Barrons Publishing Dictionary of Computer and Internet Terms 10th

data rate see BAUD.

data recovery the art and technique of recovering part or all of the information 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 directories have become corrupted.

data structures ways of arranging information in the memory of a computer. In computer programming, it is often necessary to store large numbers 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 represented 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 number). 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 locations 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 without moving other items to make room. More complex linked structures, 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 program, 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 and 23), or as combining smaller units of information. In

A 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 primary field. For example, if you are sorting the records by the number 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 database system might contain an employee file that lists the job classification 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 carried out online. See

Dazed 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 a DB-25P has 25 pins and a DB-25S has 25 holes. See

DB2 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 serial communications, with conductor 2 for input and conductor 3 for output. 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 conducted through a large set of attackers at widely distributed locations. This is often done by distributing a computer virus that will turn its victims into ZOMBIES that carry out the attack.

DDR

1.(describing computer memory) (double data rate) term used to describe a type of SDRAM computer memory that gives faster performance 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 (definition 1).

.de suffix indicating that an e-mail or web address is registered in Germany (in German, Deutschland). See TLD.

de-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 widespread 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 filename), 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.

debug

1.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

For example, multiplying power by 2 is equivalent to adding about 3 decibels; multiplying power by 10 is equivalent to adding 10 decibels; 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 level

Because 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 1101

decryption 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.

default directory see default drive see

default 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 contradiction 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 contradiction. 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 software, 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 postings 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.

Delphi 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 referring 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. government 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. Descenders

deselect 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

down 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 button 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 information 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 normally 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 desktop 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 specify the layout in great detail, use multiple input files, have comprehen-

sive 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 elegantly typeset documents that include extensive graphics. The major difference 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 necessary 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 electronic 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, running concurrently, are trying to use the same device (such as a modem or printer) at the same time. Multitasking operating systems handle contention in a number of different ways, such as SPOOLING output for printers, 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 support 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.