Barrons Publishing Dictionary of Computer and Internet Terms 10th

Ajax (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 features 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 begin

statements end

else begin

statements 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 statements embedded within larger blocks. Thus, hierarchical design is easier 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 widespread 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 completely replaced ALGOL in practical use. See also RECURSION; STRUCTURED 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 computer can understand, but the same algorithm can be written in several

different 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 stopping place is for a particular case.

There are well-understood algorithms for many common computations (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.

alias

1.(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 displays 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 command

alias dir ls -al

then from then on, dir will mean ls -al (the complete file listing command).

FIGURE 6. Aliasing

aliasing the appearance of false stairsteps or bands in an image, or false frequencies in digitized sound, due to interaction of the original signal with the sampling rate. See

align to make things line up, either horizontally or vertically. Most drawing 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 widespread 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 occupied by every file will be a multiple of 4096 bytes, regardless of how small the file is. Also called CLUSTER. See FAT32.

FIGURE 7. Align

AlltheWeb.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 alphabetic, and hence as alphanumeric. They are called national characters because they print differently on computers designed for use in different countries.

alt

1. prefix identifying “alternative” Usenet newsgroups, those that have not been voted on by the members, such as alt.folklore.urban. See

NEWSGROUP.

2. a key on a computer keyboard that is used to give an alternate meaning 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.

Altair a pioneering microcomputer marketed to hobbyists in 1975, significant 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 bookstore in 1995 in Seattle, Washington, Amazon has since expanded to sell a wide variety of products around the world. Web address: www.amazon.com

ambient 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 circuits, 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 software 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 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 illustration of ampersands in various typefaces, see Figure 8.

FIGURE 8. Ampersands

amplified speaker a speaker that includes its own amplifier to produce louder sound and stronger bass. See SOUND CARD; MULTIMEDIA.

anacronym

1.a reinterpreted abbreviation or acronym. For example, DVD originally stood for digital video disc but is now said to stand for digital versatile 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 (converted into the computer’s internal representation) in order to be stored in a computer. See also ANALOG COMPUTER; ANALOG-TO-DIGITAL CONVERTER. 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 automotive 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. Contrast

DIGITAL 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 converter 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.

analytic 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 is

http://www.vet.uga.edu/animals.html#Elephants

See HTML; WORLD WIDE WEB.

AND gate a logic gate that produces an output of 1 only when all of its inputs are 1, thus:

The 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 simultaneously on several wires, such as memory addresses. See

TION; DECODER; LOGIC CIRCUITS.

FIGURE 9. AND gate (logic symbol)

angle brackets the characters < > or, more properly, < >, used in mathematics 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 programs, 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

and graphics can also be animated by saving a series of images as an ANIMATED GIF file. See GIF89A.

FIGURE 10. Animation

anonymous 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 standardization 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 programming languages. Typically, a particular manufacturer’s version of a language will include all of the features defined in the ANSI standard, plus additional features devised by the manufacturer. To be easily transportable from one computer to another, a program should not use any features that are not in the ANSI standard. 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).

3The 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.

FIGURE 11. Antialiasing

antivirus software software that protects a computer from viruses (secretly destructive software modifications), either by blocking the modifications that a virus tries to make, or by detecting a virus as soon as possible 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 variety 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 system API, as far as possible, rather than manipulating hardware directly, because direct manipulation of hardware can interfere with other programs 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