH; ROHS.Nigerian scam see 419 SCAM.NiMH (Nickel-Metal Hydride) a type of rechargeable battery electrically similar to nickel-cadmium (NICAD) but having greater capacity. See alsoLI-ION.nm see NANOMETER.node1.	an individual computer (or occasionally another type of machine) in a network.2.	a connection point in a data structure such as a linked list or tree.3.	(draw programs) a point on a curve or line that helps define the shape of the line. See CUSP NODE; SMOOTH NODE; SPLINE.non-breaking hyphen a hyphen that does not indicate a place where a word can be broken apart. For instance, if the hyphenated word “flip-flop” falls at the end of the line, then “flip-” can appear on one line, with “flop” at the beginning of the next. But if you type “flip-flop” with a required hyphen, it will not be split up. In Microsoft Word, to type a non-breaking hyphen press Ctrl-Shift and the hyphen key together.non-breaking space a space that does not denote a place where words can be split apart at the end of a line. For instance, you might not want a per-son’s initials (as in “T. S. Eliot”) to be split at the end of a line. You should LOGIC CIRCUITS.non-volatile	332therefore use required spaces between them rather than ordinary spaces. In TEX, a required space is typed as ~ (TILDE). In Microsoft Word, a non-breaking space is typed by pressing Ctrl-Shift and the space bar together.non-volatile not erased when turned off. Disks are a non-volatile storage medium; memory (RAM) is volatile.nondocument mode a type of word processing that produces plain-text (ASCII) files with no special codes for hyphenation, page breaks, fonts, or the like. The most common way of saving a file in nondocument mode is to use the “Save as” menu and choose “text file” or “text only.” SeeTEXT FILE.NOR gate (Figure 179) a logic gate whose output is 0 when either or both of the two inputs is 1, thus:Inputs	Output0	0	10	1	01	0	01	1	0A NOR gate is equivalent to an OR gate followed by a NOT gate. NOR gates are important because all the other types of logic circuits can bebuilt from them. SeeFIGURE 179. NOR gate (logic symbol)Norton Utilities a set of programs originally written by Peter Norton and now a product of Symantec Corporation, used to provide security, recover erased files, and correct other problems with disks. See RECOV-ERING ERASED FILES. Web address: www.symantec.com/norton.NOS1.	(Network Operating System) any special operating system or operat-ing system extension that supports networking.2.	(New Old Stock) old but never sold to a customer and still in origi-nal packaging; this describes parts for obsolete equipment or the like.Compare LNIB.NOT gate (Figure 180) a logic gate whose output is 1 if the input is 0, and vice versa, thus:Inputs	Output0	11	0 LOGIC CIRCUITS.333	nudgeA NOT gate is also called an inverter because it reverses the value ofits input. SeeFIGURE 180. NOT gate (inverter)notebook a computer about the same size as a looseleaf notebook, weigh-ing less than 6 pounds (2.7 kg).Usage note: The distinction between “laptop” and “notebook” has become blurred; all present-day laptop computers are notebooks by the standards of a few years ago. Many people use “laptop” and “notebook” interchangeably.NP-complete problem see LIMITS OF COMPUTER POWER.NPC non-player character, a (simulated) person in an online game who is not controlled by a player.NPN one of the two types of bipolar TRANSISTORs (contrast PNP).NRFB (describing items for sale) “never removed from box.” See alsoEBAY; MIB.NSFW Not Safe For Work, a warning sometimes given on an online forum when the upcoming content isn’t appropriate for a work environment.NSI see NETWORK SOLUTIONS, INC.NT short for Windows NT. See WINDOWS (MICROSOFT).NTFS the file system used by Windows NT and its successors. ContrastCDFS; FAT; WINDOWS (MICROSOFT).NTSC (National Television System Committee) the type of analog color TV signal used in the United States. It was designed to be compatible with a pre-existing black-and-white system. The screen consists of 525 lines, interlaced, and a complete scan takes 1/30 second. Color informa-tion is modulated on a 3.58-MHz subcarrier. Regulations mandated the end of NTSC broadcasting in February 2009, but NTSC video signals will continue to be used in low-cost analog video monitors and the like.Contrast DIGITAL TELEVISION; HDTV; PAL; SECAM..nu suffix indicating that an e-mail or web address is registered on Niue Island in the South Pacific. Niue Island has resold most of its domain addresses for use elsewhere. See TLD.nudge to move a selected object in small increments by using the arrow keys instead of the mouse. null-terminated string	334null-terminated string a CHARACTER STRING that ends with ASCII code 0. Null-terminated strings are used in the C programming language and in many of the system routines of UNIX and Windows.Num Lock a key on PC-compatible computers that switches the NUMERIC KEYPAD between two functions: typing numbers or moving the cursor with arrow keys.number crunching (slang) arithmetical calculation, especially for scien-tific or engineering purposes.numeric keypad a separate set of keys at the end of the keyboard, con-taining the digits 0 to 9 and a decimal point key. The digits are arranged in the same way as they are on an adding machine. If you have to type large quantities of numeric data, a numeric keypad is quicker to use than the number keys on the regular keypad. Some people prefer a keyboard without a numeric keypad because it lets them place the mouse closer to where they sit. See also KEYBOARD.numerical integration the process of finding the area under a particular curve by dividing the area into many tiny rectangles, adding up the heights of individual rectangles, and then multiplying the sum by their common width. See Figure 181. Numerical integration is a good exam-ple of a calculation that is practical to do on a computer but not by hand.FIGURE 181. Numerical integrationFor example, in probability theory it is important to find the area under the bell curve defined by:y =	1	e–x2/2					2π					This area can be found with the program in Figure 182, which uses a loop to perform a numerical integration. As you might imagine, it would 335	NVRAMtake a long time to perform this calculation with a calculator, and it would be entirely impractical to do it by hand.NVIDIA a major manufacturer of graphics processors for personal com-puters, located in Santa Clara, California (www.nvidia.com). The name appears to be a pun on the Latin invidia “envy” although it is usually defined as n for numeric plus video.NVRAM (non-volatile random-access memory) the memory in which infor-mation about a computer’s configuration is stored; it is either powered by a battery or inherently able to retain data when power is off. CompareCMOS RAM, which is an older term. See also EEPROM; FLASH MEMORY.class numerical_integration{/* This Java program finds the area under the standard normal probability curve between x=0 and x=b, which is 1.0 in this example */public static void main(String args[]){double b=1.0; double a=0;double p=Math.sqrt(2*Math.PI); double dx=1/100.0;for (double x=dx; x<b; x+=dx){double y = Math.exp(-x*x/2); a += y;}double area=a*dx/p; System.out.println(”Area =” + area);}}FIGURE 182. Numerical integration algorithm SOURCE CODE.OASIS	336OOASIS (Organization for the Advancement of Structured Information Standards) an organization working on the development of e-business standards in areas such as web services (web address: www.oasis-open.org).Ob- (slang) “obligatory”; used in newsgroup postings to signify a belated return to the intended topic. See TOPIC DRIFT.obelisk the character †, a symbol used to mark footnotes. See also FOOT-NOTE. Also called a DAGGER or LONG CROSS.object1. a data item that has procedures associated with it. See OBJECT-ORI-ENTED PROGRAMMING.2. one of the parts of a graphical image. See DRAW PROGRAM.object code the output of a compiler; a program written in machine instruc-tions recognizable to the CPU, rather than a programming language usedby humans. Contrastobject linking and embedding (OLE) (in Microsoft Windows 3.1 and later versions) a method of combining information that is processed by different application programs, such as inserting a drawing or a portion of a spreadsheet into a word processing document. The main document is called the client and the document or application that supplies the embedded material is the server. OLE supersedes an older feature of Windows called dynamic data exchange (DDE).OLE can be done in either of two ways. An embedded object becomes part of the document that it is inserted into. For example, if you embed a drawing into a word processing document, the whole thing becomes one file, and to edit it, you use the word processor, which will call up the drawing program when you double-click on the drawing to edit it. A linked object has a life of its own; it remains a separate file and can be edited separately. When you edit it, the information that is linked from it into other documents is automatically updated. Thus, you can use a word processor to create a report that has links to a spreadsheet, and when you update the information in the spreadsheet, the corresponding informa-tion in the report will be updated automatically. Embedding and linking correspond to “cold links” and “hot links” in Windows 3.0 DDE. See also ACTIVEX.object-oriented graphics graphical images that are represented as instruc-tions to draw particular objects, rather than as light or dark spots on a grid. See DRAW PROGRAM.object-oriented programming a programming methodology in which the programmer can define not only data types, but also methods that are 337	object-oriented programming automatically associated with them. A general type of an object is called a class. Once a class has been defined, specific instances of that class can be created.The same name can be given to different procedures that do corre-sponding things to different types; this is called polymorphism. For example, there could be a “draw” procedure for circles and another for rectangles.Some uses for object-oriented programming include the following:1.	Graphical objects. A program that manipulates lines, circles, rec-tangles, and the like can have a separate “draw” and “move” pro-cedure for each of these types.2.	Mathematical objects. In order to work with vectors, matrices, or other special mathematical objects, the programmer has to define not only data structures for these objects, but also operations such as addition, inversion, or finding a determinant.3.	Input-output devices. The procedure to draw a line might be quite different on a printer or plotter than on the screen. Object-oriented programming provides a simple way to ensure that the right pro-cedure is used on each device.4.	Simulation. In a program that simulates traffic flow, for example, cars, trucks, and buses might be types of objects, each with its own procedures for responding to red lights, obstructions in the road, and so forth. This, in fact, is what object-oriented programming was invented for. The first object-oriented programming language was Simula, introduced in 1967.5.	Reusable software components. Object-oriented programming provides a powerful way to build and use components out of which programs can be built. For example, a programmer might use a predefined object class such as “sorted list” (a list that automati-cally keeps itself in order) rather than having to write procedures to create and sort a list.Here is an example of object-oriented programming in Java. Imagine a program that manipulates points, lines, and circles. A point consists of a location plus a procedure to display it (just draw a dot). So the programmer defines a class called pointtype as follows:class pointtype{int x; int y;void draw(Graphics g){g.drawRect(x,y,1,1);}}The class pointtype is defined to include two integer variables (x and y) and one method (draw). (The class also would include a CONSTRUC-TOR—a method called when a new object of that class is created.) object-oriented programming	338 Now variables of type pointtype can be declared, for example:pointtype: a,b;Here the objects a and b each contain an x and a y field; x and y are called instance variables. In addition, a and b are associated with the draw pro-cedure. Here’s an example of how to use them:a.x = 100; a.y = 150: a.draw(g);This sets the x and y fields of a to 100 and 150, respectively, and then calls the draw procedure that is associated with a (namely pointtype.draw). (The g stands for graphics.)Now let’s handle circles. A circle is like a point except that in addi-tion to x and y, it has a diameter. Also, its draw method is different. We can define circletype as another type that includes a pointtype, and it adds an instance variable called diameter and substitutes a different draw method. Here’s how it’s done:class circletype{pointtype p; int diameter;void draw(Graphics g){g.drawOval(p.x,p.y,diameter,diameter);}}Your program would create a new object of class circletype (call it c), define values for the variables, and then call the method circletype.draw to display the circle on the screen.It is important to remember that instance variables belong to individ-ual objects such as a, b, and c, but methods (procedures) belong to object types (classes). One advantage of object-oriented programming is that it automatically associates the right procedures with each object: c.draw uses the circle draw procedure because object c is a circle, but a.draw uses the point draw procedure because object a is a point.The act of calling one of an object’s methods is sometimes described as “sending a message” to the object (e.g., c.draw “sends a message” to c saying “draw yourself”). All object-oriented programming systems allow one class to inherit from another, so the properties of one class can automatically be used by another class. For example, there is a standard Java class called Applet which contains the code needed to display an applet on the web. When you write your own applet, it will inherit from (extend) this class, so you don’t need to recreate that code yourself. See also C++; C#; JAVA; SMALLTALK. 339	off-by-one error OBO abbreviation for “or best offer,” often used when advertising things for sale on the Internet.obscenity sexually explicit material that can be prohibited by law. In 1973 the Supreme Court of the United States ruled that material is obscene if the average person, using contemporary community standards, would find that its primary purpose is to stimulate sexual appetite (“the pruri-ent interest”); it depicts sexual behavior defined as offensive by specific laws; and it “lacks serious, literary, artistic, political or scientific value” (Miller v. California). Contrast INDECENCY. See also COMPUTER LAW;ICRA; PORNOGRAPHY.OCR see OPTICAL CHARACTER RECOGNITION.octal a way of writing numbers in base-8 notation. Octal numbers use only the digits 0, 1, 2, 3, 4, 5, 6, and 7, and the next column represents mul-tiples of 8. For example, the octal number 23 means 2 eights and 3 ones, or 19. Here are some further examples:	Binary	Octal			Decimal						001	000	10	1	 81 = 8	001	001	11	1	 81 + 1	= 9001	010	12	1	 81 + 2	= 10010	001	21	2	 81	+ 1	= 17011	001	31	3	 81	+ 1	= 25100	001	41	4	 81	+ 1	= 33101 010	100	524	5	 82	+ 2	 81 + 4 = 340Note that each octal digit corresponds to three binary digits.octet a group of exactly eight bits, regardless of whether eight bits repre-sent a character on any particular computer. Contrast BYTE.octothorpe the character #; originally a map-maker’s representation of a village with eight fields (thorpes) around a central square. Also called aPOUND SIGN.ODM (Original Design Manufacturer) a company that produces products for another firm that will sell them under its brand name.OEM (Original Equipment Manufacturer) a company that assembles com-plete pieces of equipment from parts. In some Microsoft documentation, “OEM” is used as a euphemism for “IBM” in order to avoid naming the competitor directly; but it also refers to other manufacturers.OEM character set the native character set of the IBM PC. For a chart, see IBM PC.off-by-one error a programming error caused by doing something the wrong number of times (one time too many or one time too few); also called a FENCEPOST ERROR. Office, Microsoft	340Office, Microsoft suite of office applications including Word, Excel, Outlook, and PowerPoint. Microsoft markets specialized versions of Office for home or student use as well as a premium version that includes the database program Access. Details of the various collections vary as Microsoft’s marketing targets different users. Microsoft Office is the leading business application software used on microcomputers since the 1990s. Its main competitor is OPENOFFICE.ORG 2.offset the distance, in a computer memory, between one location and another. The offset of a data item is its address relative to the address of something else (0 if they are in the same position, 5 if they are 5 bytes apart, and so forth).offset printing a way of printing on paper by means of ink transferred by a rubber roller from another surface. Offset printing is a cheap way for a print shop to produce hundreds of copies of a laser-printed original.Ogg Vorbis a format for encoding compressed digital audio that is non-pro-prietary, with better sound quality than MP3 format. For more informa-tion, see www.vorbis.com. Contrast MP3.ohm the unit of measure of electrical resistance. If an object has a resis-tance of 1 ohm, then an applied voltage of 1 volt will cause a current of 1 ampere to flow. See OHM’S LAW.Impedance is also measured in ohms. Impedance is similar to resis-tance but is defined in terms of alternating current rather than direct cur-rent. See IMPEDANCE.Ohm’s law a basic law describing the behavior of electricity. It states that the current that flows through a circuit element is equal to the voltage applied across that element divided by the resistance of that element:I = V/Rwhere I = current, in amperes; V = voltage, in volts; and R = resistance, in ohms. In effect, voltage is the force that drives a current through a resistance.OLAP (Online Analytic Processing), performing analysis of multidimen-sional hierarchical data. An OLAP software tool will typically interact with data that is stored in a large database, but it provides more advanced techniques for processing and viewing the data than are provided by a database query language such as SQL. OLAP tools also provide more flexibility and power than do traditional spreadsheets.A business typically will store data on a large number of individual transactions in a giant database. An OLAP tool will need to aggregate this data into a form that is useful for decisions. The data is inherently multidimensional, typically including dimensions for the time of the transaction, the location, the type of product, and a dimension for the type of variable (such as revenue, cost, and margin). Each dimension 341	online tradingtypically has a hierarchy; for example, the time dimension is arranged by year/quarter/month/day; the location dimension can be arranged by country/state/city/store; and the product dimension is arranged into a hierarchy of categories.To provide effective decision support, an OLAP tool should be able to generate views of the data quickly while supporting multiple users.For an example of using a spreadsheet to view a limited form of mul-tidimensional data, see PIVOT TABLE.OLE see OBJECT LINKING AND EMBEDDING.OLED (organic light-emitting diode) a type of light-emitting diode based on organic polymers instead of semiconductor crystals. See LED.OLPC (One Laptop Per Child) a nonprofit organization providing inex-pensive laptop computers to children in developing nations (web address: www.laptop.org).OLTP abbreviation for on-line transaction processing.OMG (Object Management Group) a consortium of hundreds of computer companies that develop standards for software components to interact with each other. See web address: www.omg.org. See also CORBA.on-board included within a piece of equipment. For example, it is common for a motherboard to have an on-board Ethernet interface.one-way function a function whose inverse is very hard to calculate. A function f is a one-way function if, given x, it is relatively easy to calcu-late y = f(x), but it is hard to calculate the inverse function (i.e., calculate the value of x if you are given the value of y). One-way functions are used in public key encryption schemes; see ENCRYPTION.onionskin (animation software) a translucent drawing layer placed on top of a reference image for purposes of tracing, like onionskin paper.online connected to a computer or available through a computer. For exam-ple, online help is information that can be called up immediately on a computer screen rather than having to be looked up in a book.Usage note: Online is also written with a hyphen when used before a noun, as in on-line processing, or as two separate words when used pred-icatively, as in The computer is on line.In New York City but not elsewhere, on line means “in a queue,” as inWe are standing on line—the rest of the country says standing in line. In this context it is not a computer term and is not written as a single word.online casino, online gambling see GAMBLING.online trading the buying and selling of stocks or other securities through the Internet. Instead of paying a broker to type transactions into a com-puter, you type them in yourself. Brokerage fees are much lower, and OOBE	342transactions are completed more promptly. Unfortunately, the broker’s wise counsel is absent, and fortunes have been lost through speculative day trading. See DAY TRADING.OOBE see OUT-OF-BOX EXPERIENCE.OOC abbreviation for “out of character,” used in role playing games and the like to indicate that a person’s comment is not part of the imaginary situation. Example: “OOC: That dragon reminds me that I need to feed my pet iguana.” See also IC; RPG (definition 1).OOP see OBJECT-ORIENTED PROGRAMMING.OPA (Open Patent Alliance) a group of companies formed in 2008 to promote development of WIMAX Internet use. Web address: www.openpatentalliance.com.opacity (from opaque) inability to be seen through; the opposite of trans-parency. In a graphical image, objects with low opacity are partly trans-parent. Many special effects are implemented by creating a new image, with opacity under the control of the user, and superimposing it on the existing image. See also ALPHA CHANNEL.open1.	to call a file, document, or drawing up from disk in order to work with it.2.	(in programming) to prepare a file to have data transferred into or out of it.3.	(in electronics) to put a switch into the position that does not allow current to flow.open architecture a computer architecture whose details are fully made public so that other manufacturers can make clones and compatible accessories. The architecture of the original IBM PC is open; that of the original Macintosh is not.open beta a test of incomplete software that is open to a very large group, often the entire public. See BETA TESTING.open source software software whose source code is published so that a variety of people can add contributions. This is different from propri-etary software such as Microsoft Windows, where the source code is a trade secret and only employees of the manufacturer work on the soft-ware’s development. Significant examples of open source software include the LINUX operating system, the APACHE web server, the OPENOF-FICE.ORG 2 suite, and various GNU products.open systems interconnection see DATA COMMUNICATION.OpenOffice.org an OPEN-SOURCE office software suite whose functionality rivals the industry-leading Microsoft Office suite. OpenOffice comprises 343	opt outprograms for word processing, spreadsheets, presentations, graphics, and databases. It is maintained by a worldwide organization of pro-grammers and contributors who provide the software free-of-charge. Some users report that the OpenOffice.org user interface isn’t as pol-ished as its commercial rival, however user training and support is avail-able at www.openoffice.org.OpenType a format for type fonts on personal computers developed by Microsoft in the late 1990s as a combination of TrueType and Adobe Type 1. (See TRUETYPE; TYPE 1 FONT.) OpenType support is built into Windows 2000 and its successors.Opera a popular independent web browser created by Opera Software (www.opera.com), using W3C standards. See BROWSER; FIREFOX; INTER-NET EXPLORER.operands the items on which a mathematical operation is performed. For example, in the expression 2 + 3, the operands are 2 and 3, and the oper-ation is addition.operating system a program that controls a computer and makes it possi-ble for users to enter and run their own programs.A completely unprogrammed computer is incapable of recognizing keystrokes on its keyboard or displaying messages on its screen. Most computers are therefore set up so that, when first turned on, they auto-matically begin running a small program supplied in read-only memory (ROM), or occasionally in another form (see BOOT). This program in turn enables the computer to load its operating system from disk, though some small microcomputers have complete operating systems in ROM.Under the control of the operating system, the computer recognizes and obeys commands typed by the user. In addition, the operating sys-tem provides built-in routines that allow the user’s program to perform input-output operations without specifying the exact hardware configu-ration of the computer. A computer running under one operating system cannot run programs designed to be run under another operating system, even on the same computer. For articles on specific operating systems, see CMS; CP/M; LINUX; MAC OS; MS-DOS; MVS; OS/2; OS/360; UNIX; WINDOWS (MICROSOFT); Z/OS.operations research the mathematical modeling of repetitive human activities, such as those involved in traffic flow, assembly lines, and mil-itary campaigns. Operations research makes extensive use of computer simulation.opt out to choose not to receive mass e-mailings. When giving your e-mail address to an online merchant, look carefully for an opt-out CHECKBOX somewhere on the screen, and be sure to opt out of mailings you do not want to receive.Many spammers falsely describe their mailing lists as opt-out lists; they ignore requests to opt out, because any reply tells them they have optical character recognition	344reached a good e-mail address. This is why it’s so important to never respond to spam. It’s like being hit on the head once and then asked whether you want to opt out from being hit again. See SPAM.optical character recognition (OCR) the recognition of printed or hand-written characters in an image of a piece of paper. OCR software is com-monly used with scanners so that information received on paper will not have to be retyped into the computer. A difficulty is that the computer usu-ally cannot recognize letters and digits with complete certainty, so it has to make intelligent guesses based on the spellings of known words. For example, if you type “chack” an OCR device is likely to read it as “check.” Obviously, OCR has difficulty distinguishing l from 1 or O from 0; so do humans if they don’t know the context. Information obtained through OCR should be carefully checked for accuracy. See also SCANNER.optical disc any kind of data storage disc that is read by means of light rays (visible, infrared, or ultraviolet). For examples see BLU-RAY DISC; CD; DVD.optical disk a high-density storage device that stores information by etch-ing tiny grooves in plastic with a laser. See CD-ROM and references there;WORM.optical zoom a change in the field view of a DIGITAL CAMERA achieved by changing the focal length of the lens. Unlike digital zoom, optical zoom does not sacrifice resolution (at least if the lens is of high quality).Contrast DIGITAL ZOOM.A lens marked “3 zoom” has a focal length that is three times as long at maximum as at minimum. See also FOCAL LENGTH.option buttons small circles in a dialog box, only one of which can be cho-sen at a time. The chosen button is black and the others are white. Choosing any button with the mouse causes all the other buttons in the set to be cleared. Because option buttons work like the buttons on older car radios, they are sometimes called radio buttons.FIGURE 183. Option buttonsOption key a key on the Macintosh keyboard labeled “Opt” that acts as another kind of Shift key, allowing special characters to be typed quickly. See also COMMAND KEY; MODIFIER KEY.OR gate (Figure 184) a logic gate whose output is 1 when either or both of the inputs is 1, as shown in the table: EXPONENTIAL NOTATION.345		orphanInputs	Output0	0	00	1	11	0	11	1	1See also LOGIC CIRCUITS; COMPUTER ARCHITECTURE.FIGURE 184. OR gate (logic symbol)Oracle a leading producer of database software. Oracle Corporation is headquartered in Redwood Shores, California. Web address: www.oracle.com.Orange Book1.	the official standard for compact discs that can be recorded by the user. See CD-ROM.2.	the U.S. government’s Trusted Computer System Evaluation Criteria, published in 1985 and defining standards for computer security.ORB (Object Request Broker) a system that allows objects to connect to other objects over a network. See CORBA for a description of one set of standards that define how ORBs connect different components.order of magnitude a factor-of-10 difference in size. If one number is 10 times larger than another, they differ by one order of magnitude. Personal computers have sped up by more than three orders of magni-tude—that is, a factor of more than 1,000—since the early days of the IBM PC.More formally, the order of magnitude is the exponent in exponentialnotation. See.org a suffix intended to indicate that a web or e-mail address belongs to a non-profit organization (in any country, but mostly the United States). Along with .com, .edu, .gov, .int, .net, and .mil, this is one of the origi-nal set of Internet top-level domains. Since 2000, .com, .net, and .org have been assigned almost indiscriminately to organizations of all types.Contrast .COM. See also TLD; ICANN.orphan1.	the last line of a paragraph if it appears by itself as the first line of a page. Some word processors automatically adjust page breaks to avoid creating orphans. See also WIDOW.2.	a computer product that is no longer supported by its manufacturer, or whose manufacturer is out of business. For example, the Amiga is now an orphan computer. orthogonal	346orthogonal1.	meeting at right angles. For example, in three-dimensional space, the X-AXIS, Y-AXIS, and Z-AXIS are orthogonal. The edges of a rectangular box are orthogonal.2.	usable in all combinations. For example, if the size and color of an object are orthogonal attributes, you can combine any size with any color.Orthogonality was an important design goal of ALGOL and the many programming languages that it inspired. For example, if a language con-tains both arrays and pointers, then for the sake of orthogonality, it should have arrays of arrays, arrays of pointers, pointers to arrays, and pointers to pointers.OS/2 a multitasking, virtual memory operating system with a graphical user interface for 386 and higher PC-compatible computers. OS/2 was an important predecessor of Windows 95 but is now obsolete.OS/2 was originally developed by Microsoft in cooperation with IBM. Later it became solely an IBM product, competing against Windows.OS/360 the operating system released with the IBM 360 in the early 1960s, and which formed the basis of many subsequent operating systems (OS/VS2, MVT, MVS, etc.). See also JCL; MVS; TSO; Z/OS.oscilloscope an instrument for viewing sound waves or electrical wave-forms.OSI see DATA COMMUNICATION.out of band outside the defined frequency range or channel for a commu-nication signal; more generally, outside a defined code. For example, characters with numeric values greater than 128 can be described as “out of band” if ASCII characters are expected.out-of-box experience (somewhat humorous) a user’s first experience on initially unpacking a product and trying to get it to work, without dig-ging deeply into the instructions. Favorable out-of-box experiences result in satisfied customers.(Pun on “out-of-body experience” in psychology and spiritualism.)outdent to mark the first line of a paragraph by letting it extend into the left margin; HANGING INDENT; the opposite of INDENT. The entry terms in this dictionary are outdented.outline1. a graphical image showing only the edges of an object. FIGURE 185. Outline (definition 1) 347	overflow2. a way of representing the main points of a text without giving all the details. People have been making outlines on paper for centuries, but a computer can simplify the process in two ways. First, with a word processor, it is easy to create a document by first typing an outline of it, and then going back and filling in the sections one by one. Second, soft-ware has been developed to let you display just the desired parts of an outline while concealing the rest. While you are working on one section, the details of other sections, even if they have already been written, can be removed from the screen.FIGURE 186. Outline (definition 2)Outlook popular e-mail and calendar software provided as part of the Microsoft Office suite.Outlook Express the e-mail software provided with Microsoft Windows and also made available by Microsoft for other operating systems. A more elaborate commercial version is called Outlook.output the information that a computer generates as a result of its calcula-tions. Computer output may be either printed on paper, displayed on a monitor screen, or stored on disk or tape.output device a device that shows, prints, or presents the results of a com-puter’s work. Examples of output devices include MONITORs, PRINTERs, and IMAGESETTERs.overclocking the practice of running a CPU at higher than its rated clock speed. For example, a 2.2-GHz CPU might run successfully at 2.4 GHz.Overclocking usually yields a small increase in performance and a substantial decrease in reliability. Overclocked CPUs emit more heat, requiring a larger HEAT SINK than when run at their rated speed.Sometimes, physically identical CPUs are sold with different speed ratings because the manufacturer does not want to make separate types. In this case, the lower-rated ones are less expensive but can be over-clocked with no risk of problems.overflow the error condition that arises when the result of a calculation is a number too big to be represented in the available space. For example, adding 65,535 + 1 will cause an overflow on a computer that uses 16-bit unsigned integers, because 216 – 1 = 65, 535 is the largest integer repre- overlaid windows	348sentable in that format. (Or, worse, if the computer does not detect over-flows, it may simply compute 65, 535 + 1 = 0 without letting you know anything is wrong.) Compare UNDERFLOW.overlaid windows windows that can overlap; when they do, one window hides the parts of others that are behind it (Figure 187). To bring another window to the front, move the mouse pointer into it and click the button.Contrast TILED WINDOWS. See also CASCADE.FIGURE 187. Overlaid windowsoverwrite to write over information that is already on a disk. For example, if you copy a file called ABC.TXT onto a disk that already has a file with the same name, some operating systems will ask you whether you want to overwrite the old file. If you say no, the new file will not be copied.own (slang) to conquer or defeat an opponent in a game; to break into a computer and control it. HOME PAGE; WEB349	page layout softwarePP (on a digital camera) programmed autoexposure, a mode in which the camera chooses both the lens aperture and the shutter speed. Contrast A;AV; S; TV.p-p (peak-to-peak) a way of measuring AC voltage. See PEAK-TO-PEAK.P2P1.	abbreviation for PEER-TO-PEER.2.	the use of peer-to-peer networking to share files over the Internet.packet a group of consecutive characters sent from one computer to another over a network. On most networks, all communications are in the form of packets that begin with labels indicating the machine to which they are addressed.packet radio the transmission of data (in packets) by radio. It is a fast-growing hobby among radio amateurs (“hams”) and also has commer-cial applications as a way of linking computers without wires.A typical amateur packet system consists of a computer linked by a terminal-node controller (TNC) to a VHF radio transmitter and receiver. The TNC constructs and recognizes packets. The packet radio protocol effectively prevents two systems from transmitting at the same time, and all data are error-checked. Packet systems are often used to run bulletin boards (see BBS). Unlike telephone-line BBSs, packet BBSs are inher-ently multi-user systems because each packet contains a label indicating its sender and receiver. Thus, the computer can keep track of many users concurrently.Commercial packet systems often involve portable computer termi-nals carried by delivery or service personnel. The terminals are linked by radio to a main computer many miles away. See also AX.25; PROTOCOL.page1. information available on the World Wide Web. SeePAGE.2. a section of memory that is accessible at one time. See VIRTUAL MEMORY.page fault the situation that arises when the computer needs to access an area of memory that has been swapped out to disk; it is not a malfunc-tion. See VIRTUAL MEMORY.page frame an indication of the edges of the paper displayed by your computer’s software. The area around the page frame is called the pasteboard.page layout software software specially designed for creating CAMERA-READY COPY. Page layout programs, such as Adobe InDesign and QuarkXPress, allow the desktop publisher to combine many separate DESKTOP PUBLISHING; FRAME.page printer	350files of different types into a specified design. These special designs, called TEMPLATEs or STYLESHEETs, provide a framework to put the indi-vidual elements into. Most programs come with a library of predefined stylesheets.Page layout software also allows more control over typography thanmost word processors. Seepage printer a printer that forms, in its digital memory, a graphical image of the whole page, or requires the computer to do so, before printing it out. Laser printers are page printers, and inkjet printers commonly oper-ate as page printers. Contrast LINE PRINTER.pagination to divide a document into pages for printing.paint program one type of program for drawing pictures on a personal computer. The user draws with the mouse pointer (or a graphics tablet), and commands are provided for drawing circles, lines, rectangles, and other shapes, as well as for drawing freehand and choosing colors.Paint programs treat the picture as a grid of pixels (see PIXEL; BITMAP). Shadings are easy to produce by manipulating the color of each individ-ual pixel. It is hard to move an element of the picture if it is not where you want it. Contrast DRAW PROGRAM.More sophisticated paint programs are called photopaint programs because of their ability to retouch photographs and produce realistic images. See PHOTOPAINT PROGRAM.PAL (Phase-Alternate-Line) the type of analog color TV signal used in Great Britain and many other countries, now being displaced by digital television. The screen consists of 625 lines, interlaced, and a complete scan takes 1/25 second. Color information is modulated on a 4.43-MHz subcarrier. Contrast DIGITAL TELEVISION; HDTV; NTSC; SECAM.palette1.	a set of colors chosen from a much larger set. The whole set of displayable colors is also sometimes called a palette.2.	a floating window containing specialized tools or setting controls.Palm a line of handheld computers and organizers, beginning with the pio-neering Palm Pilot in 1996. Palm, Inc. is located in Milpitas, California; web address: www.palm.com.palmtop a computer that you can hold in one hand while using it. ComparePDA.pan (animation and 3-D software) to move the viewing area left or right to see additional sections of the scene.Pantone Matching System (PMS) a color matching and calibration system designed by the Pantone company. (Web address: www.pantone.com.) There are a wide variety of products all keyed to the same numbering sys-tem. If you want a certain color, you can specify it by its Pantone number 351	parallel columns and be assured of consistent reproduction. Some software also utilizes the Pantone system. A competing system is TRUMATCH. See COLOR.paper jam a situation in which paper cannot feed through a printer because it has gotten stuck. A common cause of paper jams is that sheets of paper are stuck together when they enter the printer. See also SEPARATOR PAD.paper sizes see A4; LEGAL SIZE; LETTER SIZE; PAPER SIZES (ISO).TABLE 11PAPER SIZES, ISOEach size is made by cutting the next larger size in half.	mm (exact)	inches (approximate)				A0	841	 1189	33.1	 46.8A1	594	 841	23.4	 33.1A2	420	 594	16.5	 23.4A3	297	 420	11.7	 16.5A4	210	 297	8.3	 11.7A5	148	 210	5.8	 8.3A6	105	 148	4.1	 5.8B0	1000	 1414	39.4	 55.7B1	707	 1000	27.8	 39.4B2	500	 707	19.7	 27.8B3	353	 500	13.9	 19.7B4	250	 353	9.8	 13.9B5	176	 250	6.9	 9.8paper sizes (ISO) a set of standard sizes of paper used everywhere except the United States, of which A4 is the best known (see A4). The sizes are shown in Table 11. Each size is made by cutting the next larger size in half, and all sizes have the same height-to-width ratio (1.414:1). A0 paper has an area of 1 square meter, and B0 paper is 1 meter wide.Note that A4 paper is usually mailed in C6 or DL envelopes.These standards are administered by the International Standards Organization (ISO). They were formerly a German industrial standard (Deutsche Industrie-Norm) and were known as DIN paper sizes.parallel1.	conducting electricity along more than one path at the same time (Figure 188). Contrast SERIES.2.	transmitting different parts of the same data along more than one wire at the same time. See PARALLEL PORT.3.	using more than one CPU at the same time. A parallel computer exe-cutes more than one instruction at the same time. parallel columns adjacent columns of printed text in which the second col-umn is not a continuation of the first; instead, the second column may parallel port	352give notes, comments, or a translation into another language. Many Canadian documents are printed in parallel columns of English and French. Contrast NEWSPAPER COLUMNS.parallel port an output device that lets a computer transmit data to another device using parallel transmission—that is, several bits sent simultane-ously over separate wires. Traditionally microcomputers have used par-allel ports to communicate with printers. See IEEE 1284.FIGURE 188. Parallel circuit (two resistors)FIGURE 189. Parallel port (with symbol indicating a printer)parallel printer a printer that connects to a computer’s parallel port (rather than, for example, to a USB port or a network cable).parallel processing computation carried out at the same time on different CPUs, or on a CPU that can execute more than one instruction at the exact same time.By contrast, most multitasking is accomplished by making a single CPU switch its attention among several tasks. This is called concurrent processing or timesharing.parameter a symbol that will be replaced in a procedure, function, or method by supplied values when the procedure is called. For example, if max is a function, then in max(x,y), x and y are the parameters. SeeACTUAL PARAMETER; FORMAL PARAMETER.parens (slang) parentheses.parent an object that gives its properties to a newly created object (the CHILD). Updating the properties of the parent object affect the children, but changing the properties of the child do not affect the parent.See DRAW PROGRAM; INHERITANCE; OBJECT-ORIENTED PROGRAMMING.parent directory the directory that contains another directory.parental controls software options enabling parents to control what web sites their children access. See FILTER (definition 3).parentheses the characters ( ), also called round brackets.Usage note: The singular is parenthesis. That is, ( is a left parenthesis, ) is a right parenthesis, and () is a pair of parentheses. For use, see PRECE-DENCE. Contrast ANGLE BRACKETS; CURLY BRACKETS; SQUARE BRACKETS. 353	parsingparity the property of whether a number is odd or even. Often, when groups of bits (1’s and 0’s) are being transmitted or stored, an extra bit is added so that the total number of 1’s is always odd (or, alternatively, always even). This is called the parity of the data.One incorrectly transmitted bit will change the parity, making it pos-sible to detect the error; the parity would be unchanged only if there were two (or an even number of) incorrect bits. Thus, if errors are fre-quent, some of them will be detected and the recipient of the information will have some warning that errors are present.The memory of many PC-compatible computers is parity-checked to detect erroneously recorded bits.FIGURE 190. Parsing: structure of a sentence (1)	Sentence(2)	Noun Phrase(3)	Noun Phrase(4)	Verb Phrase(5)	Determiner(6)	Noun(7)	Noun(8)	Adjective(9)	Verb → Noun Phrase + Verb Phrase → Determiner + Noun→ Determiner + Adjective + Noun → Verb + Noun Phrase→ the → dog → cat → black → chased FIGURE 191. Parsing: grammar rules used in Figure 190parsing the analysis, by computer, of the structure of statements in a human or artificial language. For instance, Windows has to parse the commanddir b: /pto determine that dir is the name of the command, b: specifies the files to be shown, and p is another parameter (in this case, it means “pause when the screen is full”). Compilers and interpreters have to parse statements in Part 15 device	354programming languages. (See COMPILER; INTERPRETER.) Programs that accept natural-language input have to parse sentences in human languages.Parsing is done by comparing the string to be parsed to a grammar, which defines possible structures. For example, Figure 190 shows the structure of the sentence “The dog chased the black cat.” Figure 191 shows a small part of the grammar of English.Parsing can be done either top-down or bottom-up. In top-down pars-ing, the computer starts by looking for a particular constituent. It con-sults the grammar to determine what this constituent consists of, and then looks for those constituents instead, thus:Look for SentenceRule 1: Sentence consists of Noun Phrase + Verb Phrase Look for Noun PhraseRule 2: Noun Phrase consists of Determiner + Noun Look for DeterminerAccept Determiner the from input string Look for NounAccept Noun dog from input string Look for Verb Phrase (etc.)The process is complete when the input string is empty and all of the ele-ments of a sentence have been found.In bottom-up parsing, the computer accepts elements from the input string and tries to put them together, thus:Accept the, which is a DeterminerAccept dog, which is a NounDeterminer + Noun make a Noun PhraseAccept chased, which is a VerbAccept the, which is a DeterminerAccept black, which is an AdjectiveAccept cat, which is a NounDeterminer + Adjective + Noun make a Noun PhraseVerb + Noun Phrase make a Verb PhraseNoun Phrase + Verb Phrase make a SentenceParsing algorithms must be able to backtrack (back up and try alterna-tives) because the grammar provides alternatives. For example, a noun phrase may or may not contain an adjective, and a word like leaves can be a verb or a noun. Further, parsing algorithms usually use recursion to handle the recursive structure of human languages. For example, a noun phrase can contain a noun phrase, which can contain another noun phrase, as in the discoverer of the solution to the problem. See BACK-TRACKING; NATURAL LANGUAGE PROCESSING; RECURSION.Part 15 device a radio transmitter that is allowed to operate without a license under the terms of Part 15 of the Federal Communications Commission’s regulations (known to lawyers as 47 CFR 15). Examples 355	Pascalinclude cordless telephones, wireless intercoms, and some kinds of wire-less computer communication devices (wireless LANs). Because indi-vidual Part 15 transmitters are not licensed, there is no way to guarantee that they will not interfere with each other, but spread-spectrum tech-nology makes interference unlikely. By contrast, licensed transmitters can be given exclusive use of a particular frequency in a particular area.See also SPREAD-SPECTRUM.partition a part of a HARD DISK that is treated by the computer as if it were a separate disk drive. Most hard disks consist of only one partition, but multiple partitions are sometimes used with larger drives.Pascal a programming language developed by Niklaus Wirth in the early 1970s. Pascal is essentially a modernized version of ALGOL, and it has greatly influenced the design of other languages, as well as becoming popular in its own right. See TURBO PASCAL for information on a popular version.Figure 192 shows a sample program in Pascal. A Pascal program con-sists of:•	A PROGRAM statement to give the program a name (and, in older ver-sions, to declare input and output files);•	Declarations of global variables;•	Declarations of procedures and functions;•	The keyword BEGIN, the action part of the main program, the keyword END, and finally a period to mark the end.Procedures and functions, in turn, can contain their own declarations, including more procedures and functions (a kind of nesting that is not permitted in C).Pascal is not case-sensitive (e.g., X and x are equivalent). In this book, uppercase letters are used for reserved words (keywords that cannot be redefined, such as IF, THEN, and ELSE), and lowercase letters for every-thing else.Semicolons are used as separators between statements. Thus a state-ment ends with a semicolon only when what follows is the beginning of another statement. This contrasts with C and PL/I, which end every statement with a semicolon regardless of the context.Comments in Pascal are enclosed in braces, {}, or the symbols (**). A statement does not have to fit on one line; lines can be broken anywhere that blank space is permitted.Pascal provides four standard data types: real, integer, Boolean, and character. Integer variables can take on only values that are whole num-bers or the negatives of whole numbers. Real variables can take on numerical values that include fractional parts, such as 23.432. Boolean variables are logic variables that can have only two possible values: true or false. Char variables can take on single character values.An assignment statement in Pascal looks like this:x := 3; Pascal notation	356This statement gives the value 3 to the variable x. Note that the symbol for assignment is :=, not =. The arithmetic operators are + for addition,– for subtraction, * for multiplication, / for floating-point division, div for integer division, and mod for modulo (remainder from division).See also MODULA-2.PROGRAM primecheck;{This is Turbo Pascal.}VAR	n,i,max: INTEGER;continue: BOOLEAN;status: STRING[13];BEGIN REPEATwrite(’Enter a whole number (type 0 to quit): ’); readln(n);status := ’ is prime’;IF n>2 THEN continue := TRUE ELSE continue := FALSE;i := 1;max := TRUNC(SQRT(n));{max is the largest divisor that must be checked to see if n is prime}WHILE continue DO BEGINi:=i + 1;IF (n MOD i)=0 THEN BEGINstatus := ’ is not prime’; continue := FALSEEND;continue := (i < max); END; {end of WHILE loop}write(n);writeln(status); UNTIL n=0END.FIGURE 192. Pascal programPascal notation a way of combining words by running them together, all capitalized; ThisIsAnExample. Procedure names in Pascal programs are often formed this way. Contrast CAMEL NOTATION. See INTERCAPS.Pascal, Blaise (1623–1662) a French mathematician who, in 1642, built a mechanical adding machine that was one of the early forerunners of cal-culators and computers.passive FTP a variation on FTP (file transfer protocol) in which all con-nections are initiated by the client (the user’s PC), not the server. This is necessary because some routers and firewalls, for security reasons, do not allow the server to initiate connections to the client. In that situation, a user can establish an FTP connection but cannot get a list of files and folders. Passive FTP overcomes the problem. 357	patchTo select passive FTP, type the command passive in a command-line FTP session, or make the appropriate choice in setting up your FTP client software. For example, in Microsoft Internet Explorer, passive FTP is an option under Tools, Internet Options, Advanced.passive matrix an older type of liquid crystal display that produces lower contrast than newer ACTIVE MATRIX displays.password a secret sequence of typed characters that is required to use a computer system, thus preventing unauthorized persons from gaining access to the computer.If you are using a password to protect your computer:•	Protect your password. Keep it a secret and don’t share it.•	Don’t choose an obvious password. Use some imagination and forethought. What would be hard to guess? Your password is not your mantra and should not express your personality or indicate in any way who you are.•	Don’t use a word in any language; some people crack computers by automatically trying every word in a dictionary. Include digits and special characters to make your password hard to guess. See DIC-TIONARY ATTACK.•	Use the initial letters of a memorable phrase or title to create a pass-word. Again, using digits and special characters will help safeguard your password.•	Change your password regularly, but not so often you can’t keep track of it.•	When you need to provide passwords for multiple systems, distin-guish between systems that need high security and those that can tolerate low security. Use a high security unique password for sys-tems that provide access to your valuable personal data such as your e-mail account. When you need to give a password to obtain access to web information where security is not a crucial concern, use the same password for all of these locations (as long as it is different from your high-security passwords). This way you won’t have to keep track of a dozen different passwords for a dozen different organizations.paste to transfer material from a holding area into the document you are editing. In Windows and Macintosh environments, the keyboard short-cut for paste is Ctrl-V. See CLIPBOARD; COPY; CUT.PATA (parallel ATA) the original hardware implementation of the ATA hard disk interface, using parallel data transmission through a rectangu-lar 40-pin connector, later expanded to 80 conductors by using the two sides of each hole separately. If no other type of interface is specified, “ATA” usually means PATA. See ATA. Compare ESATA; SATA.patch to correct a defective piece of software by modifying one or more of the files on which it resides, rather than by installing a complete, cor-rected copy. patent	358patent legal protection for the design of a machine or mechanical process, preventing others from using the same idea without the inventor’s per-mission. Unlike a copyright, a patent protects an idea itself, not just an expression of the idea. In the United States, a patent remains in force for 20 years.Computer programs were not originally considered patentable, since they were viewed as mathematical discoveries. In recent years, however, software patents have become common, on the ground that software can be an essential part of a machine. Some early U.S. software patents were handled clumsily and appeared to cover techniques that were not actu-ally original. See also COMPUTER LAW; COPYRIGHT; SOFTWARE PATENT;TRADE SECRET.path1. a designation that specifies how to find a file on a disk that has more than one directory. In Windows, paths have either of two forms. For example,\AAA\BBB\CCCmeans, “In the root directory there is a directory called AAA. In AAA there is a directory called BBB. In BBB there is a directory or file called CCC.”If the initial backslash is left out, the path starts at the directory cur-rently in use rather than at the root directory. For example, the pathAAA\BBB\CCCmeans, “In the current directory there is a directory called AAA. In AAA there is a directory called BBB. In BBB there is a directory or file called CCC.” Paths in UNIX are written the same way but with forward slashes (/ rather than \).2.	The set of directories in which the computer will look for an exe-cutable file when the user types a command. Some software packages have to be on the path in order to work properly. In Windows, the cur-rent directory is always treated as if it were on the path; in UNIX it is not, unless explicitly included as “.” (a period).3.	a contour or outline. Objects in a draw program are defined by paths.See DRAW PROGRAM; POSTSCRIPT; VECTOR GRAPHICS.4. a line that defines the movement of an object in an animation.PC any computer whose architecture is derived, however distantly, from the original IBM PC (IBM Personal Computer) of 1981, and which is considered to be part of the same lineage, in contrast to the MACINTOSH,SUN WORKSTATION, or others.Today, PC denotes a computer that is built to run Microsoft Windows, although it may equally well run Linux, BSD UNIX, or some other oper-ating system. Despite having adopted a very similar hardware architec-ture, the Pentium-based Apple Macintosh is not considered to be a PC; instead, it is the PC’s arch-rival. 359	peakPC 100, PC 133 the special bus used by the Pentium III to communicate with RAM, at 100 or 133 MHz depending on the model.PC 2001 a specification issued by Intel and Microsoft for the design of LEGACY-FREE personal computers (i.e., computers that run the latest PC operating systems but are not burdened by the need for full compatibil-ity with the original PC hardware). See LEGACY-FREE.PC Card a newer name for PCMCIA expansion cards. See PCMCIA; see also EXPRESSCARD and note there.PC compatibility the ability of a computer to run the same programs and use the same hardware accessories as the original IBM Personal Computer (PC) or newer computers of the same general type. In order to be fully PC-compatible, a computer needs an 8088-family processor (which nowadays means a Pentium or equivalent). Some other comput-ers can software-emulate a Pentium and thereby run PC software on a completely different CPU, such as a Sun RISC CPU or a PowerPC chip.PCI (Peripheral Component Interface) an improved bus for PC-compatible computers, introduced by Intel in 1992. PCI is faster than EISA and has been expanded to 64 bits to meet the needs of the Pentium processor. PCI-bus computers can use a bus interface unit on the motherboard to connect to EISA or ISA cards, so that the same motherboard has both PCI and ISA slots. See BUS.PCI Express a faster version of the PCI bus introduced by Intel in 2004 as a replacement for both PCI and AGP, with which it is not compatible. It is not truly a bus, but rather a very high-speed serial communication sys-tem. See also EXPRESSCARD.PCIe, PCI-E see PCI EXPRESS.PCMCIA (Personal Computer Memory Card International Association) an organization that defines standards for connecting peripherals to minia-turized computers such as notebooks and laptops. The standard ISA, EISA, PCI, and VLB buses are too bulky for laptop computers; the PCM-CIA bus (now known as PC Card and CardBus) is a widely accepted stan-dard. See www.pcmcia.org. See also BUS; CARDBUS; EXPRESSCARD.PDA (personal digital assistant) a pocket-sized, special-purpose personal computer that lacks a conventional keyboard. PDAs do such things as send and receive fax messages, maintain an electronic address book and telephone directory, and serve as notepads on which files can be created for subsequent processing on a larger computer.PDF (Portable Document Format) a file format representing images of printable pages. PDF files are viewable with Adobe Reader.See ACROBAT; ADOBE SYSTEMS, INC.peak a way of measuring the voltage of alternating current from zero volts up to the most positive point. The peak voltage of any symmetrical peak-to-peak	360waveform is exactly half of the peak-to-peak voltage. See also PEAK-TO-PEAK; RMS.peak-to-peak a way of measuring the voltage of alternating current from the most positive to the most negative point. For example, the audio output of a microphone is about 0.002 volt peak-to-peak; the “line out” from a piece of audio equipment is about 1 volt peak-to-peak. Peak-to-peak mea-surement is used mostly with low-level signals that are viewed on oscil-loscopes; ordinary voltmeters usually read rms voltage. The peak-to-peak voltage of a sine wave is 2.828  the rms voltage. Contrast PEAK; RMS.PEBKAC (humorous) (Problem Exists Between Keyboard And Chair) an uncomplimentary way to indicate that a computer problem is the fault of the user. A legendary, and apparently recurrent, PEBKAC consists of the user mistaking the CD-ROM disk tray for a coffee cup holder and even-tually breaking it.peer-to-peer a network architecture in which there is no central server; all computers are equal participants.pel (picture element) PIXEL; one of the small dots of which a bitmap image is composed.pen (in draw or paint programs) a tool used to create lines. Sometimes a similar tool is shown as a pencil. Most programs allow control of the width of the stroke the pen makes, the shape of the nib of the pen, the miter limit, and the shape of the endcaps. See Figure 193.FIGURE 193. Pen toolPentium a high-performance 32-bit microprocessor introduced by Intel in 1993 as the successor to the 486. If the original numbering scheme had been continued, the Pentium would have been the 586, hence its name. The Pentium is software-compatible with the 8088 used in the original IBM PC. Subsequent models (Pentium Pro, Pentium II, Pentium III, and many others) have steadily increased in speed (to about 1000 times that of the original 8088) and have introduced fast caches, 64-bit registers, superscalar pipelining, and multiple cores. Today, the Pentium is the most popular microprocessor for general-purpose computing.per computer (describing a software license) assigned to a specific com-puter or set of computers. Most software is licensed per computer; that is, the license gives you the right to install each copy on one computer for the use of anyone who wants to use it there. Contrast PER SEAT, PER USER. 361	Perlper device (describing a computer license) PER COMPUTER.per incident (describing a service plan) charging a fee each time service is requested. Each request is called an incident.per seat (describing a software license) allowing a specific number of peo-ple to use the software at one time.In some Microsoft documentation, per seat means either PER USER or PER DEVICE, even though neither of these is exactly what others have nor-mally meant by per seat.See SEAT. Contrast PER COMPUTER, PER USER.per user (describing a software license) allowing specific people to use the software. For instance, if a software package is licensed for 12 users and installed on a server, then up to 12 user accounts of the server can access the software. Contrast PER COMPUTER, PER SEAT.peripheral a device connected to a computer. Examples of peripherals include terminals, tape drives, disk drives, and printers.Perl (Practical Extraction and Report Language) a programming language developed by Larry Wall for writing utilities that perform large amounts of string handling, text file processing, and interaction with the operat-ing system. Like AWK and REXX, which it resembles, Perl is normally interpreted, not compiled. It is designed to minimize the programming effort needed for computations that are relatively simple and quick.Figure 194 shows a simple Perl program. The overall syntax resem-bles C. Numbers and strings are interconvertible and are not distin-guished; as far as Perl is concerned, 4 = ’4’.#!/usr/local/bin/perl@path = split(/:/,$ENV{’PATH’});$n = 1 + $#path;print “You have$n directories	on your path.\n”;print	“They	are:\n”;	foreach $i (0..$#path) { print	“$path[$i] \n”; }FIGURE 194. Perl programming exampleEvery variable name begins with a character indicating its type: $for scalars (numbers and strings), @ for ordinary arrays, and % for associative arrays (arrays whose elements are retrieved by keyword rather than by position). Array elements are numbered from 0; the operator$# retrieves the index of the highest element, which is one less than the total number of elements. Lines that begin with # are comments.Under UNIX, Perl programs are often called scripts and begin with the line#!/usr/local/bin/perlso that if the program is passed to UNIX as a SHELL SCRIPT, it will be given to the Perl interpreter for execution. IBM PC;permalink	362On the WORLD WIDE WEB, Perl is often used to implement web pages that perform computations. Instead of writing a web page in HTML, the programmer writes a Perl program that generates the HTML from other sources of information, such as database files, calculations, and even the user’s menu choices. See also AWK; CGI; HTML; REXX.permalink a permanent link; in a BLOG or online newspaper, a permanent link (URL) to a particular article or page, which will remain usable after the article ceases to be today’s news.permission an attribute of a file that indicates who is allowed to read or modify it. For example, the UNIX commandchmod ugo+r-wx myfile.txtsets permissions for the user, group, and other users of file myfile.txt by adding read permission (+r) and removing write and execute permis-sion (-wx). In Windows, with NTFS, you can set permissions on a file or directory by right-clicking on it and choosing Properties, Security.personal computer a computer designed to be used by only one person, either at home or in a business setting. One of the first personal comput-ers was the Digital Equipment Corporation PDP-8, a minicomputeroften used in scientific laboratories in the early 1970s. See alsoMACINTOSH.personal digital assistant see PDA.PERT (Program Evaluation and Review Technique) a method for project planning by analyzing the time required for each step. See PROJECT MANAGEMENT.peta- metric prefix meaning 1,000,000,000,000,000 (1015). Peta- is apparently derived from the Greek word for “to fly” or “to soar.” SeeMETRIC PREFIXES.PGA1.	pin grid array, the arrangement of pins on a Pentium or similar micro-processor, making it possible to plug the processor into a socket.2.	Professional Graphics Adapter, an early high-performance graphics system for the IBM PC, marketed in the mid-1980s.PGP (Pretty Good Privacy) a public-key encryption system developed by Philip Zimmerman.phishing a type of fraud carried out by sending out e-mail that pretends to come from a bank or corporation with which the victim has an account. The e-mail message tells the victim to click on a link in order to handle some kind of urgent business. In reality, both the message and the link are counterfeit, and the victim ends up giving his or her password or credit card information to the phisher, or at the very least, visiting a web site that disseminates MALWARE. 363	photographTo protect yourself from phishing, do not click on web links that arrive in e-mail. Instead, when you want to contact your bank, type its web address yourself. Some web browsers detect links that are known to be phishing sites, but this is not perfect protection because phishers often change their addresses. See also SMISHING; WHALING.phone plug a plug of a type originally designed for telephone switchboards, now used on audio equipment. Three sizes are used, each with 2 to 4 con-ductors. The miniature 3.5-mm 3-conductor phone plug (Figure 195, cen-ter) is used for stereo connections to sound cards. Contrast RCA PLUG; RJ-11.FIGURE 195. Phone plugsphono plug See RCA PLUG.phosphorus chemical element (atomic number 15) added to silicon to cre-ate an N-type SEMICONDUCTOR.Photo CD Kodak’s proprietary format for storing digitized photographs on CD-ROM, now seldom used.photo paper1.	light-sensitive paper used in making conventional photographic prints (not involving a computer).2.	paper that has a glossy coating, but is not light-sensitive, used in inkjet printers for high-quality printing of photographs.photograph a continuous-tone image created with a camera, either digital or film. Because photographs closely reproduce what we are able to see with our eyes, we credit them with being more “real” than drawings. Photographs make any document more interesting and convey much more information to the readers than words alone.You can choose to use photographs from collections of stock pho-tographs, take photographs with a digital camera, or have some of your older photographs scanned in.It’s best to scan the photo at the finished size and at the resolution of the output device (usually your laser printer). The contrast and bright-ness of the image can be adjusted while scanning it.You’ll notice that photographs can create large files. A 31⁄2  5-inch color snapshot, saved as a 300 DPI, 24-bit (millions of colors), uncom-pressed TIFF file, creates a 3.3-megabyte file that is too large to e-mail conveniently. It’s best to save to a compressed file format (e.g., JPEG). Files of black-and-white photos are smaller; they don’t have to include as much information. ADD NOISE;PAINT PRO-photopaint program	364A photopaint program like Adobe PhotoShop or PhotoShop Elements can be used to improve digital images. Spots and flaws can be removed, distracting elements erased, backgrounds changed, or the overall color balance adjusted. (See RETOUCHING.) Then import the digital photo into your page layout or word processing program, or publish it on the World Wide Web. See DIGITAL CAMERA. Contrast GRAYSCALE.photopaint program a type of bitmap editing program with special tools and filters for manipulating photographs. You can also create illustra-tions from scratch with the drawing tools provided. CompareGRAM. Contrast DRAW PROGRAM.In many ways, photopaint programs are the professional versions of the limited paint programs that come with operating systems. They are proportionately difficult to learn and master. Be prepared to spendenough time reading the manual and experimenting. See alsoAIRBRUSH; BLUR; BRIGHTNESS; CLONE; CONTRAST; EDGE DETECT; FILTER; GIMP; MAGIC WAND; MOTION BLUR; PIXELATE.FIGURE 196. Photopaint programPHP a scripting language often used to generate web pages by computation on the server (compare CGI, definition 1; ASP). PHP originated as a set of small, efficient CGI programs to do common tasks, released by Rasmus Lerdorf in 1995. The package was called PHP Tools, short for Personal Home Page Tools. Today, PHP is said to stand for PHP Hypertext Processor (yes, the acronym includes itself; compare GNU).PHP is available free of charge for a wide range of computers. For more information see www.php.net.phreak (or phone phreak) a person who makes a game of defrauding tele-phone companies by means of fake control signals, stolen credit card numbers, and the like. See 2600.pi the Greek letter π, which stands for a special number that is approxi-mately equal to 3.14159. If the radius of a circle is r, then the circum-ference is 2πr and the area is πr 2 . 365	pilcrowpica (in typesetting) a unit of measurement equal to 12 points. There are approximately 6 picas to one inch (2.54 cm).FIGURE 197. Picapick tool a mouse cursor shaped like an arrow that is used to pick up or pick out (select) objects in an image.pickup roller in a printer, the roller that picks up pieces of paper from the paper tray. It usually has a soft, rubbery surface. The pickup roller should be cleaned or replaced if the printer is suffering frequent paper jams from the inability to pick up a piece of paper.pico- metric prefix meaning ÷1,000,000,000,000. Pico- is derived from a Spanish word that means, among other things, “small quantity.” See MET-RIC PREFIXES.PICS (Platform for Internet Content Selection) the specifications for META TAGs that label web sites with their content ratings; the standard used by rating systems such as SafeSurf and ICRA.pictograph1.	a picture that represents an idea. Computer icons are a type of picto-graph.2.	a bar graph that uses stacked or stretched symbols instead of plain vertical bars. Although this lends some visual interest to the graph, it can also be confusing if not handled correctly.pie chart a type of chart that resembles a pie and graphically shows the rel-ative size of different subcategories of a whole. See Figure 248 on page 453.piece fraction a fraction constructed of three characters: small numerals for the numerator and denominator separated by a forward slash or a hori-zontal bar (1⁄2, or 12 ). Contrast BUILT FRACTION; CASE FRACTION. See illus-tration at CASE FRACTION.pilcrow the symbol ¶, which is used to mark the beginning of a new para-graph when the text is set with continuous paragraphs. The pilcrow can also be used as a footnote symbol. See FOOTNOTE. COMPUTER ARCHITECTURE.pin	366pin1.	a movable stiff wire that presses on the ribbon of a dot-matrix printer under computer control in order to make dots on the paper.2.	a stiff prong in an electronic connector. For example, a VGA video connector has 15 pins; a serial port connector has 9 or 25 pins.PIN (Personal Identification Number) a number used as a password by a computer user.ping the command, on a TCP/IP network, that sends a test data packet to another site and waits for a response.ping flooding the practice of maliciously disrupting a computer by pinging it continuously (i.e., flooding it with test data packets to which it must respond). Also known as SMURFING. See DENIAL-OF-SERVICE ATTACK.pipe1. a way of stringing two programs together so that the output of one of them is fed to the other as input. For example, the Windows commandC:\> dir | sort | moreinvokes dir (which lists the names of the files on a disk), feeds its output to sort (which puts the items in alphabetical order), and feeds that out-put to more (which displays it one screenful at a time).See also BROKEN PIPE.2. the character | (the pipe symbol).pipeline a device within a CPU that enables it to fetch (read) instructions in advance of executing them, so that whenever an instruction is completed, the next instruction is ready to execute. This is a way of partly overcom-ing the Von Neumann bottleneck. See alsopiracy the unauthorized copying of software, which is forbidden by law.See COPYRIGHT; SOFTWARE LICENSE.pitch (typesetting) the number of characters per inch in a particular size and style of type. Fixed-pitch type has every character the same width; pro-portional-pitch type has some characters wider than others (e.g., M wider than I), and the pitch can be measured only approximately as the aver-age of many different letters. See Figure 104, page 193.pivot table a multi-dimensional data table that can be rearranged to allow different views of the data. For example, suppose you need to keep track of a budget with two divisions and three spending categories for three months. Here is the original data: 367	pivot table DIVISION	MONTH	CATEGORY	AMOUNTNORTH	January	EMPLOYEES	564NORTH	January	SUPPLIES	320NORTH	January	RENT	40NORTH	February	EMPLOYEES	602NORTH	February	SUPPLIES	348NORTH	February	RENT	40NORTH	March	EMPLOYEES	620NORTH	March	SUPPLIES	352NORTH	March	RENT	40SOUTH	January	EMPLOYEES	212SOUTH	January	SUPPLIES	180SOUTH	January	RENT	20SOUTH	February	EMPLOYEES	240SOUTH	February	SUPPLIES	200SOUTH	February	RENT	20SOUTH	March	EMPLOYEES	265SOUTH	March	SUPPLIES	160SOUTH	March	RENT	20It would be easier to understand the data if it were arranged in a pivot table, like this:NORTH					EMPLOYEES	SUPPLIES	RENT	TOTALJanuary	564	320	40	924February	602	348	40	990March	620	352	40	1012TOTAL	1786	1020	120	2926SOUTH					EMPLOYEES	SUPPLIES	RENT	TOTALJanuary	212	180	20	412February	240	200	20	460March	265	160	20	445TOTAL	717	540	60	1317Sometimes it helps to rearrange the data. Here are two different ways of doing this:January					EMPLOYEES	SUPPLIES	RENT	TOTALNORTH	564	320	40	924SOUTH	212	180	20	412TOTAL	776	500	60	1336February					EMPLOYEES	SUPPLIES	RENT	TOTALNORTH	602	348	40	990SOUTH	240	200	20	460TOTAL	842	548	60	1450 PIX				368March					EMPLOYEES	SUPPLIES	RENT	TOTALNORTH	620	352	40	1012SOUTH	265	160	20	445TOTAL	885	512	60	1457**************************************************	EMPLOYEES					NORTH	SOUTH	TOT	January	564	212	776	February	602	240	842	March	620	265	885	TOT:	1786	717	2503	SUPPLIES					NORTH	SOUTH	TOT	January	320	180	500	February	348	200	548	March	352	160	512	TOT:	1020	540	1560	RENT					NORTH	SOUTH	TOT	January	40	20	60	February	40	20	60	March	40	20	60	TOT:	120	60	180	Or, you might wish to consolidate all of the spending categories and create a view like this:ALL CATEGORIES					January	February	March	TOTALNORTH	924	990	1012	2926SOUTH	412	460	445	1317TOTAL	1336	1450	1457	4243Excel contains a wizard that automatically creates pivot tables.PIX (Private Internet Exchange) a type of secure hardware FIREWALL devel-oped in 1994 by John Mayes and Brantley Coile at Cisco Systems. It will translate network addresses, so the internal network can use IP addresses of its own choosing without worrying about clashes with IP addresses in the worldwide Internet. It was one of the first implementations of NAT (network address translation).pixel one of the individual dots that make up a graphical image. For exam-ple, an xGA color screen in high-resolution mode consists of a 1024  768 pixel array. A program can draw pictures on the screen by control-ling the color of each pixel. See GRAPHICS. 369	platformpixelate, pixelize to transform a bitmap image into rectangular blocks of uniform color, as if the pixels were much larger than before.FIGURE 198. Pixelated imagePKCS (Public-Key Cryptography Standards) a set of standards developed by RSA Laboratories and others. Information on the web is available at www.rsa.com/rsalabs/pubs/PKCS.PL/1 (PL/I) a powerful programming language developed by IBM in the early 1960s to accompany its System/360 computer. The name stands for Programming Language One.PL/1 can be described as a combination of ALGOL 60 block struc-ture, FORTRAN arithmetic, and COBOL data structuring. PL/1 is the language of choice for writing complex programs on IBM mainframe computers, but it has received little use on other types of machines.planar1.	(adjective) flat. For example, planar transistors are made of flat pieces of silicon.2.	(adjective) situated on the motherboard of a computer (e.g., planar RAM).3.	(noun) motherboard. See MOTHERBOARD; contrast DAUGHTERBOARD;RISER.plane1.	in geometry, all the points on a flat surface. Thus a plane is a two-dimensional space on which things have length and width but no thick-ness.2.	in computer graphics, one of several images that are superimposed to produce the final image. For example, many video cards have separate planes (internal bitmaps) for red, green, and blue. The complete image is a combination of the images stored on the three planes. See CHANNEL.plasma glowing ionized gas. See GAS PLASMA DISPLAY.platform a piece of equipment or software used as a base on which to build something else. For example, a mainframe computer can serve as a plat-form for a large accounting system. Microsoft Windows serves as a platform for application software. plenum-rated	370plenum-rated (describing cable) suitable for use in places where air circu-lates, such as above suspended ceilings. (A plenum is a place full of air, the opposite of a vacuum.) Plenum-rated cable is fire resistant and does not give off noxious fumes when overheated. Contrast RISER-RATED.plotter a device that draws pictures on paper by moving pens according to directions from a computer. See GRAPHICS.Plug and Play a standard way of configuring PC-compatible computer hardware automatically, developed by Microsoft and a number of other companies in the mid-1990s. Plug and Play hardware is compatible with conventional hardware (ISA, PCI, PCMCIA, USB, etc.) but has addi-tional capabilities. Each card or accessory inserted into a computer con-tains identifying information that can be read by the BIOS and the operating system. Thus, the computer can see all the installed acces-sories and can configure itself to use them appropriately.plug-in an accessory program that provides additional functions for a main application program. Plug-ins have to be loaded at the same time as the main program; they then show up as an option in an appropriate menu. Plug-ins are also added to a web browser to allow it to view additional file formats, such as multimedia shows.PMS see PANTONE MATCHING SYSTEM.PNP one of the two types of bipolar TRANSISTORs (contrast NPN).PnP abbreviation for PLUG AND PLAY.podcasting (from iPod and broadcast, but not confined to the Apple iPod) the practice of preparing audio and video programs like radio and TV broadcasts, but distributing them through the Internet for playback on MP3 players, iPods, and similar devices. See IPOD.FIGURE 199. Point (in typography)point1.	in geometry, an exact location; in graphics, a precise location or sin-gle PIXEL.2.	a unit of typographical measurement equal to 1⁄72 inch. The height of type is usually expressed in points. However, this is not a measurement of the size of the letters, but rather of the wooden blocks on which the metal type was mounted for printing presses. This usually included some space at the top of the tallest capital letters and below the descenders. Therefore, dif- 371	Polish notationferent typefaces of the same point size may actually differ in size. To this day, even digitized typefaces show some of the same idiosyncrasies. A desire to be faithful to the original designs has prevented the type’s apparent size from being regularized. See LEADING; TYPEFACE; TYPESETTING MISTAKES.FIGURE 200. 24-point type samplespoint-of-sale system a computer used in place of a cash register where merchandise is sold. Besides keeping track of cash, the computer can keep track of inventory and print informative invoices and receipts, and perhaps automatically deduct funds from the customer’s account.point release a minor upgrade of a piece of software, such as the upgrade from version 2.0 to 2.1.point-to-point protocol see PPP; PROTOCOL.pointer1.	an arrow-like symbol that moves around a computer screen under the control of the user. For example, to execute a command in a windowed operating system, use the mouse to move the pointer to the icon repre-senting that command, and then quickly press the mouse button twice.2.	a data item consisting of an address that tells where to find a desired item. For examples, see LINKED LIST; TREE. Pascal, C, and many other programming languages provide a specific data type called a pointer variable that can be used to keep track of data structures that vary in size as the program is executed.3.	a device for pointing to a board or projection screen using a stick or a laser beam.pointing device a computer peripheral that allows you to control your com-puter in a graphical user interface (GUI). The most familiar pointing device is a MOUSE, but some people prefer a TRACKBALL or a GRAPHICS TABLET. The pointing device on a LAPTOP is often a TOUCHPAD or a button that works like a miniature joystick.Polish notation a way of writing algebraic expressions that does not require parentheses to state which operations are done first. It is named in honor of its inventor, Jan Lukasiewicz (1878–1956), whose name most English-speaking mathematicians cannot pronounce. The ordinary algebraic expression4 + (5 – 3) + 2translates into Polish notation as4 5 3 – + 2 + polygon	372To evaluate the expression, work through it from left to right until you encounter an operation (a plus or minus), then perform that operation on the numbers immediately to the left of the operator, replace the numbers and the operator with the result of the operation, and keep going in the same manner. Thus,4 5 3 – + 2 +simplifies to4 2 + 2 +6 2 +8This is technically known as reverse Polish notation (RPN); the expression can also be written in the other direction and worked through from right to left. Many calculators and programming language inter-preters translate expressions into Polish notation in order to evaluate them. Also, Hewlett-Packard calculators use Polish notation rather than parentheses on the ground that Polish notation is easier to work with once the user becomes accustomed to it.polygon a closed geometric figure with any number of straight sides. Triangles, squares, pentagons (five-sided), hexagons (six-sided), hep-tagons (seven-sided), and octagons (eight-sided) are all examples of polygons.FIGURE 201. Polygonspolymorphism the use of different procedures, each with the same name, which are associated with different object types. For example, proce-dures named draw could be associated with the types point, circle, and square. Calling draw for any particular object then activates the right drawing procedure for that type. See OBJECT-ORIENTED PROGRAMMING.Ponzi scheme see PYRAMID SCHEME.pop to remove the topmost item from a stack. See STACK.POP1.	Post Office Protocol, a standard protocol for delivering e-mail to per-sonal computers. See PROTOCOL.2.	point of purchase (point of sale). For example, a POP computer is a computer used as a cash register. See POINT-OF-SALE SYSTEM.3.	point of presence, a place where an INTERNET SERVICE PROVIDER can be accessed, such as a local telephone number. 373	portpop-under ad an advertisement that appears automatically underneath (behind) the WEB PAGE you are viewing. Although pop-under ads are less intrusive than pop-up ads, they are still unwelcome because of the clut-ter they generate. See BANNER AD; MOUSETRAP; POP-UP AD.pop-up ad an advertisement that appears automatically in a separate win-dow when you access a WEB PAGE. See ADWARE. Compare BANNER AD;POP-UNDER AD.pop-up menu see PULL-DOWN MENU.pornography publications or images whose primary purpose is to stimu-late sexual appetite. See also INDECENCY; OBSCENITY.Even the most innocent Internet user occasionally stumbles upon pornography. Some pornographers send advertisements to all the e-mail addresses they can obtain or make up (see SPAM). (Such e-mail does not indicate that the recipient has been viewing pornographic web pages, even if it says that it does.) Others rig their web pages so that SEARCH ENGINEs will lead users to them who are looking for something else.Many individuals find pornography strongly addictive. The purpose of pornography is to make money, and the only reason it is given away free is to get people “hooked” so that they will pay for more.In the United States, most of the pornography on the Internet is legal, but sexually explicit images of children are not, and images of sexual behavior may not be legal, depending on local laws. (See COMPUTER LAW; OBSCENITY.) Law enforcement is hampered by the difficulty of deter-mining the physical locations of web sites. Note however that because the laws prohibit distribution of obscene material, a link to an obscene site can itself be illegal.There have been various efforts to keep pornography from reaching children and unwilling audiences. The Communications Decency Act of 1996 was struck down because it assumed, incorrectly, that Internet ser-vice providers can control the information sent through their computers; replacement legislation has been proposed. Schools and libraries some-times use “filtering” software to block access to known pornographic web sites, but the filtering process is imperfect. See ICRA.Pornography is associated with credit card fraud and other crimes. Several Internet pornography vendors have been caught making unau-thorized charges to credit card numbers, presumably expecting the vic-tims to be too embarrassed to complain. In some cases the numbers were stolen or made up and belonged to people who had never had any con-tact with the vendors.It is naive to describe pornography as “victimless.” The most obvious victims are the individuals who become addicted, and whose relation-ships and even marriages are damaged. The young people hired to pose for pornographic pictures are also victims of exploitation.port1. to adapt a program from one kind of computer to another. For exam-ple, some PC programs have been ported to the Macintosh. port replicator	3742.	a connection where a computer can be connected to an external device, such as a modem, printer, or tape drive. See PARALLEL; SCSI; SERIAL; USB.3.	a unique number used by a microprocessor to identify an input-out-put device. For example, the hexadecimal number 3F8 is the port address for part of the circuitry that controls the first serial port (COM1) on PC-compatible computers.4.	a number identifying the type of connection requested by a remote computer on the Internet. See URL.port replicator a DOCKING STATION for a portable computer that contains serial and parallel ports (and perhaps other ports, such as USB) which substitute for those in the computer itself.portable1.	able to be carried around. A portable computer is larger than a laptop computer, but is still easily movable.2.	(said of programs) able to run on more than one type of computer.Portable Document File see PDF.portal a web site designed for people to visit when they are looking for links to other sites. Examples include www.msn.com and www.yahoo.com as well as the home pages of various Internet service providers.portrait the position in which a sheet of paper is taller than it is wide, like a portrait painting. Most printers print with the paper in portrait orienta-tion. If the printed image can be turned sideways, the result is called landscape orientation. See Figure 148, page 276.POS1.	point of sale. See POINT-OF-SALE SYSTEM.2.	programmable option select, the system for recording the configura-tion of a computer into CMOS RAM so that the computer can boot suc-cessfully. See CMOS RAM.POSIX (usually understood as “portable operating system interface based on UNIX” though this is not its official definition) an IEEE standard set of operating system functions available to software. The POSIX stan-dard makes it possible to write programs that will run under any POSIX-compliant operating system by simply recompiling them. Windows 95, 98, and Me are partially POSIX-compliant; Windows NT, 2000, XP, and Vista are more so. POSIX is a trademark of the IEEE. See IEEE; Z/OS.post to place a message in a NEWSGROUP, BBS, WEB PAGE, or other public dis-cussion forum.posting a message placed in a NEWSGROUP, BBS, or other public discussion forum.PostScript a programming language for controlling laser printers and other graphical output devices, developed by Adobe Systems of Palo Alto, California. A PostScript printer accepts not only characters to be printed 375	PostScript but also commands to change the size of type fonts or to draw lines or cir-cles in specific positions. An application designed to work with PostScript will automatically send PostScript codes to the printer. The user can also write programs in the PostScript language. Figure 202 shows an example. The text following the percent signs is treated as comments, but some of the comments (such as BoundingBox) are used by some types of software. This program is suitable for encapsulation into larger programs; to print it by itself, add the command showpage after the last line.%!PS-Adobe-2.0 EPSF-2.0 %%BoundingBox: 1 800 1 800%	PostScript program to print a gray square%	and the words ’PostScript Example’ /Helvetica-Bold findfont 12 scalefont setfont newpath72	720	moveto	%	start	drawing	rectangle182	720	lineto				182	648	lineto				72	648	lineto				closepath				.75	setgray	%	choose	75% grayfill					0 setgray	%	restore color	to black85	700	moveto				(PostScript) show	%	print	text	85	685	moveto				(Example) show				FIGURE 202. PostScript programFIGURE 203. PostScript output (from the sample program)PostScript works with a coordinate system with the origin at the lower left-hand corner of the page, with units 1⁄72 of an inch long. The program demonstrates the use of the moveto command, the lineto command (which draws a line from the previous point to the indicated point), and the fill command (which fills an area with a desired shade of gray). Text is printed by enclosing it in parentheses and then using the com-mand show. Before printing, the appropriate font must be selected; fonts can be scaled to different point sizes. PostScript also lets the program-mer use variables and define abbreviations and procedures. Figure 203 shows the output of Figure 202. POTS	376Many printers expect a Ctrl-D (UNIX end-of-file mark) at the end of every PostScript job. Some software also generates a Ctrl-D at the begin-ning of the job, to clear out anything that may have previously been sent to the printer.PostScript was introduced in 1985; Level 2 PostScript, an extended version of the language, was introduced in 1991 and is now standard. Encapsulated PostScript (EPS) is a file format for using the PostScript language to exchange graphics between programs. EPS files must con-tain a BoundingBox comment, must follow certain other restrictions, and can contain bitmap previews of the image. Notoriously, software that imports some EPS files does not necessarily understand the entire PostScript language; many programs confine themselves to the Adobe Illustrator (AI) subset of EPS.POTS (Plain Old Telephone Service) humorous name for conventional ana-log telephone lines compatible with all telephones made since the 1920s. Designing modems for POTS lines has been a challenging engineering problem. At present, the highest possible data rate is thought to be about 56 kbps, but other, much lower, rates were thought to be the highest pos-sible rate in past years. Contrast CABLE MODEM; DSL; ISDN; T1 LINE.pound key the key on a telephone marked with the symbol #. It is often used to signal the end of an international telephone number.pound sign1.	the character #. Also called an OCTOTHORPE.2.	the character £ denoting British pounds.power cycle to switch off electric power to a device, then switch it on again. This is one way to REBOOT a computer. However, it should be done only in emergencies because many operating systems (including UNIX and Windows) will lose data if not shut down properly. After turning power off, always wait a few seconds for capacitors to discharge and disks to spin down before turning the power on again.power line protection measures taken to protect a computer from prob-lems caused by the AC power supplied by the wall outlet. Several things can go wrong:1.	Brief bursts (“spikes”) of excessive voltage can damage the com-puter. These spikes come from lightning or from large electric motors switching off. They are easily absorbed by a surge protec-tor (see SURGE PROTECTOR).2.	Power failures cause the computer to shut down or restart sud-denly, losing the data that you were working on. A surge protector cannot prevent this. If the problem is frequent, you may want to invest in an uninterruptible power supply (UPS).3.	The computer can emit radio or TV interference through the power line. See RFI PROTECTION. power supply the part of a computer or other electronic equipment that supplies power to the other parts. The power supply generally includes 377	precisionstepdown  transformers	and voltage  regulators.  See  also  SWITCHINGPOWER SUPPLY.The watt and ampere ratings of a power supply represent the maxi-mum that it can deliver; the actual power consumed will depend on the devices attached to it. Correct practice is to use a power supply with the correct voltages and correct or higher watt and ampere ratings.PowerPC a family of high-performance 32- and 64-bit microprocessors developed jointly by IBM, Motorola, and Apple to compete with the Intel microprocessors and Microsoft software that were dominating the market. From 1994 to 2006, PowerPC CPUs were used in the Apple Macintosh. Despite the name, the PowerPC microprocessor has never been used in Windows-based PCs.PowerPoint presentation software sold with Microsoft’s Office suite. The user can create an outline version of a talk and then display it one page at a time or one line at a time, and graphics can be included as well. Speaker notes or handouts can also be printed. See PRESENTATION GRAPHICS.PowerToys a set of small Windows utilities distributed free of charge by Microsoft to allow additional customization of the operating system.PPM (pages per minute) a measure of the speed of a printer.PPP (Point-to-Point Protocol) a communications protocol often used inDIAL-UP NETWORKING. Compare SLIP. See PROTOCOL.PPPoE (Point to Point Protocol over Ethernet) a protocol allowing the use of PPP (as would be used in dial-up networking) when the user is con-nected to the network through an Ethernet port (as would be the case for a user connected to the Internet via a cable modem).precedence the property of arithmetic operations that determines which operations are done first in a complex expression.Typically, exponentiations are done first, then multiplications and divisions, and finally additions and subtractions. For example, the Excel expression=5+4*3^2means 5 + 4  9 = 5 + 36 = 41. You can use parentheses to change the order of the operations when you need to, since any operation in paren-theses will be done first. For example,=5+(4*3)^2means 5 + 122  =5 + 144 = 149.precision the exactness with which a quantity is specified. For numbers, the precision is the number of significant digits that the computer keeps track of when it carries out arithmetic operations. See ROUNDING ERROR. For examples in Java, see DOUBLE PRECISION; LONG. preferences	378Precision is entirely separate from accuracy. If I weigh 175 pounds and you say that I weigh 150.03535 pounds, your assertion is precise but not accurate.preferences settings for a computer program to allow for individual differ-ences. The preferences menu is sometimes a rather obscure catchall for adjustments to mouse tracking, the double-click rate, the NUDGE rate, and the brush style. Take the time to become familiar with the “Preferences” settings in your software; sometimes a problem can be quickly solved by making a small adjustment.preflight the step just before printing an image or document. During pre-flight checking, software can detect missing fonts, images too large for the paper, and the like. The name apparently alludes to the preflight checks performed on aircraft just before taking off.prepend to append at the beginning; to put in front. For example, if you prepend // to a line in a C++ program, that line becomes a comment.prepress the preparation of material to be printed in quantity on a printing press. Many prepress functions are highly automated and well suited to being performed with computers. For example, there are software pack-ages that take a PostScript file and slightly increase the outline of color areas to create TRAP. The graphic arts industry now uses a digital pro-duction system where the printing plates are prepared directly from com-puter files. See also PDF.presentation graphics the use of software to create business or educational presentations that integrate text, drawings, charts, music, sound effects, and animation. Examples of presentation software include PowerPoint (Microsoft) and Freelance Graphics (Lotus). Typically, these programs include drawing tools and the ability to make charts out of data taken from a spreadsheet program. Users can standardize the color scheme and graphic style for all of the images. By viewing miniature images of the presentation slides all on the same screen, the user can obtain an overview of the finished product and sort the order of the slides. Presentations are usually shown using a video projector. See also MULTI-MEDIA; TRANSITION EFFECT.press to depress a mouse button and hold it down until the mouse action is completed. Contrast CLICK. FIGURE 204. Press 379	print spoolerpretzel (slang) nickname for   , the Macintosh command key.preview a viewing mode that displays the appearance of the finished doc-ument or drawing. In order to let you work quickly, many drawing pro-grams show you only an outline of the objects on screen. (This is called WIREFRAME mode.) When you want to see what the final printed piece would look like, you have to use the “Preview” command. Then all of the objects appear with their fills so that you can check whether they are layered correctly.Most drawing programs will allow you to work in preview mode, but the time spent redrawing the screen after each action can be quite irri-tating. If it becomes a problem, work in the wireframe view and preview frequently to check your work. Another method is to open two windows containing the same file, one in wireframe and the second in preview.Most word processors also allow you to preview your document.primary key a field in a database record with a unique identifier. When sorting a database, the items will be sorted first according to the primary key. Sometimes you also wish to define a secondary key. For example, you may wish to sort a list of club members first by their membership date (the primary key), and then alphabetically by their last name (the secondary key).primary mouse button the button used to select objects on a mouse with more than one button. For a right-handed user, this is usually the left mouse button. The SECONDARY MOUSE BUTTON is the button used to call up the action menu.Left-handed users have the option of reversing the default order for mouse buttons; they can use the right mouse button as the primary but-ton and the left button as the secondary.primitive a basic element or concept in terms of which larger elements or concepts are formed. For example, in programming languages such as FORTH and Lisp, it is common for programmers to create their own state-ments by defining them in terms of primitives provided by the language.print head1.	The part of an INKJET PRINTER that actually contacts the paper in order to print. It contains tiny holes through which ink is sprayed; if some of these holes are clogged, printing will be streaky and the print head should be cleaned as specified in the instructions.2.	The part of an IMPACT PRINTER that presses on the ribbon, which con-tacts the paper.print server a computer through which other computers access a printer over a network. A print server may be an ordinary computer or a small circuit board or box mounted inside or just outside the printer.print spooler a program that stores computer output in memory so that the user’s program can finish creating the output without waiting for the printer	380printer to print it. The spooler then sends the stored output to the printer at the proper speed. Print spoolers are built into Windows and UNIX.See also SPOOLING; BUFFER.printer a device for putting computer output on paper or other appropriate media such as transparencies and adhesive labels. See DAISYWHEEL PRINTER; DOT-MATRIX PRINTER; ELECTROSTATIC PRINTER; IMPACT PRINTER;INKJET PRINTER; LASER PRINTER; LINE PRINTER; PAGE PRINTER; PICKUP ROLLER; THERMAL PRINTER.private key the key (password, code word) that the recipient of a message uses to decrypt a message that was encrypted with the recipient’s PUBLIC KEY. Only the recipient of a message knows his or her own private key.See ENCRYPTION..pro a suffix indicating that a web or e-mail address belongs to a licensed professional, such as a physician, lawyer, or accountant (in any country).Contrast .COM. See also ICANN; TLD.procedure a SUBROUTINE; a smaller program that is part of a main program. The procedure is executed when the main program calls it.Procedures eliminate the need to program the same thing more than once. If you know that one task will be performed more than once in your program, it is better to write a procedure to handle the task rather than duplicate the program code when the task is needed again. Also, a large program is easier to understand if it consists of procedures, each with a well-defined purpose. See TOP-DOWN PROGRAMMING.A procedure that returns a value is often called a FUNCTION. In object-oriented languages such as Java, a procedure is called a METHOD.process a series of instructions that a computer is executing in a multitask-ing operating system. Many processes execute concurrently. From the user’s viewpoint, processes may be programs or parts of programs (such as the editing routine and the printing routine in a word processor that can print while editing). See MULTITASKING; UNIX.process color four-color printing on a commercial printing press, usually used for producing a full-color publication. See CMYK. Contrast SPOT COLOR.processor see COPROCESSOR; CPU; MICROPROCESSOR.profile1.	in any software package or operating system, a file of saved infor-mation that contains settings chosen by the user.2.	in Windows, the folder containing information specific to one user, including account information, numerous settings and preferences, e-mail files, the DESKTOP and all the files on it, and the like. See DOCU- MENTS AND SETTINGS. See also ROAMING USER PROFILES.3. a user’s home page on a social networking site, displaying basic bio-graphical information and pictures. 381	programming languageprogram a set of instructions for a computer to execute. A program can be written in a programming language, such as C or Java, or in an assem-bly language. See APPLICATION PROGRAM; UTILITY.programmable function key a key on a computer keyboard whose func-tion depends on the software being run. In many cases, programmable function (PF) keys can be defined as equivalent to combinations or sequences of other keys.programmatically (adverb) by means of a computer program. For exam-ple, in Windows, the volume level of the speaker can be changed pro-grammatically; that is, software can change it.programmer a person who prepares instructions for computers.programming the process of composing instructions for a computer to carry out. A programmer needs to develop a well-defined concept of how to solve a problem. (See ALGORITHM.) Then this concept must be trans-lated into a computer language. (See PROGRAMMING LANGUAGE.) Finally, the program needs to be typed into the computer, tested, and debugged before being placed into service.programming language a language used to give instructions to computers. During the 1960s and 1970s, a huge variety of programming languages were developed, most of which are no longer in wide use. Moreover, a substantial amount of programming is now done with special program development tools (e.g., Visual Basic), or in programming languages that pertain to specific pieces of software (e.g., Maple) rather than by simply writing instructions in a general-purpose language.The following is a rough classification of programming languages. Most of these languages are treated in separate articles in this book.1.	General-purpose languages for large, complex programs: PL/I, C, C++, Pascal, Modula-2, Ada, Java, C#.2.	General-purpose languages for smaller programs: BASIC, Visual Basic, Pascal, Python.3.	Mathematical calculation, science, and engineering: FORTRAN, APL, Maple, and the general-purpose languages named above.4.	Business data processing: COBOL, RPG. Where microcomputers are involved, BASIC, C, and languages associated with specific database products are also widely used.5.	Artificial intelligence and programs of extreme logical complex-ity: Lisp and Prolog.6.	String handling and scripting: SNOBOL, REXX, Awk, Perl, Python, VBSCRIPT, JavaScript.Another useful classification is based on the way the program is organized.1.	Sequential languages treat the program as a series of steps, with an occasional GOTO statement as a way of breaking out of the sequence. In this category are FORTRAN, BASIC, and COBOL project	382(though COBOL also allows programs to be written in a style more like a block-structured language).2.	Block-structured languages encourage structured programming by allowing the programmer to group statements into functional units. (See STRUCTURED PROGRAMMING.) This category originated with Algol and now includes Pascal, Modula-2, C, PL/I, and Ada.3.	Object-oriented languages allow the programmer to define new data types and associate procedures with them. Languages of this type include C++, Java, C#, object-oriented extensions of Pascal, and Smalltalk.4.	Symbolic languages allow the program to examine and modify itself, treating instructions as data. Lisp and Prolog fall into this category.Usage note: Although names of some programming languages are normally written in all capital letters, names of most languages are not, even if they are acronyms. Usage varies from language to language. With some, usage has shifted over the years. See also BASIC (Usage note).See also ARRAY; BACKTRACKING; COMMENT; EXTREME PROGRAMMING;LOGO; LOOP; OBJECT ORIENTED PROGRAMMING; RECURSION; SOFTWARE ENGI-NEERING; SORT; STRUCTURED PROGRAMMING; TOP-DOWN PROGRAMMING.project the set of all files needed to produce the ready-to-use version of a program. Typically, the compiler accepts procedures from several differ-ent files and combines them into one executable (EXE) file. See MAKE; LINK.FIGURE 205. Project management: Gantt chart for moving to a new homeproject management the scheduling of a complex project involving many different tasks. A typical task requires some resources and a certain amount of time; it also requires that certain other tasks have already been finished. You may sometimes schedule two tasks to be performed simul- 383	prontaneously if they don’t overtax the supply of available resources, but when the tasks are sequential, you must schedule them in the proper order. For example, the engines on the wings of an airplane cannot be installed until the wings have been built. A project manager program takes the information the user enters for each task and then determines how to schedule the tasks. The results are often presented in the form of a diagram called a Gantt chart (see Figure 205).Prolog a programming language developed in the early 1970s by Alain Colmerauer at the University of Marseilles and standardized by the ISO in 1995. Prolog is used for writing computer programs that model human thinking. It exemplifies logic programming, a kind of programming developed by Robert Kowalski of the University of London.In ordinary programming, a program describes the steps that a com-puter is to work through in order to solve a problem. In logic program-ming, the program gives the computer facts about the problem, plus rules by means of which other facts can be inferred. The computer then applies a fixed procedure to solve the problem automatically.For example, Prolog can chain together the fact “Atlanta is in Georgia” and the rule “X is in the U.S.A. if X is in Georgia” to answer the question, “Is Atlanta in the U.S.A.?” In Prolog, the fact and the rule are:located_in(atlanta,georgia). located_in(X,usa) :-located_in(X,georgia).The question (called a query) is typed:?-located_in(atlanta,usa).and is answered “yes.”One of the most important properties of Prolog is its ability to back-track, that is, to back up and try alternative solutions. This is necessary whenever the search starts pursuing a chain of rules that do not lead to a solution.Prolog is not confined to the simple kind of logic described here. It can implement all types of algorithms, including sorting, numerical computation, and parsing. See also ANONYMOUS VARIABLE; ARTIFICIAL INTELLIGENCE; BACKTRACKING; SINGLETON VARIABLE.PROM (Programmable Read-Only Memory) a type of computer memory that can be programmed once but not reprogrammed. See also EPROM.promiscuous mode a mode in which a computer reads all the data packets on the network, not just those addressed to it. See SNIFFER.prompt a symbol that appears on a computer terminal screen to tell the user that the computer is ready to receive input. See also COMMAND PROMPT.pron deliberate misspelling of porn, often used to discuss pornography when a chat filter blocks other words for it. prop	384prop an object placed within the scene of an animation.properties the attributes of any object. Under Windows, menus titled “Properties” are the usual way of changing settings. In Windows, right-click on an object to change its properties. For example, the properties of an icon specify what it looks like and what should happen when the user clicks on it.proportional pitch the use of characters with different widths in a single typeface. For example, in proportional-pitch type, M is wider than I. Compared to a fixed-pitch typewriter or printer, this improves the appearance of the type and makes it more readable. Most books and newspapers are set in proportional type. See TYPE; TYPEFACE. For an illus-tration, see PITCH.Because the letters are of different widths, it is not possible to count letter spaces in proportional-pitch type the way one does on a typewriter.See TYPESETTING MISTAKES.proprietary owned by a specific company or individual. A feature of a computer is proprietary if one company has exclusive rights to it.protocol a standard way of carrying out data transmission between com-puters. See HANDSHAKING; ATM (definition 4); DHCP; FTP; GOPHER; HTTP;IIOP; IMAP; IPV6; IPXSPX; KERBEROS; KERMIT; NETBEUI; POP; PPP; REALAU-DIO; SAMBA; SMTP; SOAP; TCP/IP; TELNET; TWAIN; VOIP; X.25.proxy an item that represents something else. See also PROXY SERVER.proxy server a computer that saves information acquired from elsewhere on the INTERNET and makes it available to other computers in its imme-diate area. For example, if several users connect to the same WEB SITE through a proxy server, each page of information will be downloaded from that site only once and then provided to all the users.A disadvantage of proxy servers is that they make it impossible to count HITs accurately.PS 21.	an advanced version (Level 2) of the POSTSCRIPT graphics language.2.	Sony Playstation 2, a video game machine.PS/2 see IBM PC.PS/2 keyboard, PS/2 mouse a keyboard or mouse with a small round con-nector of the type originally used on the IBM PS/2 but now widely used on other PC-compatible computers.pseudocode an outline of a computer program, written in a mixture of a programming language and English. Writing pseudocode is one of the best ways to plan a computer program. For example, here is a pseudocode outline of a Pascal program to find the largest of a set of 10 numbers: 385	pullbegin repeatread a number;test whether it is the largest found so far until 10 numbers have been read;print the largest end.Here is the program that results from translating all of the pseudocode into genuine Pascal:PROGRAM findthelargest (INPUT, OUTPUT); VAR num, largest, count : INTEGER; BEGINcount := O; largest := O; REPEATcount := count + 1; read(num);IF num > largest THEN largest := num; UNTIL count = 10;writeln(’The largest number was ’,largest)END.(This program assumes that the largest number will be greater than zero.)The advantage of pseudocode is that it allows the programmer to con-centrate on how the program works while ignoring the details of the lan-guage. By reducing the number of things the programmer must think about at once, this technique effectively amplifies the programmer’s intelligence.PST the file extension on archived e-mail messages in Microsoft Outlook.public domain the status of literature, art, music, or software that was not copyrighted, or whose copyright has expired and not been renewed.A computer program is in the public domain if it is not covered by any kind of copyright. Few substantial public-domain programs exist, but the term “public domain” is often used incorrectly to describe other kinds of freely copyable software (see FREE SOFTWARE). See also COPYRIGHT.public key a publicly revealed password used for encoding private mes-sages to a particular recipient. The recipient then uses his or her own secret PRIVATE KEY to decrypt the message. See ENCRYPTION.public html typical name for the directory in which an individual user’s WEB PAGE is stored under UNIX.pull the process whereby the user retrieves information from a network at the user’s request, as in traditional web browsing; contrast PUSH (definition 2). pull-down menu	386pull-down menu a menu that appears when a particular item in a menu bar is selected. See also MENU BAR.FIGURE 206. Pull-down menupunched card a stiff paper card on which holes can be punched to encode data that can be read by a computer. In the 1960s, punched cards were the dominant way of feeding programs into computers, but they have now been replaced by interactive keyboards. Punched cards are still used in some voting systems. (see CHAD).Standard punched cards had 80 columns and each card corresponded to one line of a text file. The use of punched cards for data processing actually preceded the invention of the computer by more than 50 years. Herman Hollerith realized that it took several years to process data from the 1880 census. He calculated that, unless a faster method was found, the U.S. Census Bureau would still be working on the results from the 1890 census when it came time to start the 1900 census. Hollerith devel-oped a system in which census data were punched on cards, and machines were used to sort and tabulate the cards. Even earlier, in 1801, a system of punched cards was used to direct the weaving pattern on the automatic Jacquard loom in France.purge to discard data that is no longer wanted; in an e-mail reading pro-gram, to discard messages that have been marked for deletion.push1.	to place an item on a stack. See STACK.2.	to deliver information to a client machine without waiting for the user to request it. Push technology makes the World Wide Web work rather like TV; the user selects a “channel” and views whatever is being sent out at the moment. This contrasts with the way web browsers traditionally work, where the user manually selects information to retrieve from the Web. (Contrast PULL.) Push technology is useful for delivering informa-tion that has to be updated minute by minute, such as stock market quotes or news bulletins. (See RSS.) However, the user demand for push technol-ogy is not what had once been expected. Users do not want to give up control of the Internet in order to watch it passively like television. push technology See PUSH (definition 2). 387	Pythonpushdown stack, pushdown store a data structure from which items can only be removed in the opposite of the order in which they were stored.See STACK.pushing the envelope working close to, or at, physical or technological limits. See ENVELOPE.PvE (Player versus Environment) a type of game where players overcome challenges given to them by the game itself rather than by other players.PvP (Player versus Player) a type of game where players compete against each other.pwn comical misspelling of own in the slang sense. See OWN.pyramid scheme (Ponzi scheme) a get-rich-quick scheme in which you receive a message containing a list of names. You’re expected to send money to the first person on the list, cross the first name off, add your name at the bottom, and distribute copies of the message.Pyramid schemes are presently common on the Internet, but they are illegal in all 50 states and in most other parts of the world. They can’t work because there is no way for everyone to receive more money than they send out; money doesn’t come out of thin air. Pyramid schemers often claim the scheme is legal, but that doesn’t make it so. See alsoCOMPUTER LAW.Python a programming language invented by Guido van Rossum for quick, easy construction of relatively small programs, especially those that involve character string operations. Figure 207 shows a simple Python program that forms the plurals of English nouns.Python is quickly replacing Awk and Perl as a scripting language. Like those languages, it is run by an interpreter, not a compiler. That makes it easy to store programs compactly as source code and then run them when needed. It is also easy to embed operating system commands in the program.Python is also popular for teaching programming to non-program-mers, since even a small, partial knowledge of the language enables peo-ple to write useful programs.The syntax of Python resembles C and Java, except that instead of enclosing them in braces, groups of statements, such as the interior of a while loop, are indicated simply by indentation.Powerful data structures are easy to create in Python. These include lists and dictionaries, where a dictionary is an array whose elements are identified by character strings. Operations such as sorting, searching, and data conversion are built in.Free Python interpreters and more information can be obtained from www.python.org. See also AWK; INTERPRETER; PERL; STRING OPERATIONS. Python	388 #	File plural6.py -M. Covington 2002#	Python function to form English pluralsdef pluralize(s):”Forms the plural of an English noun”# Exception dictionary. More could be added.e = { ”child”	: ”children”,”corpus”	: ”corpora”,”ox”	: ”oxen” }# Look up the plural, or form it regularlyif e.has_key(s): return e[s]elif	s[-1] == ”s” \or	s[-2:] ==	”sh” \or	s[-2:] ==	”ch”:	return s +	”es”else:		return s +	”s”FIGURE 207. Python program 389	QuicksortQQoS Quality of Servicequad-core having four CPU cores. See CORE (definition 1).quantum computing a possible method for creating future computers based on the laws of quantum mechanics. Classical computers rely on physical devices that have two distinct states (1 and 0). In quantum mechanics, a particle is actually a wave function that can exist as a superposition (combination) of different states. A quantum computer would be built with qubits rather than bits. Theoretical progress has been made in designing a quantum computer that could perform computations on many numbers at once, which could make it possible to solve prob-lems now intractable, such as factoring very large numbers. However, there are still practical difficulties that would need to be solved before such a computer could be built.quantum cryptography an experimental method for securely transmitting encryption keys by using individual photons of polarized light. A funda-mental principle of quantum mechanics, the Heisenberg uncertainty principle, makes it impossible for anyone to observe a photon without disturbing it. Therefore, it would be impossible for an eavesdropper to observe the signal without being detected. See ENCRYPTION.qubit a quantum bit. See QUANTUM COMPUTING.query language a language used to express queries to be answered by a database system. For an example, see SQL.queue1.	a data structure from which items are removed in the same order in which they were entered. Contrast STACK.2.	a list, maintained by the operating system, of jobs waiting to be printed or processed in some other way. See PRINT SPOOLER.Quicken a popular financial record keeping program produced by INTUIT.Quicksort a sorting algorithm invented by C. A. R. Hoare and first pub-lished in 1962. Quicksort is faster than any other sorting algorithm avail-able unless the items are already in nearly the correct order, in which case it is relatively inefficient (compare MERGE SORT).Quicksort is a recursive procedure (see RECURSION). In each iteration, it rearranges the list of items so that one item (the “pivot”) is in its final position, all the items that should come before it are before it, and all the items that should come after it are after it. Then the lists of items pre-ceding and following the pivot are treated as sublists and sorted in the same way. Figure 208 shows how this works:(a) Choose the last item in the list, 41, as the pivot. It is excluded from the searching and swapping that follow. QuickTime	390(b), (c) Identify the leftmost item greater than 41 and the rightmost item less than 41. Swap them.(d), (e), (f), (g) Repeat steps (b) and (c) until the leftmost and right-most markers meet in the middle.(h), (i) Now that the markers have met and crossed, swap the pivot with the item pointed to by the leftmost marker.(j) Now that the pivot is in its final position, sort each of the two sub-lists to the left and in right of it. Quicksort is difficult to express in lan-guages, such as BASIC, that do not allow recursion. The amount of memory required by Quicksort increases exponentially with the depth of the recursion. One way to limit memory requirements is to switch to another type of sort, such as selection sort, after a certain depth is reached. (See SELECTION SORT.) Figure 209 shows the Quicksort algo-rithm expressed in Java.FIGURE 208. Quicksort in actionQuickTime a standard digital video and multimedia framework originally developed for Macintosh computers, but now available for Windows-based systems. The QuickTime Player plays back videos and other multimedia presentations and is available as a free download from www.apple.com/downloads.The premium version of QuickTime provides video editing capability as well as the ability to save QuickTime movies (.mov files). CompareAVI FILE; MOV. 391	quit class quicksortprogram{/* This Java program sorts an array using Quicksort. */static int a[] = {29,18,7,56,64,33,128,70,78,81,12,5}; static int num = 12; /* number of items in array */ static int max = num-1; /* maximum array subscript */static void swap(int i, int j){int t=a[i]; a[i]=a[j]; a[j]=t;}static int partition(int first, int last){/*	Partitions a[first]...a[last] into 2 sub-arrays using a[first] as pivot. Value returned is position where pivot ends up. */int pivot = a[first]; int i = first;int j = last+1;do{do { i++; } while ((i<=max) && (a[i]<pivot)); do { j——; } while ((j<=max) && (a[j]>pivot)); if (i<j) { swap(i,j); }}while (j>i);swap(j,first); return j;}static void quicksort(int first, int last){/* Sorts the sub-array from a[first] to a[last]. */ int p=0;if (first<last){p=partition(first,last); /* p = position of pivot */ quicksort(first,p-1);quicksort(p+1,last);}}public static void main(String args[]){quicksort(0,max);for (int i=0; i<=max; i++){System.out.println(a[i]);}}}FIGURE 209. Quicksort quit to clear an application program from memory; to EXIT. Most software prompts you to save changes to disk before quitting. Read all message boxes carefully. race condition, race hazard	392Rrace condition, race hazard in digital circuit design, a situation where two signals are “racing” to the same component from different places, and although intended to be simultaneous, they do not arrive at exactly the same time. Thus, for a brief moment, the component at the destination receives an incorrect combination of inputs.radial fill a way of filling a graphical object with two colors such that one color is at the center, and there is a smooth transition to another color at the edges. See FOUNTAIN FILL. Contrast LINEAR FILL.FIGURE 210. Radial fillradian measure a way of measuring the size of angles in which a complete rotation measures 2π radians. The trigonometric functions in most com-puter languages expect their arguments to be expressed in radians. To convert degrees to radians, multiply by π/180 (approximately 1/57.296).radio buttons small circles in a dialog box, only one of which can be cho-sen at a time. The chosen button is black and the others are white. Choosing any button with the mouse causes all the other buttons in the set to be cleared. Radio buttons acquired their name because they work like the buttons on older car radios. Also called OPTION BUTTONS.FIGURE 211. Radio buttonsradix the base of a number system. Binary numbers have a radix of 2, and decimal numbers have a radix of 10.radix sort an algorithm that puts data in order by classifying each item immediately rather than comparing it to other items. For example, you might sort cards with names on them by putting all the A’s in one bin, all the B’s in another bin, and so on. You could then sort the contents of each bin the same way using the second letter of each name, and so on. 393	railroad diagramThe radix sort method can be used effectively with binary numbers, since there are only two possible bins for the items to be placed. For other sorting methods, see SORT and references there.ragged margin a margin that has not been evened out by justification and at which the ends of words do not line up.This isan example offlush-left, ragged-right type.See also FLUSH LEFT, FLUSH RIGHT.RAID (redundant array of inexpensive disks) a combination of disk drives that function as a single disk drive with higher speed, reliability, or both. There are several numbered “levels” of RAID.RAID 0 (“striping”) uses a pair of disk drives with alternate sectors written on alternate disks, so that when a large file is read or written, each disk can be transferring data while the other one is moving to the next sector. There is no error protection.RAID 1 (“mirroring”) uses two disks which are copies of each other. If either disk fails, its contents are preserved on the other one. You can even replace the failed disk with a new, blank one, and the data will be copied to it automatically with no interruption in service.RAID 2 (rarely used) performs striping of individual bits rather than blocks, so that, for instance, to write an 8-bit byte, you need 8 disks. Additional disks contain bits for an error-correcting code so that any missing bits can be reconstructed. Reliability is very high, and any sin-gle disk can be replaced at any time with no loss of data.RAID 3 (also uncommon) performs striping at the byte rather than bit or sector level, with error correction.RAID 4 performs striping at the level of sectors (blocks) like RAID 0, but also includes an additional disk for error checking.RAID 5 is like RAID 4 except that the error-checking blocks are not all stored on the same disk; spreading them among different disks helps equalize wear and improve speed. RAID 5 is one of the most popular configurations. If any single disk fails, all its contents can be recon-structed just as in RAID 1 or 2.RAID 6 is like RAID 5 but uses double error checking and can recover from the failure of any two disks, not just one.Caution: RAID systems do not eliminate the need for backups. Even if a RAID system is perfectly reliable, you will still need backups to retrieve data that is accidentally deleted and to recover from machine failures that affect all the disks at once.railroad diagram a diagram illustrating the syntax of a programming lan-guage or document definition. Railroad-like switches are used to indi-cate possible locations of different elements. Figure 212 shows an example illustrating the syntax of an address label. First name, last RAM	394name, and City-State-Zip Code are required elements, so all possible routes include those elements. Either Mr. or Ms. is required, so there are two possible tracks there. A middle name is optional, so one track bypasses that element. There may be more than one address line, so there is a return loop track providing for multiple passes through that element.FIGURE 212. Railroad diagram.RAM (Random-Access Memory) a memory device whereby any location in memory can be found as quickly as any other location. A computer’s RAM is its main working memory. The size of the RAM (measured in megabytes or gigabytes) is an important indicator of the capacity of the computer. See DRAM; EDO; MEMORY.random-access device any memory device in which it is possible to find any particular record as quickly, on average, as any other record. The computer’s internal RAM and disk storage devices are examples of ran-dom-access devices. Contrast SEQUENTIAL-ACCESS DEVICE.random-access memory see RAM.random-number generator a computer program that calculates numbers that seem to have been chosen randomly. In reality, a computer cannot generate numbers that are truly random, since it always generates the numbers according to a deterministic rule.However, certain generating rules produce numbers whose behavior is unpredictable enough that they can be treated as random numbers for practical purposes. Random-number generators are useful in writing programs involving games of chance, and they are also used in an impor-tant simulation technique called MONTE CARLO SIMULATION.rapid prototyping the construction of prototype machines quickly with computer aid. Computerized CAD-CAM equipment, such as milling machines and THREE-DIMENSIONAL PRINTERS, can produce machine parts directly from computer-edited designs.raster graphics graphics in which an image is generated by scanning an entire screen or page and marking every point as black, white, or another color, as opposed to VECTOR GRAPHICS.A video screen and a laser printer are raster graphics devices; a pen plotter is a vector graphics device because it marks only at specified points on the page.raster image processor (RIP) a device that handles computer output as a grid of dots. Dot-matrix, inkjet, and laser printers are all raster image processors. BITMAP; RASTER GRAPHICS;395	real estate rasterize to convert an image into a bitmap of the right size and shape tomatch a raster graphics output device. SeeVECTOR GRAPHICS.RAW in digital photography, unprocessed; the actual binary data from the camera, with, at most, only the processing that is unavoidably done by the camera itself. Although commonly written uppercase (RAW), this is simply the familiar word raw, meaning “uncooked.”Raw image files contain more detail than JPEG compressed images but are much larger and can only be read by special software.ray tracing the computation of the paths of rays of light reflected and/or bent by various substances.Ray-tracing effects define lighting, shadows, reflections, and trans-parency. Such computations often are very lengthy and may require sev-eral hours of computer time to produce a RENDERING (realistic drawing of three-dimensional objects by computer).RCA plug (also called PHONO PLUG) an inexpensive shielded plug some-times used for audio and composite video signals (see Figure 213); it plugs straight in without twisting or locking. Contrast BNC CONNECTOR.FIGURE 213. RCA plugRDRAM (Rambus dynamic random access memory) a type of high-speed RAM commonly used with the Pentium IV, providing a bus speed on the order of 500 MHz. Contrast SDRAM.read to transfer information from an external medium (e.g., a keyboard or diskette) into a computer.read-only pre-recorded and unable to be changed. See ATTRIBUTES; CD-ROM; LOCK; WRITE-PROTECT.read-only memory computer memory that is permanently recorded and cannot be changed. See ROM.readme (from read me) the name given to files that the user of a piece of software is supposed to read before using it. The readme file contains the latest information from the manufacturer; it often contains major corrections to the instruction manual.real estate (informal) space on a flat surface of limited size, such as a motherboard (on which different components consume different amounts of real estate) or even a computer screen. Compare SCREEN ESTATE. real number	396real number any number that can be represented either as an integer or a decimal fraction with any finite or infinite number of digits. Real num-bers correspond to points on a number line.Examples are 0, 2.5, 345, –2134, 0.00003,	1	,	2 , and π. However,					3			−1 is not a real number (it does not exist anywhere among the positive or negative numbers). Contrast COMPLEX NUMBER.On computers, real numbers are represented with a finite number of digits, thus limiting their accuracy. See ROUNDING ERROR.In many programming languages, “real number” means “floating-point number.” See DOUBLE; FLOATING-POINT NUMBER.real-time programming programming in which the proper functioning of the program depends on the amount of time consumed. For instance, computers that control automatic machinery must often both detect and introduce time delays of accurately determined lengths.RealAudio a communication protocol developed by Real Networks (www.realaudio.com) that allows audio signals to be broadcast over the Internet. The user hears the signal in real time, rather than waiting for an audio file to be downloaded and then played. RealAudio is used to dis-tribute radio broadcasts. See INTERNET RADIO; PROTOCOL.RealPlayer a widely used program for playing RealAudio files, distributed by Real Networks. See REALAUDIO.ream 500 sheets of paper.reboot to restart a computer (i.e., turn it off and then on again). Many oper-ating systems, including UNIX and Windows, have to be shut down properly before power to the computer is turned off; otherwise, data will be lost. See BOOT.record a collection of related data items. For example, a company may store information about each employee in a single record. Each record consists of several fields—a field for the name, a field for a Social Security number, and so on.The Pascal keyword record corresponds to struct in C. See STRUCT.recovering erased files retrieval of deleted files whose space has not yet been overwritten by other data.In Windows and on the Macintosh, deleted files usually go into a TRASH can or RECYCLE BIN from which they can be retrieved. The disk space is not actually freed until the user empties the trash. Until then, the files can be restored to their original locations.Even after the trash can or recycle bin has been emptied, the physical disk space that the file occupied is marked as free, but it is not actually overwritten until the space is needed for something else. If you erase a file accidentally, you can often get it back by using special software. As soon as you realize you want to recover a file, do everything you can to 397	recursionstop other programs from writing on the same disk so that nothing else will be written in the space that the file occupied.recursion the calling of a procedure by itself, creating a new copy of the procedure.To allow recursion, a programming language must allow for local variables (thus, recursion is not easy to accomplish in most versions of BASIC). Each time the procedure is called, it needs to keep track of val-ues for the variables that may be different from the values they had the last time the procedure was called. Therefore, a recursive procedure that calls itself many times can consume a lot of memory.Recursion is the natural way to solve problems that contain smaller problems of the same kind. Examples include drawing some kinds of fractals (see FRACTAL); parsing structures that can have similar structures inside them (see PARSING); sorting (see QUICKSORT); and calculating the determinant of a matrix by breaking it up into smaller matrices.A recursive procedure can be used to calculate the factorial of an inte-ger. (See FACTORIAL.) Figure 214 shows a program that does so.class factorial_program {/* Java program to find the factorial of awhole number (4 in this example) by recursion */ static int factorial(int x){System.out.println(”Now looking for factorial of ” + x); int z=1;if (x<1){z=1;}else{z=x*factorial(x-1); /* this is the recursive step */}System.out.println(”The factorial of ”+x+” is ” + z); return z;}public static void main(String args[]){System.out.println(factorial(4));}}FIGURE 214. RecursionA simple example of recursion involves finding factorials, defined as follows:1.	The factorial of 0 or 1 is 1.2.	The factorial of any larger whole number x is x times the factorial of x – 1. Recycle Bin	398This definition is recursive in step 2, because to find a factorial, you have to find another factorial. It can be translated directly into a recursive computer program (Figure 214). Admittedly, this is not the fastest way to do the computation, but it is a classic example.In the program, the recursion occurs when the function factorial calls itself. Note that the else clause is crucial. That clause gives a non-recursive definition for the factorial of zero. If it were not there, the pro-gram would end up in an endless loop as the function factorial kept calling itself until the computer ran out of memory. Any time recursion is used, it is necessary to make sure that there is some condition that will cause the recursion to halt.Following is an example of the output from this program when the number 4 is given as the input. In practice, you would want to remove the two println statements from the function, but they are included here to make it possible to see the order of execution.Now looking for factorial of 4 Now looking for factorial of 3 Now looking for factorial of 2 Now looking for factorial of 1 Now looking for factorial of 0 The factorial of 0 is 1The factorial of 1 is 1 The factorial of 2 is 2 The factorial of 3 is 6 The factorial of 4 is 24 24Any iterative (repetitive) program can be expressed recursively, and recursion is the normal way of expressing repetition in Prolog and Lisp.Recycle Bin in Windows, the place where deleted files are stored, corre-sponding to the TRASH on the Macintosh. You can put a file in the Recycle Bin by dragging it there or by choosing “delete” in Explorer and similar applications. Files in the Recycle Bin still occupy disk space and are retrievable. To reclaim the disk space, empty the Recycle Bin.FIGURE 215. Recycle BinRed Book the Philips/Sony standard format for audio compact discs.See CD.Red Hat a company headquartered in Raleigh, N.C., that sponsors the Red Hat and Fedora distributions of Linux. Red Hat distributions were orig-inally freeware, but the current product, Red Hat Enterprise Linux, is commercially licensed and supported. It is recommended for organiza-tions that need commercial support for Linux. The freeware Red Hat 399	reflectionproject continues under the name Fedora. For more information see www.redhat.com. See also FEDORA. Compare DEBIAN, UBUNTU. (It’s not related to BLACK HAT or WHITE HAT despite the similar name.)redirect in HTML, an instruction to go directly to another web page with-out requiring the user to click. This is achieved with an HTML instruc-tion such as:<META HTTP-EQUIV=”Refresh” CONTENT=”0; URL=www.termbook.com”>This means: “Refresh (reload) this page immediately (after 0 seconds) from a different address, namely www.termbook.com.”redline to mark a portion of a printed document that has been changed. Redlining is usually a line (originally red) in the margin or a gray shad-ing applied to the marked area. It is used with manuals, laws, regula-tions, and the like, where changes need to be marked.redo to reverse the effect of the most recent UNDO command.redundancy1.	unnecessary repetition; lack of conciseness. Data files can be com-pressed by removing redundancy and expressing the same data more concisely. See DATA COMPRESSION.2.	the provision of extra information or extra hardware to increase reli-ability. For example, a simple way to protect a message from errors in transmission is to transmit it twice. It is very unlikely that both copies will be corrupted in exactly the same way. See ERROR-CORRECTING CODE.Redundant hardware is extra hardware, such as one disk drive serving as a backup for another. See also RAID.reentrant procedure a procedure that has been implemented in such a way that more than one process can execute it at the same time without con-flict. See MULTITASKING.refactoring the process of reorganizing a computer program without changing its functionality. Refactoring a program usually means divid-ing it up into conceptual units and eliminating repetitious code.referential integrity in a database, the requirement that everything men-tioned in a particular field of one table must be defined in another table. As an example, consider a database with two tables, one listing cus-tomers and their addresses, and the other listing orders the customers have placed. A referential integrity requirement might specify that every customer who appears in the order table must also be listed in the cus-tomer table.reflection the ability of a computer program to obtain information about itself. Some computer languages, such as LISP and PROLOG, support extensive reflection; the program can treat itself as data. In Microsoft.NET Framework, the reflection subsystem allows a running program to obtain information about its classes (object types). See CLASS. reflow	400reflow (rewrap) to rearrange a written text so that the ends of lines come out more even. For example, reflowing will change this text:Four score and seven years ago our forefathersbrought forth upon this continent a newnation, conceived in liberty...into this:Four score and seven years ago our forefathers brought forth upon this continent a new nation, conceived in liberty...Reflowing changes the total number of lines and thus the positions of the page breaks. See WORD WRAP.refresh1.	to update the contents of a window to show information that has changed; to REPAINT the screen.2.	to RELOAD the contents of a WEB PAGE from the machine on which it resides.3.	to regenerate the display on a CRT screen by scanning the screen with an electron beam. Though it seems to, the screen does not glow contin-uously; instead, it is refreshed 30 to 90 times per second, rather like a movie screen.4.	to freshen the contents of a memory chip. Dynamic RAM chips have to be refreshed many times per second; on each refresh cycle, they read their own contents and then store the same information back into the same memory location.refresh rate the rate at which a CRT screen is repeatedly scanned to keep the image constantly visible; typically 30 to 90 hertz (cycles per second). A faster refresh rate gives an image less prone to flicker. Since LCD dis-plays hold the image in memory, the refresh rate is not critical.regional settings the settings in an operating system that pertain to the user’s location, such as language, currency, and time zone.register1.	a row of flip-flops used to store a group of binary digits while the computer is processing them. (See FLIP-FLOP.) A flip-flop can be in either of two states, so one flip-flop can store 1 bit. A register consisting of 16 flip-flops can store words that are 16 bits long.2.	to inform a manufacturer of a purchase (see REGISTRATION, definition 1). The word register has many other meanings in business and education; only those specific to computers are covered here. registrar an organization authorized to register TLD. For example, the domain covingtoninnovations.com belongs to three of the authors of this book because they have registered it with a registrar. 401	relational databaseregistration1.	the act of informing the manufacturer of a product that you have pur-chased and installed it. Registering software is a good idea because it makes you eligible for customer support and upgrades from the manu-facturer.2.	the alignment of color plates in a multi-color printing job on a print-ing press. If the colors are not perfectly aligned, there may be MOIRÉS, or there may be unintentional gaps between colors. See TRAPPING.3.	the recording of information in the Windows Registry or similar con-figuration files. See REGISTRY.Registry the part of Windows that stores setup information for the hard-ware, software, and operating system. It takes over most of the functions performed by INI files in earlier versions of Windows.The information in the Registry is supplied by the Control Panel and the setup routines for particular applications. You can also view the con-tents of the Registry directly by choosing Run from the START MENU and typing regedit. This is rarely necessary unless unusual problems arise.See HIVE.FIGURE 216. Registry Editor (Regedit)regular expression a way of defining a possible series of characters. Table 12 gives some examples. In UNIX, the grep command searches a file for character strings that match a regular expression; regular expressions are also used in several programming languages and in some editors.See AWK; GREP; PERL.Regular expressions are efficient to process because the computer can always tell whether a string matches a regular expression by working through the string and the expression from left to right, one item at a time. This is simpler than the methods used to parse Backus-Naur form or other kinds of syntactic description. See BACKUS-NAUR FORM; PARSING.relational database a database that consists of tables made up of rows and columns. For example: relative address	402		TABLE 12		REGULAR EXPRESSIONS					Expression	Matches				abc	The string abc	a.c	Like abc but with any character in place of b	a*bc	Zero or more a’s, followed by bc	a*b+c	Zero or more a’s, one or more b’s, and c\*	An asterisk\\A backslash[BbCx]	The character B, b, C, or x[A-E2-4]	The character A, B, C, D, E, 2, 3, or 4[^A-E2-4]	Any character except A, B, C, D, E, 2, 3, or 4[Ff]ill	Fill or	fill		^abc	abc at beginning of line		abc\$	abc at end of line															Name			City	State							Downing, D.		Seattle	Washington		Covington, M.		Athens	Georgia							The table defines a relation between the things in each row. It says that Seattle is the city for Downing, Athens is the city for Covington, and so on.One important operation in a relational database is to join two tables (i.e., cross-reference information between them). For example, the names in this table could be cross-referenced to another table containing names and salaries; the result would be a table relating name, city, state, and salary.A database with only one table is called a flat-file database. Every relational database has a query language for expressing commands to retrieve data. See PIVOT TABLE; QUERY LANGUAGE; SQL.relative address1.	in computer memory, a memory address measured relative to another location. To convert a relative address into an absolute (true) address it is necessary to add the address of the point it is measured from.Compare OFFSET.2.	in a spreadsheet program, a cell address that indicates the position of a cell relative to another cell. If this formula is copied to another loca-tion, the address will be changed so that it refers to the cell in the same position relative to the new cell. In Lotus 1-2-3 and Microsoft Excel, a cell address is treated as a relative address unless it contains dollar signs. (See ABSOLUTE ADDRESS.) For example, if the formula 2*D7 is entered into the cell E9, the D7 in the formula really means, “the cell that is one col- 403	renderingumn to the left and two rows above.” If this formula is now copied to cell H15, the formula will now become 2*G13, since G13 is the cell that is one column to the left and two rows above the cell H15.relative URL a URL for a document in the same directory as the current document. For example, if a web page contains the link <a href=”doc1.html”> it will look for the document doc1.html in the same directory as the page containing the link. If you copy both of these files to a different directory or different machine, the link will still work.Contrast ABSOLUTE URL.release1.	the edition or version number of a software product. Most commonly, whole-number increments in the release number (e.g., from 1.0 to 2.0) signify a major upgrade to the program. Fractional increases are for minor upgrades and bug fixes.2.	to let go of the mouse button. Contrast CLICK; PRESS.reload to obtain a fresh copy of information that is already in a computer; an example is reloading a WEB PAGE that may have changed recently, rather than viewing a copy stored in a CACHE on your own computer.remote located on a computer far away from the user. Contrast LOCAL.Remote Desktop a feature of some versions of Microsoft Windows that allows one computer to serve as the screen, keyboard, and mouse of another; thus, any computer can be operated remotely. This is particularly handy for administering servers that may be located in a different room.To enable remote access to a computer, go to Control Panel, System, Remote, and turn on remote access. Add one or more user accounts to the Remote Desktop Users security group. If the computers involved are sep-arated by a firewall, make sure port 3389 traffic is allowed between them.Once you have made a computer accessible, you can “remote in” to it from a second computer by going to Programs, Accessories, Communication, Remote Desktop Connection, and typing its network address. The host computer’s desktop will be a window on the screen of the client computer.Common versions of Windows allow one or two remote users at a time. Server versions can be licensed to allow larger numbers of users.remoting the spreading of a computational task across multiple computers in different locations.remove spots a paint program filter that erases spots from digitized pho-tographs and pictures. Technically, it removes all pixel groups below a certain size; image detail may be lost.render (3-D program) to apply a color, texture, and lighting to a WIRE-FRAME model.rendering the technology of drawing three-dimensional objects realisti-cally on a computer. It involves computations of texture and light reflec- REFLOW; WRAPrepaginate	404tions. Rendering is performed automatically by VRML viewers. See alsoRAY TRACING; VRML.repaginate to allow a word processor or page layout program to reposition page breaks by working forward from the current cursor position. Seealsorepaint to regenerate the display on all or part of a computer screen.repeat keyword used to define one kind of loop in Pascal. The word REPEAT marks the beginning of the loop, and the word UNTIL marks the end. Here is an example:REPEAT writeln(x); x := 2*x;writeln(’Type S if you want to stop.’); readln(c); {c is of type CHAR}UNTIL c = ’S’;The computer always executes the loop at least once because it does not check to see whether the stopping condition is true until after it has exe-cuted the loop. See DO. Contrast WHILE.repeater a device that receives signals by network cable or by radio and retransmits them, thereby overcoming limits of cable length or radio range. A repeater can also conserve BANDWIDTH by retransmitting only the data packets that are addressed to sites in its area.required hyphen a hyphen that does not indicate a place where a word can be broken apart. For instance, if the hyphenated word “flip-flop” falls at the end of the line, then “flip-” can appear on one line, with “flop” at the begin-ning of the next. But if you type “flip-flop” with a required hyphen, it will not be split up. In Microsoft Word, to type a required hyphen (also called a NON-BREAKING HYPHEN), press Ctrl-Shift and the hyphen key together.required space a space that does not denote a place where words can be split apart at the end of a line. For instance, you might not want a per-son’s initials (as in “T. S. Eliot”) to be split at the end of a line. You should therefore use required spaces between them rather than ordinary spaces. In TEX, a required space is typed as ~ (TILDE). In Microsoft Word, a required space (also called a NON-BREAKING SPACE) is typed by pressing Ctrl-Shift and the space bar together.resample to change the size of a bitmap image or the sampling rate of a digital audio file, using interpolation to fill in the intermediate samples (Figure 217). See also INTERPOLATION (definition 2).reseat to remove an integrated circuit (IC) or a printed circuit board from its socket and reinsert it. This often yields a better electrical connection. 405	resolutionFIGURE 217. Resampling (interpolation)to enlarge an imagereserve price a secret minimum bid in an auction. Ordinarily, the minimum bid (the lowest price that the seller will take) is announced to would-be buyers. However, auction services such as eBay allow the seller to spec-ify a secret minimum bid, called a reserve price. The amount of the reserve price is not disclosed, but bids below it do not result in a sale.See AUCTION; EBAY.reserved word a word that has a special meaning in a particular program-ming language and cannot be used as a variable name. For example, in C and its derivatives, if is a reserved word. COBOL has dozens of reserved words. FORTRAN and PL/I have none, since in these lan-guages it is always possible to tell from the context whether or not a word is a variable name.resistance the measure of how difficult it is for electric current to flow through a circuit or component. Resistance is measured in a unit called the ohm. See OHM’S LAW.resize to change the size or dimensions of; to SCALE.To resize an object interactively with the mouse in most environments, select the object, and then drag one of the HANDLEs in the desired direc-tion. Dragging a corner handle will keep the vertical and horizontal aspects of the object in the same proportion to each other (like reducing or enlarging something on a photocopier). Dragging one of the handles at the midpoint of the BOUNDING BOX will affect only one dimension of the object. This way, you can stretch or shrink the object to the desired shape.resolution a measure of the amount of detail that can be shown in the images produced by a printer or screen. For instance, many laser print-ers have a resolution of 600 dots per inch (dpi), which means that they print characters using a grid of black and white squares each 1/600 of an inch across. This means that their resolution is 300 lines per inch when printing line art, or 100 lines per inch when printing halftone shadings (such as photographs), which use pixels in groups of six.Inkjet printers often have very high resolution (e.g., 2800 dots per inch), which means they control the position of the ink sprayer to a pre-cision of 1/2800 inch. The actual dots of colored ink are much larger than 1/2800 inch in size. However, halftoning is not needed; each dot can be any color or shade of gray. resource	406The human eye normally resolves about 150 lines per inch at normal reading distance, but a person examining a page critically can distin-guish two or three times this much detail.The resolution of a screen is given as the total number of pixels in each direction (e.g., 1024  768 pixels across the whole screen). The equivalent number of dots per inch depends on the size of the screen. Present-day video screens resolve about 100 dots per inch; they are not nearly as sharp as ink on paper.A big advantage of draw programs, as opposed to paint programs, is that they can use the full resolution of the printer; they are not limited to printing what they display on the screen. However, some paint programs can handle very detailed images by displaying only part of the image at a time. See DRAW PROGRAM; PAINT PROGRAM; VECTOR GRAPHICS.resource1.	anything of value that is available for use. Resources can refer to computers on a network, preallocated memory blocks in an operating system, or money in a budget.2.	a modifiable part of a program, separate from the program instruc-tions themselves. Resources include menus, icons, and fonts.resource leak see LEAK.restart (in Windows) to REBOOT.restore to make a window go back to its previous size after being mini-mized or maximized. In Windows, the restore button is to the right of the minimize button on the title bar and alternates with the maximize (full-screen) button. Or, right-click the application’s icon on the taskbar; the top choice of the pop-up menu is “Restore.”See also MAXIMIZE; MINIMIZE; WINDOW.FIGURE 218. Restore buttonretouching the alteration of a digital image to change its content, e.g., by removing visible blemishes on the skin of a person in a portrait. See PHO-TOPAINT PROGRAM. Because they are so easily retouched, digital images are not usable as evidence (in science or in courtrooms) unless their authen-ticity can be proven. Retouching is different from image processing, which involves applying a uniform transformation to the entire image to enhance the visibility of information already contained in the image. 407	reverseretrocomputing the hobby of preserving old computer technology, either by maintaining the machines themselves or by emulating them on newer equipment. See Figure 219.FIGURE 219. Retrocomputing: a 1981 computer emulated under Windows 2000return1.	the keyboard key that transmits ASCII code 13 (CR), normally the same as the Enter key. See CR.2.	to give a value as a result of a computation. For example, in many programming languages, sqrt(2) returns the square root of 2.Returning a value is not the same as printing it out; returning a value makes it available for further computation, as in sqrt(2)+3.3.	in C and related languages, the statement that causes the computer to exit a function or subroutine and return to the program that called it. For example, return x; means “exit, returning the value of x as the value of the function,” and return; means “exit, returning no value.”Return key the key on a computer keyboard that tells the computer that the end of a line has been reached. On most keyboards the Return key is marked Enter. On IBM 3270 terminals, the Return and Enter keys are separate.reusable components pieces of software that can be used in other pro-grams. For example, Java classes are reusable; they can be used by pro-grams other than the one for which they were originally created.reverse (in graphics) to replace white with black and black with white. A reversed block of type can be a dramatic design element—however, leg-ibility can become a factor. A large block of reverse text is difficult to read. Typefaces with hairline strokes do not reverse well. The letters may spread and fill in if the font size is too small. Always evaluate a proof of reverse type carefully.Type can also be reversed out of a color or a tint. Check that there is enough contrast between the type and the background for the text to be read. reverse engineer	408FIGURE 220. Reversed typereverse engineer to find out how something works by examining and dis-assembling the finished product.reverse Polish notation see POLISH NOTATION.revert to reload from disk a previously saved version of a file, losing all intermediate changes. Revert is therefore a super-undo command. Save your file before attempting a potentially dangerous command (search and replace or applying a filter), and you will have the option of revert-ing to the older file in case something goes wrong.rewrap See REFLOW.REXX a programming language used to write procedures that contain operating system commands. REXX is used in OS/2 .CMD files, IBM PC-DOS 7.0 .BAT files, and some IBM mainframe operating systems.Compare AWK; PERL.RF (radio-frequency) a frequency in the range that is typical of radio waves, approximately 0.5 to 2000 megahertz. Contrast AF.RFC1.	(radio-frequency choke) an inductor (coil) designed to keep high-fre-quency signals from flowing into power supply lines and other intercon-nections. See RFI PROTECTION.2.	(Request For Comment) one of numerous documents defining the standard for the Internet. All are supposedly unofficial, although most are followed universally. For example, RFC 822 specifies the format for E-MAIL messages in transit. RFCs are available online at www.cis.ohio-state.edu/hypertext/information/rfc.html and other sites.RFI protection protection of electronic equipment from radio-frequency interference.Computers use the same kind of high-frequency electrical energy as radio transmitters. This often causes RFI (radio-frequency interference), also called EMI (electromagnetic interference). All computers interfere with nearby radio and TV reception to some extent, and sometimes the problem is severe. On rare occasions, the opposite happens — a strong signal from a nearby radio transmitter disrupts the computer, or two computers interfere with each other. See EMC.Here are some suggestions for reducing RFI:1.	If possible, move the radio or TV receiver away from the com-puter, and plug it into an outlet on a different circuit. 409	RIMM2.	Supply power to the computer through a surge protector that includes an RFI filter (see SURGE PROTECTOR).3.	Ground the computer properly (see SURGE PROTECTOR).4.	Use high-quality shielded cables to connect the parts of the com-puter system together. Make sure all cable shields and ground wires are connected properly. This is especially important for the monitor cable and the printer cable. If possible, wind the cable into a coil to increase its inductance.5.	Check that the computer has the appropriate approval from the FCC (Federal Communications Commission). Some computers are not approved for use in residential areas. See CLASS A; CLASS B;FCC.RFID (radio-frequency identification) the use of radio signals to recognize, from a few feet away, a tiny device (“RFID tag”) that can be built into price tags, library books, parking permits, ID cards, passports, or the like. RFID tags are even implanted under the skin of dogs for positive identi-fication so that they can be returned to their owners if lost and found.The RFID tag consists of an antenna and an integrated circuit, but no battery. The antenna picks up enough power from the transmitter that it can energize the integrated circuit and transmit a response, typically just an identifying number. The RFID tag itself contains almost no informa-tion; its usefulness comes from a database linking its ID number to other records.RFP (Request For Proposal) an invitation to submit a price quotation, sales pitch, or grant proposal.ribbon in the redesigned user interface of Microsoft Office 2007, the part of the screen containing tabs providing access to commands.ribbon bar a row of small icons arranged just below the menu bar of a win-dow. Each icon gives the user access to a frequently used command.rich text text that contains codes identifying italics, boldface, and other special effects. WORD PROCESSING programs deal with rich text rather than plain ASCII or UNICODE text. See RTF. Contrast NONDOCUMENT MODE; TEXT FILE.Rich Text Format see RTF.right-click to CLICK the SECONDARY MOUSE BUTTON (usually the right but-ton). In Windows, right-clicking the mouse will pop up an action menu that includes access to the “Properties” dialog for the selected object.RIM (Research In Motion) the producer of the BLACKBERRY. Web address: www.rim.com.RIMM (Rambus inline memory module) a memory module similar to a SIMM, but containing Rambus high-speed memory (RDRAM). Ring 0, Ring 1, Ring 2, Ring 3	410Ring 0, Ring 1, Ring 2, Ring 3 levels of privilege for processes running on a Pentium-family microprocessor. Normally, parts of the operating sys-tem run at Ring 0 (the maximum privilege level), and everything else runs at Ring 3.rip1.	(from raster image processing) to convert a PostScript or vector graphics file to a bitmap file suitable for a particular output device, such as a color printer.2.	to convert an audio file from audio CD format to a digital format such as MP3.RISC (Reduced Instruction Set Computer, pronounced “risk”) a CPU design with a small number of machine language instructions, each of which can be executed very quickly. The Sun Sparcstation and the PowerPC are examples of RISC computers. The opposite of RISC is CISC.RISC architecture was developed for speed. A RISC computer can exe-cute each instruction faster because there are fewer instructions to choose between, and thus less time is taken up identifying each instruction.RISC and CISC computers can run the same kinds of software; the only difference is in what the software looks like in machine code. CISC is faster than RISC if memory access is relatively slow; the RISC machine has to fetch more instructions from memory than the CISC machine to do the same work. RISC is faster than CISC if memory access is very fast. See also CISC; COMPUTER ARCHITECTURE; POWERPC.riser a small circuit board inserted perpendicularly into the motherboard, containing slots for cards. Compare DAUGHTERBOARD. See also CARD (definition 2); MOTHERBOARD.riser-rated (describing cable) suitable for use inside walls and in open areas but not in places where air circulates, such as above suspended ceilings. Riser-rated cable is fire-resistant but can give off noxious fumes when overheated. Contrast PLENUM-RATED.river a series of white spaces between words that appear to flow from line to line in a printed document, like the white patch in the following exam-ple. Rivers result from trying to justify type when the columns are too narrow or the available software or printer is not versatile enough. SeeJUSTIFICATION.Quo usque tandem abutere, Catilina, patientia nostra? quamdiu etiam furor iste tuus nos eludet?RJ-11 the 4-pin modular connector used to connect telephones and modems to the telephone line (see Figure 221, right). One RJ-11 con- 411	rloginnector can support two telephone lines, one on the inner pair of pins and one on the outer pair.RJ-45 the 8-pin modular connector used on the ends of 10base-T and 100base-T cables (see Figure 221, left); it resembles a 4-pin telephone connector but is wider. The color code for wiring RJ-45 connectors is shown in Table 13. See also CATEGORY 3 CABLE, CATEGORY 5 CABLE.FIGURE 221. RJ-45 connector (left ) andRJ-11 connector (right )TABLE 13RJ-45 CONNECTOR WIRING FOR 10BASE-T and 100BASE-T NETWORKST568A	T568B	1 white-orange	1 white-green2 orange	2 green3 white-green	3 white-orange4 blue	4 blue5 white-blue	5 white-blue6 green	6 orange7 white-brown	7 white-brown8 brown	8 brown	Pins are numbered from left to right as seen with the plug pointing away from you, contacts up.Note that one twisted pair (on pins 3 and 6) goes to nonadjacent pins.Normal cables are T568A or T568B at both ends. A crossover cable is T568A at one end and T568B at the other.RL abbreviation for “real life” in e-mail and online games.rlogin (remote login) the UNIX command that allows you to use your com-puter as a terminal on another computer. Unlike telnet, rlogin does more than just establish a communication path: it also tells the other com-puter what kind of terminal you are using and sends it your user name. ARTIFICIAL INTELLIGENCE.RMI	412RMI (Remote Method Invocation) technique for calling a method in a Java class located on a machine (such as a web server) different from the machine (such as the browser client) on which the current application is running.rms (root-mean-square) the most common method of measuring the voltage of an alternating current; the square root of the mean of the square of the instantaneous voltage. This method of measurement is used because power (wattage) depends on the voltage squared; thus, 120 volts AC rms will light a light bulb to the same brightness as 120 volts DC. With a sine wave, the rms voltage is 0.707  the peak voltage or 0.353 the peak-to-peak voltage. Contrast PEAK; PEAK-TO-PEAK.roaming user profiles in Windows NT and its successors, a facility that allows each user’s desktop, account information, and files to be stored on a server so that they are accessible from any networked PC at which the user logs on. See PROFILE (definition 2).robot1. a computer that moves itself or other objects in three-dimensional space under automatic control. Robots are now widely used in manufac-turing. See also2.	(slang; also bot) a computer program that performs a human-like communication function such as replying to E-MAIL or responding to messages in a NEWSGROUP. See also DAEMON.3.	a program that searches the World Wide Web, gathering information for indexing in search engines. See CRAWLER; SEARCH ENGINE; SPIDER. See also META TAG.robust reliable even under varying or unforeseen conditions. Contrast BRIT-TLE.Rock Ridge a compatible extension to the ISO 9660 CD-ROM format, allowing longer filenames, commonly used in UNIX systems. On com-puters that do not support Rock Ridge format, the discs can still be read, and the files still have unique names, but the names are shortened.Compare JOLIET FILE SYSTEM.ROFL online abbreviation for “rolling on the floor laughing.” See alsoROTFL.RoHS (Restrictions on Hazardous Substances) a directive adopted by the European Community and effective on July 1, 2006, requiring the almost complete elimination of lead, mercury, cadmium, hexavalent chromium, polybrominated biphenyls, and polybrominated diphenyl ethers in electronic equipment sold in Europe. Similar restrictions are being adopted elsewhere.The main effect of RoHS is to mandate the use of lead-free solder and to eliminate nickel-cadmium batteries. See NICD; SOLDER. 413	rolloverrole-playing game a game in which the player controls a fictional charac-ter that is not merely a representation of themselves (contrast AVATAR). Computer games that are designated as role-playing usually have design elements in common with table-top games like Dungeons and Dragons, such as the use of “experience points” and “leveling up” to measure characters’ increases in power. MUDs, MOOs, and MMORPGs are all role-playing games.roll-up menu a dialog box that can be “rolled up” to just the size of its title bar to keep it visible but reduce its size when it is not in use. It is very similar in concept to a TOOLBOX.FIGURE 222. Roll-up menuroller (as part of a printer) see PICKUP ROLLER; TRANSFER ROLLER.rollerball see TRACKBALL.rollover1.	an important change in the date or another gradually increasing num-ber, such as the date rollover from 1999 to 2000.2.	an explanatory note that appears as the mouse cursor is placed onto (rolls over) a key word, icon, or graphic even though the mouse has not been clicked. Rollovers are used by operating systems and application programs, but are especially common on web pages. FIGURE 223. Rollover (definition 2) rollover	414 JavaScript can be used to provide rollover effects on a web page. The following example uses the status line at the bottom of the browser win-dow to include a description of a link when the mouse passes over it:<html><head><title>Mouse Rollover example</title><script language=’javascript’> <!—— hidefunction rollOn(choicedescription) { window.status=choicedescription;}function rollOut() { window.status=” ”;}--> </script></head> <body><h1>Example of a Mouse Rollover, using the status line</h1> <ul><li><a href=”#choice1”onMouseOver=”rollOn(’Here is text that describes choice 1’); return true;”onMouseOut=”rollOut(); return true;”> Choice 1 </a><li><a href=”#choice2”onMouseOver=”rollOn(’Here is text that describes choice 2’); return true”onMouseOut=”rollOut(); return true;”> Choice 2 </a></ul><a name=”choice1”><h2>Here is choice 1</h2></a> Here is some text for choice 1.<br><a name=”choice2”><h2>Here is choice 2</h2></a> Here is some text for choice 2.<br> </body></html>The next example changes the display of the image when the mouse rolls over the links:<html><head><title>Mouse Rollover example</title><script language=‘javascript’> <!—— hidefunction rollOn(choicenum) { if (choicenum==1){ document.image1.src=”image_for_choice_1.jpg”; } else {document.image1.src=”image_for_choice_2.jpg”; }}function rollOut() { document.image1.src=”default_image.jpg”;}--></script></head> 415	rootkit<body><h1>Example of a Mouse Rollover with a changing image</h1> <ul><li><a href=”#choice1” onMouseOver=”rollOn(1); return true;” onMouseOut=”rollOut(); return true;”> Choice 1 </a><li><a href=”#choice2” onMouseOver=”rollOn(2); return true;” onMouseOut=”rollOut(); return true;”> Choice 2 </a></ul><img name=”image1” height=”200” width=”300” src=”default_image.jpg”><a name=”choice1”><h2>Here is choice 1</h2></a> Here is some text for choice 1.<br><a name=”choice2”><h2>Here is choice 2</h2></a> Here is some text for choice 2.<br> </body></html>See also HTML; JAVA; JAVASCRIPT; RIGHT-CLICK.ROM (Read-Only Memory) a computer memory that contains instructions that do not need to be changed, such as permanent parts of the operating system. The computer can read instructions out of ROM but cannot store new data in it. See also CD-ROM; EPROM; PROM.ROM BIOS see BIOS.roman the kind of type that books are normally typeset in, as opposed to italics or boldface. The type you are reading now is roman type. SeeTYPEFACE.root the account name used by the system administrator under UNIX. (From ROOT DIRECTORY.)root directory the main directory of a disk, containing files and/or subdi-rectories. See DIRECTORY.root hub the set of USB ports located inside a computer. See USB.root-mean-square see RMS.rootkit a software package that tampers with the innermost kernel of an operating system, concealing its presence and its effects unusually well because it can intercept any attempt to detect it.Originally, a “root kit” was a set of cracking programs designed to run under the ROOT (system administrator) account of UNIX while leaving no trace of their presence.Today, most rootkits are for Windows. The only sure way to detect them is to boot an entirely separate operating system, such as a copy of rot13	416Windows on a CD, and scan the installed Windows kernel to verify its authenticity.Compare VIRUS. See also COMPUTER SECURITY; CRACKER; MALWARE.rot13 (rotate 13) a type of ENCRYPTION commonly used on the Internet to conceal answers to puzzles and the like. To encode a message, the first 13 letters of the alphabet are swapped with the last 13. Performing the same swap again decodes the message. This is not a secure code, of course, but it provides a way to make things temporarily unreadable.Qba’g lbh jbaqre jung guvf fnlf?rotate in draw programs, to turn an object around a specific center. By default, the center of rotation is at the center of the object itself. You can drag this center to wherever you want it to be and then rotate the object around it. Rotation can be done interactively with the mouse, or if you require more precision, you can set the angle of rotation in degrees.FIGURE 224. RotateROTFL online abbreviation for “rolling on the floor laughing.” See alsoROFL.round brackets the  characters  (),  more  properly  called  parentheses.Contrast ANGLE BRACKETS; CURLY BRACKETS; SQUARE BRACKETS.rounding the act of replacing a number with the nearest number that has a smaller number of significant digits. For example, 2.76 rounded to one decimal place is 2.8. The rule is that if the first digit to be discarded is 5 or greater, the last digit that is kept should be increased. Thus 2.74 → 2.7, but 2.76 → 2.8. Contrast TRUNCATION.An alternative way to do rounding—easier on the computer—is to add 0.5 (or 0.05, or 0.005, etc.) to the number before discarding digits. For example:2.74 + 0.05 = 2.79 → 2.7 2.76 + 0.05 = 2.81 → 2.8The addition changes 7 to 8 in the desired cases; then the subsequent dig-its can simply be discarded.In C and C++, you can round x to the nearest integer by evaluating floor(x+0.5), where floor finds the integer just below x. You can round x to two decimal places by evaluating floor(100*x+0.5)/100 (unless the result is thrown off by rounding error; see next entry). 417	RS-232There is a big difference between rounding a number, which actually changes it, and simply printing it with a limited number of decimal places. In the latter case, the number itself is not changed, and its origi-nal value remains available for further computation.rounding error an error that occurs because the computer cannot store the true value of most real numbers; instead, it can store only an approxi-mation of a finite number of digits.If you wish to write 1⁄3 as a decimal fraction, you can approximate it as 0.333333333, but it would require an infinite number of digits to express it exactly. The computer faces the same problem except that internally it stores the numbers in binary, and it can lose accuracy in con-verting between binary and decimal. For example, 0.1 has no exact rep-resentation on a computer; if you add the computer’s representation of 0.1 to 0 ten times, you will not get exactly 1. To avoid rounding error, some computer programs represent numbers as decimal digits. SeeBINARY-CODED DECIMAL.Route 128 a highway that skirts the west side of Boston, Massachusetts, passing through the cities of Needham and Waltham. Route 128 has been the home of a number of computer companies, including Digital Equipment Corporation and Lotus.router a network component that joins several networks together intelli-gently. A router is often used to link an incoming DSL or cable modem connection to a home network. A router is more powerful than a bridge because instead of just choosing network segments based on previous traffic, a router can look up the best route to a distant site. The Internet relies heavily on routers. Compare BRIDGE; HUB; SWITCH (definition 2). See also DNS (definition 1).RPG1.	abbreviation for role-playing game.2.	(Report Program Generator) a programming language developed by IBM in the 1960s in an attempt to simplify programming for business applications.RPG was often the first programming language taught to trainees because RPG programming can be reduced to a fixed procedure for fill-ing out forms. However, RPG programs are markedly less readable than programs in other languages, and complex algorithms are difficult to express in RPG.RPM (revolutions per minute) a measure of speed of rotation. For exam-ple, many newer hard disks rotate at 10,000 RPM.RPN see POLISH NOTATION.RS-232 an Electronics Industries Association (EIA) recommended stan-dard for transmitting serial data by wire. This standard is now officially known as EIA-232D. Computer serial ports follow the RS-232 standard. RS-422, RS-423A	418			TABLE 14		RS-232 PIN CONNECTIONS (25-PIN)				(Pin numbers are embossed on the connector.)Pin	Signal	Direction	Explanation			1	GND	Both	Frame ground—ties together chassis of			terminal and modem; often omitted.2	TxD	To modem	Transmitted data.3	RxD	To terminal	Received data.4	RTS	To modem	Request to send—high when terminal			is on and able to communicate.5	CTS	To terminal	Clear to send—high when computer			on other end is able to receive.6	DSR	To terminal	Data set ready—high when modem is			on and functioning.7	SG	Both	Signal ground—reference point for all			signal voltages.8	CD	To terminal	Carrier detect—high when a connec-			tion to another computer has been			established.20	DTR	To modem	Data terminal ready—high when			terminal is on and functioning. Most			modems hang up phone when DTR			goes low.22	RI	To terminal	Ring indicator—high when telephone			is ringing.			RS-422, RS-423A two standards, recommended by the Electronics Industries Association (EIA), which define a format for transmitting ser-ial data by wire, intended to replace the older RS-232 format. The new format offers faster data rates and greater immunity to electrical noise.RS/6000 see WORKSTATION.RSA encryption a public key encryption algorithm named after the initials of its developers (Ron Rivest, Adi Shamir, and Leonard Adleman). The security of the system relies on the difficulty of factoring very large numbers. More information about RSA encryption can be found at www.rsa.com. See also ENCRYPTION.RSACi (Recreational Software Advisory Council on the Internet) the origi-nal web site content rating service, which became ICRA in 1999. See ICRA.RSN Real Soon Now. (sarcastic) at an indefinite time in the future, proba-bly much later than promised. See VAPORWARE.RSS (Really Simple Syndication, or sometimes Rich Site Summary), a sys-tem that makes it easy to be notified of new information on web sites. The CLONE TOOL.419	runRSS material comes from a special XML file on the web site, and the per-son who maintains the web site also updates that file when there is new material to announce. Blogs often make their content available this way. The original version of RSS was developed by Netscape in the late 1990s.RTF (Rich Text Format) a format created by Microsoft for interchange of files between different word processing programs. RTF files consist of ASCII text with codes that indicate formatting and typefaces. For example,{\rtf This is a {\b sample} of {\i RTF.}}is RTF code for:This is a sample of RTF.RTF works somewhat like the lowest-level codes in TEX, but it is designed to be produced by programs, not humans. Many popular word processors can save their files in RTF. Compare TEX; contrast TEXT FILE.RTFM (read the friendly manual) an exhortation to computer users who are unwilling to look things up for themselves. Compare RYFM.rubber stamp a paint program tool that duplicates a selected area of adrawing. See the illustration atRuby a scripting language for object-oriented programming similar in intent to PERL and PYTHON. Ruby was developed by Yukihiro Matsumoto in 1995. See www.ruby-lang.org/en.rule a procedure defined in advance that will automatically perform a par-ticular action when a certain type of e-mail is received; for example, for-warding all messages from a particular person to another person, or moving all messages on a specific topic into the relevant folder.ruler a strip at the top (and sometimes the side) of the on-screen work area, marked in units of measurement. Like its physical counterparts, an on-screen ruler is used to help you measure things. Rulers can be found in word processors, drawing programs, and page layout programs. You have the option to display the ruler, or to hide it and give yourself some more working room. You can also have the ruler show units of measure-ment other than inches (PICAs might be more useful for many applica-tions). A ruler is especially useful when using a high degree of zoom because it helps you keep your orientation.In a word processor, the ruler is used to set the TABs and MARGINs.FIGURE 225. Ruler run to make the computer execute a program. A distinction is often made between compile time (the time when the program is compiled; see COM-PILER) and run time (the time when the program is run). DATA COM-run-length encoding	420run-length encoding a way of storing data in less than the usual amount ofspace by using special codes to indicate repeated bytes. SeePRESSION.run-time error an error that occurs when a program is being executed; an EXCEPTION. For example, a run-time error might occur if division by 0 is attempted or if the subscript for an array is outside the allowable bounds for that array. A run-time error may cause the program to stop execution, or it may be handled by an error-trapping routine (see TRAPPING).Contrast COMPILE-TIME ERROR.Run. . . a menu choice under Windows’s START BUTTON that allows you to run a program or open a file by typing its filename. For instance, to run myprog.exe from drive A, choose “Run. . . ” and type a:\myprog.exe or simply myprog (it isn’t necessary to type the extension). This is a com-mon way to run setup programs when installing new software.running head a small headline that appears at the top or bottom of each page. Running heads are usually there to remind the reader which chap-ter (or section) he or she is reading. In a dictionary like this one, the run-ning heads tell you the first and last entries on the page spread.RYFM (read your friendly manual, pronounced “riff-um”) Compare RTFM. 421	sans-serifSS (on a digital camera) shutter-priority autoexposure, the mode in which the user sets the shutter speed (exposure time) and the camera chooses the lens opening (f-ratio); same as TV. See F-RATIO. Contrast A; AV; P; TV.Sad Mac (pre OS X only) the icon of a frowning Macintosh computer that announces the Mac has found a hardware problem and cannot finish booting.Safari Apple Computer’s web browser for the Macintosh OS and Windows. The free download is available at www.apple.com/safari/download.Compare FIREFOX; INTERNET EXPLORER; OPERA.safe mode a way of running Windows with many special hardware drivers disabled, to work around problems with improperly installed hardware or software. Safe mode is used only for testing and to recover from an improper shutdown.Samba a widely used, free, open-source software package for linking UNIX systems to Windows file and printer sharing, and for achieving various other kinds of interoperability between networks. For details, see www.samba.org. The name is derived from SMB (Server Message Block), an important part of the protocol that Samba follows. See CIFS; PROTOCOL.sampling rate the number of times per second that sound waves are sam-pled and digitized. The highest frequency that can be reproduced is half the sampling rate. High-fidelity audio is usually sampled at 44.1 kHz (44,100 samples per second) in order to reproduce frequencies up to 22.05 kHz, just above the limit of human hearing. Speech can be stored more compactly by sampling at a lower rate, 22.05 or 11.025 kHz.Another parameter is the number of bits of data stored in each sam-ple. High-fidelity audio requires 16 bits per sample; speech can be repro-duced adequately with 8 bits. One minute of sound, sampled at 44.1 kHz with 16 bits per sample, requires about 5 megabytes of disk space (10 megabytes for stereo). See also MP3.SAN see STORAGE AREA NETWORK.sandbox a safe environment to play in; a protective mechanism that pre-vents a program from accessing or changing memory or disk space out-side of its own permitted area. This is a security feature preventing programs from damaging the system they run on. For example, a Java applet loaded from the World Wide Web runs in a sandbox where it is prohibited access to the hard disk on the browser’s computer.sans-serif a typeface that does not have serifs, such as this one. Serifs are small perpendicular marks at the ends of the strokes (sans is French for “without”). See Figure 226. Contrast SERIF. See also TYPEFACE. VECTOR PROCES-FONT; VEC-SAP	422FIGURE 226. Sans-serif typeSAP a large developer of e-business software, headquartered in Walldorf, Germany. The name stands for the German words for “Systems, Applications, Products.” See www.sap.com.SATA (serial ATA) a newer hardware implementation of the ATA hard disk interface, using serial data transmission through a narrow 7-pin connec-tor, giving greater speed than PATA. SATA disk drives also have a new, narrow 15-pin power connector instead of the traditional 4-pin one. SeeATA. Compare ESATA; PATA.saturation the intensity of a color. A highly saturated color is vivid and brilliant color; to dull a color (decrease its saturation), you add small amounts of its COMPLEMENT, making it closer to gray. See COLOR; HSB.save to transfer information from the computer’s memory to a storage device such as a disk drive. Saving data is vital because the contents of the computer’s memory are lost when power is turned off. The opposite process is known as loading, retrieving, or opening.Save As...1.	to save a document or drawing under a different name. The first time you save an untitled work, you will use “Save As . . . ” instead of “Save.”2.	to save a file in a different format (e.g., to save a CorelDraw file as a Windows Metafile). See also EXPORT.SBC see SINGLE-BOARD COMPUTER.scalable able to be used on a large or small scale without major changes. For example, much of the appeal of the UNIX operating system is its scalability; it can be used on small or large computers with little change in the way it works.scalable font a font that can be used to print characters of any size. Most printers include scalable fonts; also, TrueType, OpenType, and Type 1 fonts are scalable. The shapes of the characters in a scalable font arestored in the form of vector graphics rather than bitmaps. SeeTOR GRAPHICS.scalar a quantity represented by a single number, as opposed to an ARRAY or LIST.scalar processor a computer that operates on only one piece of data at atime. Most computers are scalar processors. ContrastSOR. See also SUPERSCALAR PROCESSOR.scale to change the size of a graphical object without changing its shape.Contrast STRETCH. 423	scattergraph, scatter plot FIGURE 227. ScaleScanAlert a company that developed technology that could be used to ver-ify that a web site has protection from hackers, and providing Hacker Safe certificates to web pages that passed the tests. However, it still is difficult to verify that a web page is truly safe. In 2008 ScanAlert was acquired by McAfee (web address: www.mcafee.com).scanner1. a device that enables a computer to read a printed or handwritten page. The simplest scanners give the contents of the page to the com-puter as a graphic image—a handy way of putting pictures into the com-puter (see DESKTOP PUBLISHING).There are many nuances involved in scanning artwork. In some ways, it is quite similar to photography and if you are comfortable with a cam-era, it will make the transition to working with a scanner very smooth. Scanners can adjust the contrast and brightness of the image. Controls for setting the highlight and shadow area are usually provided. Color images can be color-corrected at the scanning stage.With appropriate software, scanners can read the letters of typewrit-ten text, transmitting them into the computer as if they were typed on the keyboard (OCR). This process, however, is seldom 100% accurate. You will find it necessary to proofread scanned copy very carefully.2. Several other kinds of electronic devices are called scanners, includ-ing bar code readers (see BAR CODE) and devices for scanning the radio spectrum.scattergraph, scatter plot a graph that shows points of data plotted on an x-y coordinate system. Also called an x-y graph. FIGURE 228. Scattergraph scientific notation	424scientific notation see EXPONENTIAL NOTATION. scissors1.	a tool available in paint programs that allows you to define an area of the picture that you wish to separate from its surroundings. The result-ing cutout may be resized (scaled), stretched, rotated, or moved.The scissors tool is one of several SELECTION TOOLS available to you. Others include the LASSO, MAGIC WAND, and box selection tools. To save space, these tools may be grouped together in your toolbox and only one may be visible at a time.2.	icon for “cut the selected item to the Clipboard.” See also COPY; PASTE.FIGURE 229. Scissors iconSCO a software company that sued several companies claiming that it owned a copyright to part of the code in the open-source operating sys-tem Linux. Its claims were denied (SCO v. Novell).SCR1.	(silicon controlled rectifier) an electronic component that functions as a latching switch for direct current. Compare TRANSISTOR.Usage note: The hyphenation silicon-controlled rectifier is incorrect. The term means a controlled rectifier made of silicon, not a rectifier con-trolled by silicon.2.	(as a filename extension) screen saver. In Windows, an .scr file is just like an .exe file except that it can be installed as a screen saver; the assumption is that it will display graphics. A common way to trick peo-ple into running a malicious program is to give it the extension .scr, which they won’t recognize, instead of .exe. See EXE FILE; MALWARE.scratch disk, scratch tape a disk or tape that one can erase and reuse at any time because it does not contain anything that needs to be kept for future use.screen1.	a computer display; a monitor.2.	a screenful of information.3.	to process a grayscale image for printing by breaking down the various shades of gray into very small dots. See HALFTONE.screen capture see SCREEN SHOT.screen estate (informal) space on a computer screen, especially when thought of as a limited resource. A program that consumes too much screen estate will be hard to use concurrently with others. Compare REAL ESTATE. 425	scrollscreen saver a program that automatically blanks the screen of a computer, or displays a moving picture, when the computer has not been used for several minutes.The original purpose of screen savers was to prevent “burn-in” (i.e., to keep the screen coating from wearing out in places where bright parts of the image were constantly displayed). Modern screens are less vul-nerable to burn-in, and today, screen savers serve mainly to protect the privacy of users who are away from their desks, and to save energy when used in conjunction with monitors that turn themselves off when the screen goes blank. Screen savers also provide entertainment. See alsoGREEN PC; ENERGY STAR.screen shot, screen snapshot an image of the current screen, saved as a bitmap.Snapshots of the screen are easy to obtain: on the Macintosh, hold down Command and Shift, then press 3. This places a bitmapped image of the screen in a PICT file in your root directory. In Windows, pressing the Print Screen key puts a copy of the entire screen onto the CLIPBOARD. Holding down Alt while pressing Print Screen saves a bitmap of the active window only.script1.	a style of type that resembles cursive handwriting (not italics), as shown in Figure 230.2.	a file containing commands to be executed, such as a SHELL SCRIPT or a script of dialing commands for a communication program. See alsoJAVASCRIPT; VBSCRIPT; PERL; PYTHON; CGI.3. a file or printout containing a copy of information that was displayed on the screen.<