Barrons Publishing Dictionary of Computer and Internet Terms 10th

autodimensioning

36

autodimensioning 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 contains 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 registry 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. Autojoin

automagically (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.25

AV

1.abbreviation for audio-visual; pertaining to the recording and reproduction of sounds and pictures.

2.(for “aperture value,” on a digital camera) aperture-priority autoexposure. See A. Contrast P, TV, S.

avatar an image representing the user of a visual chat or virtual reality program. Avatars range from simple images to complex, personalized threedimensional 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 initials. 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 standard. See PACKET RADIO; X.25.

B-spline

38

B

B-spline a smooth curve that approximately connects two points. B-splines can be joined together to make a smooth curve passing close to any number 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 + 2/3

c = –t3/2 + t2/2+ t/2 + 1/6 d = t3/6

x = ax1 + bx2 + cx3 + dx4 y = ay1 + by2 + cy3 + dy4

That 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-TIER

ARCHITECTURE.

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	backtracking

backbone 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 execution is possible only in multitasking operating systems. See MULTITASKING; 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 combinations 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 combinations is to set up three nested loops as follows:

FOR x[1]:=1 to 5 DO FOR x[2]:=1 to 5 DO

FOR x[3]:=1 to 5 DO

IF 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 increments x[2] and starts the innermost loop over again. Similarly, the middle 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 understand 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 discussed 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 specify 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 also

CD-R.

Backus-Naur form (BNF) a notation used to describe the syntax of languages. BNF was devised by J. Backus and Peter Naur and introduced in the first official description of ALGOL 60 (see ALGOL); it is sometimes 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 written 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 concise and definitive and because parsers can be generated directly from them. See PARSING.

Backus normal form see BACKUS-NAUR FORM.

backward compatible

42

backward compatible able to substitute for an older computer or operating system. For example, Windows Vista is mostly backward compatible 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 graph

bank switching the ability to use more than one set of memory chips at different times, while giving them the same addresses. This makes it possible 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.

banner

1.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. Compare

POP-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 code

bar graph a type of chart that displays information by representing quantities as rectangular bars of different heights. Sometimes symbols are stacked or stretched to the appropriate heights to lend some visual interest to the chart (see PICTOGRAPH, definition 2).

bare metal

44

Bar graphs are usually preferred for representing and contrasting data collected over a period of time.

FIGURE 27. Bar graph

bare 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.

base

1.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 elements 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) consisting of seemingly random letters and digits with no spaces or punctuation. 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, without 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 carriers. See BROADBAND.

BASIC (Beginner’s All-purpose Symbolic Instruction Code) a computer language designed by John Kemeny and Thomas Kurtz in 1964 and popularized 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 PROGRAMMING and a wide variety of DATA STRUCTURES. Today, BASIC is one of the most complex programming languages in wide use, incorporating features from Pascal, FORTRAN, and C. Line numbers are no longer necessary. However, BASIC is still easy for beginners to use, and the newest BASIC compilers still accept programs written in Kemeny and Kurtz’s

original 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 program

20 PRINT ”Temperature in Fahrenheit”;

30 INPUT F

40 LET C=(F-32)*5/9

50 PRINT F;” F equals ”;

60 PRINT C;” C”

70 END

Temperature in Fahrenheit? 98 98 F equals 36.66667 C

FIGURE 28. BASIC program and its output

BAT 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 commands

dir 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 also

AUTOEXEC.BAT.