daemon

n. A program associated with UNIX systems that performs a housekeeping or maintenance utility function without being called by the user. A daemon sits in the background and is activated only when needed, for example, to correct an error from which another program cannot recover.

daisy chain

n. A set of devices connected in series. In order to eliminate conflicting requests to use the channel (bus) to which all the devices are connected, each device is given a different priority, or, as in the Apple Desktop Bus, each device monitors the channel and transmits only when the line is clear.

daisy wheel

n. A print element consisting of a set of formed characters with each character mounted on a separate type bar, all radiating from a center hub. See also daisy-wheel printer, thimble, thimble printer.

daisy-wheel printer

n. A printer that uses a daisy-wheel type element. Daisy-wheel output is crisp and slightly imprinted, with fully formed characters resembling typewriter quality. Daisy-wheel printers were standard for high-quality printing until being superseded by laser printers. See also daisy wheel, thimble, thimble printer.

DARPANET

n. Short for Defense Advanced Research Projects Agency Network. See ARPANET.

DASD

n. Acronym for direct access storage device. A data storage device by which information can be accessed directly, instead of by passing sequentially through all storage areas. For example, a disk drive is a DASD, but a tape unit is not, because, with a tape unit, the data is stored as a linear sequence. See also direct access. Compare sequential access.

data

n. Plural of the Latin datum, meaning an item of information. In practice, data is often used for the singular as well as the plural form of the noun. Compare information.

data acquisition

n. The process of obtaining data from another source, usually one outside a specific system.

data bank

n. Any substantial collection of data.

database

n. A file composed of records, each containing fields together with a set of operations for searching, sorting, recombining, and other functions.

database administrator

n. One who manages a database. The administrator determines the content, internal structure, and access strategy for a database, defines security and integrity, and monitors performance. Also called database manager. Acronym: DBA.

database analyst

n. One who provides the analytic functions needed to design and maintain applications requiring a database.

database designer

n. One who designs and implements functions required for applications that use a database.

database engine

n. The program module or modules that provide access to a database management system (DBMS).

database management system

n. A software interface between the database and the user. A database management system handles user requests for database actions and allows for control of security and data integrity requirements. Also called database manager. See also database engine. Acronym: DBMS.

database publishing

n. The use of desktop publishing or Internet technology to produce reports containing information obtained from a database.

database structure

n. A general description of the format of records in a database, including the number of fields, specifications regarding the type of data that can be entered in each field, and the field names used.

data bit

n. In asynchronous communications, one of a group of from 5 to 8 bits that represents a single character of data for transmission. Data bits are preceded by a start bit and followed by an optional parity bit and one or more stop bits. See also asynchronous transmission, bit, communications parameter.

data cable

n. Fiber-optic or wire cable used to transfer data from one device to another.

datacom

n. Short for data communications. See communications.

data compression

n. A means of reducing the amount of space or bandwidth needed to store or transmit a block of data, used in data communications, facsimile transmission, and CD-ROM publishing. Also called data compaction.

data-driven processing

n. A form of processing where the processor or program must wait for data to arrive before it can advance to the next step in a sequence.

data encryption key

n. A sequence of data that is used to encrypt and decrypt other data. See also decryption, encryption, key (definition 3). Acronym: DEK.

data entry

n. The process of writing new data to computer memory.

data/fax modem

n. A modem that can handle both serial data and facsimile images to either send or receive transmissions.

data field

n. A well-defined portion of a data record, such as a column in a database table.

data file

n. A file consisting of data in the form of text, numbers, or graphics, as distinct from a program file of commands and instructions. Compare program file.

data format

n. The structure applied to data by an application program to provide a context in which the data can be interpreted.

datagram

n. One packet, or unit, of information, along with relevant delivery information such as the destination address, that is sent through a packet-switching network. See also packet switching.

data interchange format

n. A format consisting of ASCII codes in which database, spreadsheet, and similar documents can be structured to facilitate their use by and transfer to other programs. See also ASCII. Acronym: DIF.

data link

n. A connection between any two devices capable of sending and receiving information, such as a computer and a printer or a main computer and a terminal. Sometimes the term is extended to include equipment, such as a modem, that enables transmission and receiving. Such devices follow protocols that govern data transmission. See also communications protocol, data-link layer, DCE, DTE.

data-link layer

n. The second of seven layers in the ISO/OSI model for standardizing computer-to-computer communications. The data-link layer is one layer above the physical layer. Its concern is packaging and addressing data and managing the flow of transmissions. It is the lowest of the three layers (data-link, network, and transport) involved in actually moving data between devices. See also ISO/OSI model.

data mining

n. The process of identifying commercially useful patterns or relationships in databases or other computer repositories through the use of advanced statistical tools.

data network

n. A network designed for transferring data encoded as digital signals, as opposed to a voice network, which transmits analog signals.

data processing

n. 1. The general work performed by computers. Also called ADP, automatic data processing, EDP, electronic data processing. See also centralized processing, decentralized processing, distributed processing. 1. More specifically, the manipulation of data to transform it into some desired result. Acronym: DP.

data protection

n. The process of ensuring the preservation, integrity, and reliability of data. See also data integrity.

data set

n. 1. A collection of related information made up of separate elements that can be treated as a unit in data handling. 2. In communications, a modem. See also modem.

data transfer

n. The movement of information from one location to another, either within a computer (as from a disk drive to memory), between a computer and an external device (as between a file server and a computer on a network), or between separate computers.

data transmission

n. The electronic transfer of information from a sending device to a receiving device.

data type

n. In programming, a definition of a set of data that specifies the possible range of values of the set, the operations that can be performed on the values, and the way in which the values are stored in memory. Defining the data type allows a computer to manipulate the data appropriately. Data types are most often supported in high-level languages and often include types such as real, integer, floating point, character, Boolean, and pointer. How a language handles data typing is one of its major characteristics. See also cast, constant, enumerated data type, strong typing, type checking, user-defined data type, variable, weak typing.

data value

n. The literal or interpreted meaning of a data item, such as an entry in a database, or a type, such as an integer, that can be used for a variable.

data warehouse

n. A database, frequently very large, that can access all of a company's information. While the warehouse can be distributed over several computers and may contain several databases and information from numerous sources in a variety of formats, it should be accessible through a server. Thus, access to the warehouse is transparent to the user, who can use simple commands to retrieve and analyze all the information. The data warehouse also contains data about how the warehouse is organized, where the information can be found, and any connections between data. Frequently used for decision support within an organization, the data warehouse also allows the organization to organize its data, coordinate updates, and see relationships between information gathered from different parts of the organization. See also database, decision support system, server (definition 1), transparent (definition 1).

date stamping

n. A software feature that automatically inserts the current date into a document.

DCA

n. 1. Acronym for Document Content Architecture. A formatting guideline used in IBM's Systems Network Architecture (SNA) that enables the exchange of text-only documents between differing types of computers. DCA provides for two types of document formatting: Revisable-Form-Text DCA (RFTDCA), which allows for modification of formatting, and Final-Form-Text DCA (FFTDCA), which cannot be modified. See also DIA, SNA. 2. Acronym for Directory Client Agent. See DUA.

DCD

n. Acronym for Data Carrier Detected. A signal in serial communications that is sent from a modem to its computer to indicate that the modem is ready for transmitting. Also called RLSD (Received Line Signal Detect). See also RS-232-C standard.

DCOM

n. Acronym for Distributed Component Object Model. The version of Microsoft's Component Object Model (COM) specification that stipulates how components communicate over Windows-based networks. It permits the distribution of different components for a single application across two or more networked computers, running an application distributed across a network so that the distribution of components is not apparent to the user, and remotely displaying an application. Also called Distributed COM. See also COM (definition 2), component (definition 2).

DDE

n. Acronym for Dynamic Data Exchange. An interprocess communication method featured in Microsoft Windows and OS/2. DDE allows two or more programs that are running simultaneously to exchange data and commands. In Windows 3.1, DDE was largely supplanted by OLE, which is an extension of DDE. In Windows 95 and Windows NT, OLE and ActiveX are more commonly used. See also ActiveX, interprocess communication, OLE.

dead key

n. A key used with another key to create an accented character. When pressed, a dead key produces no visible character (hence its name) but indicates that the accent mark it represents is to be combined with the next key pressed. See also key (definition 1).

debug

vb. To detect, locate, and correct logical or syntactical errors in a program or malfunctions in hardware. In hardware contexts, the term troubleshoot is the term more often used, especially when the problem is a major one. See also bug, debugger.

debugger

n. A program designed to aid in debugging another program by allowing the programmer to step through the program, examine the data, and monitor conditions such as the values of variables. See also bug (definition 1), debug.

DECchip 21064

n. A Digital Equipment Corporation microprocessor introduced in February 1992. The DECchip 21064 is a 64-bit, RISC-based, superscalar, superpipelined chip with 64-bit registers, a 64-bit data bus, a 64-bit address bus, and a 128-bit data path between the microprocessor and memory. It also has a built-in 8-KB instruction cache, a built-in 8-KB data cache, and a floating-point processor. The DECchip 21064 contains 1.7 million transistors and operates at 3.3 volts. The 200-MHz version runs at a peak rate of 400 MPS. The chip's architecture is SMP compliant, so that several chips can be used in a parallel (multiprocessor) configuration. See also floating-point processor, MIPS, pipelining (definition 1), RISC, superpipelining, superscalar.

decentralized processing

n. The distribution of computer processing facilities in more than one location. Decentralized processing is not the same as distributed processing, which assigns multiple computers to the same task to increase efficiency.

decibel

n. Abbreviated dB. One tenth of a bel (named after Alexander Graham Bell), a unit used in electronics and other fields to measure the strength of a sound or signal. Decibel measurements fall on a logarithmic scale and compare the measured quantity against a known reference. The following formula gives the number of decibels between two values:

decision tree

n. Similar to a decision table, an analysis instrument where possible outcomes of some condition are represented as branches, which may in turn generate other branches. See also branch (definition 1), tree.

declarative markup language

n. In text processing, a system of text-formatting codes that indicates only that a unit of text is a certain part of a document. Document formatting is then done by another program, called a parser. SGML and HTML are examples of declarative markup languages. See also HTML, SGML. Acronym: DML.

decoder

n. 1. A device or program routine that converts coded data back to its original form. This can mean changing unreadable or encrypted codes into readable text or changing one code to another, although the latter type of decoding is usually referred to as conversion. Compare conversion. 2. In electronics and hardware, a type of circuit that produces one or more selected output signals based on the combination of input signals it receives.

decompiler

n. A program that attempts to generate high-level source code from assembly language code or machine code. This can be a difficult task, as some assembly language code has no corresponding high-level source code. See also disassembler. Compare compiler (definition 2).

decrement¹

n. The amount by which a number is decreased. Compare increment¹.

decryption

n. The process of restoring encrypted data to its original form. Compare encryption.

DECstation

n. 1. A small computer system used primarily for word processing, introduced by Digital Equipment Corporation in 1978. 2. One of a series of personal computers introduced by Digital Equipment Corporation in 1989. 3. One of a series of single-user UNIX workstations introduced by Digital Equipment Corporation in 1989 and based on RISC processors. See also RISC.

dedicated channel

n. A communications link reserved for a particular use or a particular user.

dedicated line

n. 1. A communications channel that permanently connects two or more locations. Dedicated lines are private or leased lines, rather than public ones. T1 lines, which are used by many organizations for Internet connectivity, are examples of dedicated lines. Also called leased line, private line. Compare switched line. 2. A telephone line that is used for one purpose only, such as to receive or send faxes or to serve as a modem line.

de facto standard

n. A design, program, or language that has become so widely used and imitated that it has little competition, but whose status has not been officially recognized as standard by an organization such as the American National Standards Institute (ANSI) or the International Organization for Standardization (ISO). See also standard. Compare de jure standard.

default¹

n. A choice made by a program when the user does not specify an alternative. Defaults are built into a program when a value or option must be assumed for the program to function.

default²

vb. In reference to programs, to make a choice when the user does not specify an alternative.

default button

n. The control that is automatically selected when a window is introduced by an application or operating system, typically activated by pressing the Enter key.

default drive

n. The disk drive that an operating system reads to and writes from when no alternative is specified.

default home page

n. On a Web server, the file that is returned when a directory is referenced without a specific filename. This is specified by the Web server software and is typically the file called index.html or index.htm.

default printer

n. The printer to which a computer sends documents for printing unless an alternative is specified.

Defense Advanced Research Projects Agency

n. The U.S. government agency that provided the original support for the development of the interconnected networks that later grew into the Internet. See also ARPANET. Acronym: DARPA.

defragmentation

n. The process of rewriting parts of a file to contiguous sectors on a hard disk to increase the speed of access and retrieval. When files are updated, the computer tends to save these updates on the largest continuous space on the hard disk, which is often on a different sector than the other parts of the file. When files are thus "fragmented," the computer must search the hard disk each time the file is accessed to find all of the file's parts, which slows down response time. Windows 95 and Windows NT include defragmentation utilities (or defraggers) as part of the operating system. For the MAC OS, Windows 3.x, and DOS systems, defragmentation utilities must be purchased separately. See also optimization (definition 1). Compare fragmentation.

degradation

n. 1. In communications, a deterioration of signal quality, as from line interference. 2. In computer systems, a reduction in level of performance or service. Degradation in microcomputer performance is indicated by slow response times or frequent pauses for disk access because memory is insufficient to hold an entire program plus the data the program is using.

delete

vb. To eliminate text, a file, or part of a document with the intention of removing the information permanently. There are several ways to delete. On-screen characters and parts of documents can be deleted with the Delete key, the Backspace key, or with a program's Delete command. Files can be deleted through a command to the operating system.

Delete key

n. 1. On IBM and PC-compatible computers, a key whose function changes depending on the application program. Usually it erases the character under the cursor, although in some applications it can erase selected text or graphics. 2. On Apple Macintosh computers, a key on the ADB and Extended keyboards that erases the character preceding the insertion point or erases highlighted text or graphics.

delimiter

n. A special character that sets off, or separates, individual items in a program or set of data. In the following example, commas separate the fields in a database record (each non-numeric field is enclosed by double quotation marks).

demand paging

n. The most common implementation of virtual memory, in which pages of data are read into main memory from an auxiliary storage device only in response to interrupts that result when software requests a memory location that the system has saved to auxiliary storage and reused for other purposes. See also paging, swap (definition 2), virtual memory.

demo

n. 1. Short for demonstration. A partial or limited version of a software package distributed free of charge for advertising purposes. Demos often consist of animated presentations that describe or demonstrate the program's features. See also crippled version. 2. A computer in a store that is available for customers to test, to see if they wish to buy it.

demodulation

n. In communications, the means by which a modem converts data from modulated carrier frequencies (waves that have been modified in such a way that variations in amplitude and frequency represent meaningful information) over a telephone line to the digital form needed by a computer, with as little distortion as possible. Compare modulation (definition 1).

demonstration program

or demo program n. 1. A prototype that shows the on-screen look and sometimes the proposed capabilities of a program under development. See also prototyping. 2. A scaled-down version of a proprietary program offered as a marketing tool.

DES

n. Acronym for Data Encryption Standard. A specification for encryption of computer data developed by IBM and adopted by the U.S. government as a standard in 1976. DES uses a 56-bit key. See also encryption, key (definition 3).

descending sort

n. A sort that arranges items in descending order--for example, with Z preceding A and higher numbers preceding lower ones. See also alphanumeric sort. Compare ascending sort.

deselect

vb. To reverse the action of selecting an option, a range of text, a collection of graphical objects, and so on. Compare select.

desk accessory

n. A type of small program on Macintosh computers and in windowing programs for IBM and PC-compatible machines that acts as the electronic equivalent of a clock, calendar, calculator, or other small appliance found on a typical desktop. Desk accessories are conveniences that can be activated when needed and then either put away or moved to a small part of the screen. A special type of desk accessory, a control panel, provides the user with the ability to change the date and time as well as to control screen colors, mouse movements, and other parameters. Also called desktop accessory. See also control panel. Acronym: DA.

desktop

n. An on-screen work area that uses icons and menus to simulate the top of a desk. A desktop is characteristic of the Apple Macintosh and of windowing programs such as Microsoft Windows. Its intent is to make a computer easier to use by enabling users to move pictures of objects and to start and stop tasks in much the same way as they would if they were working on a physical desktop. See also graphical user interface.

desktop component

n. In a web browser's desktop, a small screen region that displays summary or update information about a web site which has been subscribed to as a channel. For example, a desktop component might display a stock ticker, a list of news stories, or a pop-up broadcast message. Desktop components are available only in browsers that support channel definitions, such as Microsoft Internet Explorer version 4.0.

desktop computer

n. A computer that fits conveniently on the surface of a business desk. Most personal computers as well as some workstations can be considered desktop computers. Compare portable computer.

desktop conferencing

n. The use of computers for simultaneous communication among geographically separated participants in a meeting. This communication may include input to and display from application programs as well as audio and video communication. See also data conferencing, teleconferencing, video conferencing.

desktop publishing

n. The use of a computer and specialized software to combine text and graphics to create a document that can be printed on either a laser printer or a typesetting machine. Desktop publishing is a multiple-step process involving various types of software and equipment. The original text and illustrations are generally produced with software such as word processors and drawing and painting programs and with photograph-scanning equipment and digitizers. The finished product is then transferred to a page-makeup program, which is the software most people think of as the actual desktop publishing software. This type of program enables the user to lay out text and graphics on the screen and see what the results will be; for refining parts of the document, these programs often include word processing and graphics features in addition to layout capabilities. As a final step, the finished document is printed either on a laser printer or, for the best quality, by typesetting equipment.

desktop video

n. The use of a personal computer to display video images. The video images may be recorded on video tape or on a laser disc or may be live footage from a video camera. Live video images can be transmitted in digital form over a network in video conferencing. Acronym: DTV.

destination

n. The location (drive, folder, or directory) to which a file is copied or moved. Compare source (definition 1).

detection

n. Discovery of a certain condition that affects a computer system or the data with which it works.

developer's toolkit

n. A set of routines (usually in one or more libraries) designed to allow developers to more easily write programs for a given computer, operating system, or user interface. See also library (definition 1), toolbox.

development cycle

n. The process of application development from definition of requirements to finished product, including the following stages: analysis, design and prototyping, software coding and testing, and implementation.

device

n. A generic term for a computer subsystem. Printers, serial ports, and disk drives are often referred to as devices; such subsystems frequently require their own controlling software, called device drivers. See also device driver.

device address

n. A location within the address space of a computer's random access memory (RAM) that can be altered either by the microprocessor or by an external device. Device addresses are different from other locations in RAM, which can be altered only by the microprocessor. See also device, input/output, RAM.

device dependence

n. The requirement that a particular device be present or available for the use of a program, interface, or protocol. Device dependence in a program is often considered unfortunate because the program either is limited to one system or requires adjustments for every other type of system on which it is to run. Compare device independence.

device driver

n. A software component that permits a computer system to communicate with a device. In most cases, the driver also manipulates the hardware in order to transmit the data to the device. However, device drivers associated with application packages typically perform only the data translation; these higher-level drivers then rely on lower-level drivers to actually send the data to the device. Many devices, especially video adapters on PC-compatible computers, will not work properly--if at all--without the correct device drivers installed in the system.

device independence

n. A characteristic of a program, interface, or protocol that supports software operations that produce similar results on a wide variety of hardware. For example, the PostScript language is a device-independent page description language because programs issuing PostScript drawing and text commands need not be customized for each potential printer. Compare device dependence.

device manager

n. A software utility that allows viewing and changing hardware configuration settings, such as interrupts, base addresses, and serial communication parameters.

Device Manager

n. In Windows 95, a function within the System Properties utility that indicates device conflicts and other problems and allows a user to change the properties of the computer and each device attached to it. See also property, property sheet.

device name

n. The label by which a computer system component is identified by the operating system. MS-DOS, for example, uses the device name COM1 to identify the first serial communications port.

DHCP

n. Acronym for Dynamic Host Configuration Protocol. A TCP/IP protocol that enables a network connected to the Internet to assign a temporary IP address to a host automatically when the host connects to the network. See also IP address, TCP/IP. Compare dynamic SLIP.

Dhrystone

n. A general-performance benchmarking test, originally developed by Rheinhold Weicker in 1984 to measure and compare computer performance. The test reports general system performance in dhrystones per second. It is intended to replace the older and less reliable Whetstone benchmark. The Dhrystone benchmark, like most benchmarks, consists of standard code revised periodically to minimize unfair advantages to certain combinations of hardware, compiler, and environment. Dhrystone concentrates on string handling and uses no floating-point operations. Like most benchmarking tests, it is heavily influenced by hardware and software design, such as compiler and linker options, code optimizing, cache memory, wait states, and integer data types. See also benchmark². Compare sieve of Eratosthenes, Whetstone.

DHTML

n. Short for Dynamic HyperText Markup Language.

DIA

n. Acronym for Document Interchange Architecture. A document exchange guideline used in IBM's Systems Network Architecture (SNA). DIA specifies methods of organizing and addressing documents for transmission between computers of different sizes and models. DIA is supported by IBM's Advanced Program-to-Program Communication (APPC) and by Logical Unit (LU) 6.2, which establish the capabilities and types of interactions possible in an SNA environment. See also DCA (definition 1), SNA.

dialect

n. A variant of a language or protocol. For example, Transact-SQL is a dialect of structured query language (SQL).

dialog

n. 1. In computing, the exchange of human input and machine responses that forms a "conversation" between an interactive computer and the person using it. 2. The exchange of signals by computers communicating on a network.

dialog box

n. In a graphical user interface, a special window displayed by the system or application to solicit a response from the user. See also windowing environment. Compare integrator.

dial-up

adj. Of, pertaining to, or being a connection that uses the public switched telephone network rather than a dedicated circuit or some other type of private network.

dial-up access

n. Connection to a data communications network through a public switched telecommunication network.

dial-up service

n. A telephone connection provider for a local or worldwide public switched telephone network that provides Internet or intranet access, advertisement via a Web page, access to news services, or access to the stock market and other resources.

DIB

n. 1. Acronym for device-independent bit map. A file format designed to ensure that bitmapped graphics created using one application can be loaded and displayed in another application exactly the way they appeared in the originating application. See also bitmapped graphics. 2. Acronym for Directory Information Base. A directory of user and resource names in an X.500 system. The DIB is maintained by a Directory Server Agent (DSA). Also called white pages.

DIBengine

n. Software, or a combination of hardware and software, that produces DIB files. See also DIB (definition 1).

digit

n. One of the characters used to indicate a whole number (unit) in a numbering system. In any numbering system, the number of possible digits is equal to the base, or radix, used. For example, the decimal (base-10) system has 10 digits, 0 through 9; the binary (base-2) system has 2 digits, 0 and 1; and the hexadecimal (base-16) system has 16 digits, 0 through 9 and A through F.

digital

adj. 1. Related to digits or the way they are represented. 2. In computing, analogous to binary because the computers familiar to most people process information coded as combinations of binary digits (bits). Compare analog.

digital audio tape

n. A magnetic tape storage medium for recording digitally encoded audio information. Acronym: DAT.

digital audio/video connector

n. An interface on some high-end video cards or TV tuner cards that allows the simultaneous transmission of digital audio and video signals. Also called DAV connector. See also interface (definition 3), video adapter.

digital camera

n. A type of camera that stores photographed images electronically instead of on traditional film. A digital camera uses a CCD (charge-coupled device) element to capture the image through the lens when the operator releases the shutter in the camera; circuitry within the camera then stores the image captured by the CCD in a storage medium such as solid-state memory or a hard disk. After the image has been captured, it is downloaded by cable to the computer using software supplied with the camera. Once stored in the computer, the image can be manipulated and processed much like the image from a scanner or related input device. See also charge-coupled device.

digital data transmission

n. The transfer of information encoded as a series of bits rather than as a fluctuating (analog) signal in a communications channel.

digital display

n. A video display capable of rendering only a fixed number of colors or gray shades. Examples of digital displays are IBM's Monochrome Display, Color/Graphics Display, and Enhanced Color Display. See also CGA, EGA, MDA. Compare analog display.

Digital ID

n. A digital ID (also called a digital certificate), which is issued by a certified security authority, such as VeriSign, or from your administrator, provides a means for proving your identity on the Internet. With a digital ID you can add a digital signature to an e-mail message so the intended recipients can make sure that the message actually came from you and has not been tampered with. You can also encrypt messages, using a special mathematical formula, so only the intended recipient can read your messages and attachments. To send a secure message over the Internet using certificates, both the sender and the receiver must have a valid certificate. See also certificate, personal certificate, web site certificate.

digital photography

n. Photography by means of a digital camera. Digital photography differs from conventional photography in that a digital camera does not use a silver halide-based film to capture an image. Instead, a digital camera captures and stores each image electronically. See also digital camera.

digital signal

n. A signal, such as one transmitted within or between computers, in which information is represented by discrete states--for example, high and low voltages--rather than by fluctuating levels in a continuous stream, as in an analog signal.

digital signal processor

n. An integrated circuit designed for high-speed data manipulation and used in audio, communications, image manipulation, and other data acquisition and data control applications. Acronym: DSP.

digital signature

n. A personal authentication method based on encryption and secret authorization codes used for "signing" electronic documents. See alsocertificate, digital ID, certificate, personal certificate, web site certificate.

Digital Simultaneous Voice and Data

n. A modem technology, patented by Multi-Tech Systems, Inc., that allows a single telephone line to be used for conversation together with data transfer. This is accomplished by switching to packet-mode communications when the need for voice transfer is detected; digitized voice packets are then transferred along with data and command packets. Acronym: DSVD.

digital subscriber line

n. An ISDN BRI (Basic Rate Interface) line or channel. See also BRI, ISDN. Acronym: DSL.

digital-to-analog converter

n. A device that translates digital data to an analog signal. A digital-to-analog converter takes a succession of discrete digital values as input and creates an analog signal whose amplitude corresponds, moment by moment, to each digital value. Compare analog-to-digital converter. Acronym: DAC.

digital video disc

n. The next generation of optical disc storage technology. With digital video disc technology, video, audio, and computer data can be encoded onto a compact disc (CD). A digital video disc can store greater amounts of data than a traditional CD. A standard single-layer, single-sided digital video disc can store 4.7 GB of data; a two-layer standard increases the single-sided disc capacity to 8.5 GB. Digital video discs can be double-sided with a maximum storage of 17 GB per disc. A digital video disc player is needed to read digital video discs; this player is equipped to read older optical storage technologies. Advocates of the digital video disc intend to replace current digital storage formats, such as laser disc, CD-ROM, and audio CD, with the single digital format of the digital video disc. Also called digital versatile disc. See also digital video disc-ROM. Acronym: DVD.

digiterati

n. The digital counterpart of literati, an undefined collection of individuals recognized as knowledgeable, hip, or up-to-date regarding all things digital.

digitize

vb. To convert any continuously varying (analog) source of input, such as the lines in a drawing or a sound signal, to a series of discrete units represented in a computer by the binary digits 0 and 1. Analog-to-digital converters are commonly used to perform this translation. See also aliasing, analog-to-digital converter.

DikuMUD

n. 1. Multi-user dungeon (MUD) software developed by five individuals at the Computer Science Institute at Copenhagen University (whose acronym in Danish is DIKU). DikuMUD uses multimedia and is object-oriented, but the classes are hard-coded. The software is covered by a license agreement that forbids its distribution for money. See also MUD, multimedia, object-oriented. 2. A game that uses the DikuMUD software.

dimmed

adj. Shown on the screen in gray characters instead of black characters on white or white characters on black. Menu options are dimmed in a graphical user interface to indicate that under current circumstances they are not available--for example, "Cut" when no text has been highlighted or "Paste" when there is no text in the clipboard.

DIN connector

n. A multipin connector conforming to the specification of the German national standards organization (Deutsch Industrie Norm). DIN connectors are used to link various components in personal computers.

dingbat

n. A small graphical element used for decorative purposes in a document. Some fonts, such as Zapf Dingbats, are designed to present sets of dingbats. See also font. Compare bullet.

diode

n. A device that passes current in only one direction. A diode is usually a semiconductor. See also semiconductor.

diode-transistor logic

n. A type of circuit design that employs diodes, transistors, and resistors to perform logic functions. Acronym: DTL.

DIP

n. Acronym for dual in-line package. A standard for packaging integrated circuits in which the microminiature electronic circuits etched on a silicon wafer are enclosed in a rectangular housing of plastic or ceramic and connected to downward-pointing pins protruding from the longer sides of the chip. Designed to facilitate circuit board manufacturing, this design does not work well for modern chips requiring very large numbers of connections. Compare leaderless chip carrier, pin grid array, SIP, surface-mount technology. See document image processing.

DIP switch

n. One or more small rocker- or sliding-type switches contained in the plastic or ceramic housing of a dual in-line package (DIP) connected to a circuit board. Each switch on a DIP switch can be set to one of two positions, closed or open, to control options on the circuit board. See also DIP.

dir

n. An MS-DOS command that instructs a computer to display a list of files and subdirectories in the current directory or folder. If the command is followed by a path, the computer displays a list of files and subdirectories in the specified directory or folder. See also command, MS-DOS, path (definition 2).

direct access

n. The ability of a computer to find and go straight to a particular storage location in memory or on disk to retrieve or store an item of information. Note that direct access is not the same as direct memory access (DMA), which is the ability to transfer information directly between an input/output channel and memory rather than taking the longer and more circuitous route of I/O channel to microprocessor to memory. See also random access. Compare direct memory access.

direct cable connection

n. A link between the I/O ports of two computers that uses a single cable rather than a modem or other active interface device. In most cases, a direct cable connection requires a null modem cable.

direct-connect modem

n. A modem that uses standard telephone wire and connectors and plugs directly into a telephone jack, eliminating the need for an intermediary telephone. Compare acoustic coupler.

direct current

n. Electrical current whose direction of flow does not reverse. The current may stop or change amplitude, but it always flows in the same direction. Compare alternating current. Acronym: DC.

direct memory access

n. Memory access that does not involve the microprocessor and is frequently used for data transfer directly between memory and an "intelligent" peripheral device, such as a disk drive. Acronym: DMA.

directory

n. A catalog for filenames and other directories stored on a disk. A directory is a way of organizing and grouping the files so that the user is not overwhelmed by a long list of them. The topmost directory is called the root directory; the directories within a directory are called subdirectories. Depending on how an operating system supports directories, filenames in a directory can be viewed and ordered in various ways--for example, alphabetically, by date, by size, or as icons in a graphical user interface. What the user views as a directory is supported in the operating system by tables of data, stored on the disk, that indicate characteristics and the location of each file. In the Macintosh and Windows 95 operating systems, directories are called folders.

Directory Access Protocol

n. The protocol that governs communications between X.500 clients and servers. See also CCITT X series.

directory replication

n. The copying of a master set of directories from a server (called an export server) to specified servers or workstations (called import computers) in the same or other domains. Replication simplifies the task of maintaining identical sets of directories and files on multiple computers because only a single master copy of the data must be maintained. See also directory, server.

directory tree

n. A graphic display listing the directories and subdirectories on a hard disk in tree form, with subdirectories shown as branches of the main directory. See also branch (definition 1), directory, tree structure.

DirectX

n. Windows 95 software that gives applications direct access to a computer's sound and graphics hardware.

dirty

adj. Of, pertaining to, or characteristic of a communications line that is hampered by excessive noise, degrading the quality of the signal. See also noise (definition 2).

dirty bit

n. A bit used to mark modified data in a cache so that the modifications may be carried over to primary memory. See also bit, cache.

disable

vb. To suppress something or to prevent it from happening. Disabling is a method of controlling system functions by disallowing certain activities. For example, a program might temporarily disable nonessential interrupts (requests for service from system devices) to prevent interruptions during a critical point in processing. Compare enable.

disassembler

n. A program that converts machine code to assembly language source code. Most debuggers have some kind of built-in disassembler that allows the programmer to view an executable program in terms of human-readable assembly language. See also decompiler. Compare assembler.

disassociate

vb. In Windows 95 and NT, to remove an association between a file and some application. Compare associate.

disc

n. A round, flat piece of nonmagnetic, shiny metal encased in a plastic coating, designed to be read from and written to by optical (laser) technology. It is now standard practice to use the spelling disc for optical discs and disk in all other computer contexts, such as floppy disk, hard disk, and RAM disk. See also compact disc.

disconnect

vb. To break a communications link.

discussion group

n. Any of a variety of online forums in which people communicate about subjects of common interest. Forums for discussion groups include electronic mailing lists, Internet newsgroups, and IRC channels.

disk

n. A round, flat piece of flexible plastic (floppy disk) or inflexible metal (hard disk) coated with a magnetic material that can be electrically influenced to hold information recorded in digital (binary) form. In most computers a disk is the primary means of storing data on a permanent or semi-permanent basis. Floppy disks are encased in a protective plastic jacket to protect them from damage and contamination. A hard disk is enclosed in a rigid case and can be exposed only in a dust-free environment. Types of disks used with microcomputers include floppy disks, microfloppy disks, hard disks, and removable cartridges that can be used with some hard disk drives and units, such as the Bernoulli box. Compare compact disc, disc.

disk buffer

n. A small amount of memory set aside for the purpose of storing data read from, or soon to be written to, a disk. Because disk devices are slow compared with the CPU, it is not efficient to access the disk for only one or two bytes of data. Instead, during a read, a large chunk of data is read and stored in the disk buffer. When the program wants information, it is copied from the buffer. Many requests for data can be satisfied by a single disk access. The same technique can be applied to disk writes. When the program has information to store, it writes it into the disk buffer area in memory. When the buffer has been filled, the entire contents of the buffer are written to the disk in a single operation.

disk cache

n. A portion of a computer's random access memory (RAM) set aside for temporarily holding information read from disk. A disk cache does not hold entire files, as does a RAM disk (a portion of memory that acts as if it were a disk drive). Instead, a disk cache is used to hold information that either has recently been requested from disk or has previously been written to disk. If the required information remains in a disk cache, access time is considerably faster than if the program must wait for the disk drive mechanism to fetch the information from disk. See also cache. Compare disk buffer.

disk controller

n. A special-purpose chip and associated circuitry that directs and controls reading from and writing to a computer's disk drive. A disk controller handles such tasks as positioning the read/write head, mediating between the drive and the microprocessor, and controlling the transfer of information to and from memory. Disk controllers are used with floppy disk drives and hard disks and can either be built into the system or be part of a card that plugs into an expansion slot.

disk copy

n. The process of duplicating a source disk's data and the data's organizational structure onto a target disk. See also backup.

disk crash

n. The failure of a disk drive. See also crash¹.

disk drive

n. An electromechanical device that reads from and writes to disks. The main components of a disk drive include a spindle on which the disk is mounted, a drive motor that spins the disk when the drive is in operation, one or more read/write heads, a second motor that positions the read/write head(s) over the disk, and controller circuitry that synchronizes read/write activities and transfers information to and from the computer. Two types of disk drives are in common use: floppy disk drives and hard disk drives. Floppy disk drives are designed to accept removable disks in either 5.25-inch or 3.5-inch format; hard disk drives are faster, high-capacity storage units that are completely enclosed in a protective case.

disk driver

n. A device driver that is added to a system to support a specific manufacturer's disk device. See also device driver.

disk interface

n. 1. The circuitry that connects a disk drive to a computer system. 2. A standard for connecting disk drives and computers. The ST506 standard for connecting hard disks to computers is a disk interface standard.

diskless workstation

n. A station on a computer network that is not equipped with a disk drive and that uses files stored in a file server. See also file server.

disk mirroring

n. A technique in which all or part of a hard disk is duplicated onto one or more other hard disks, each of which ideally is attached to its own controller. With disk mirroring, any change made to the original disk is simultaneously made to the other disks, so that if the original disk becomes damaged or corrupted, the mirror disks will contain a current, undamaged collection of the data from the original disk. Also called disk duplexing. See also fault tolerance.

disk partition

n. A logical compartment on a physical disk drive. A single disk might have two or more logical disk partitions, each of which would be referenced with a different disk drive name. Multiple partitions are divided into a primary (boot) partition and one or more extended partitions.

disk striping

n. The procedure of combining a set of same-size disk partitions that reside on separate disks (from 2 to 32 disks) into a single volume, forming a virtual "stripe" across the disks that the operating system recognizes as a single drive. Disk striping enables multiple I/O operations in the same volume to proceed concurrently, thus offering enhanced performance. See also disk striping with parity, input/output.

disk striping with parity

n. The technique of maintaining parity information across a disk stripe so that if one disk partition fails, the data on that disk can be recreated using the information stored across the remaining partitions in the disk stripe. See also disk striping, fault tolerance, parity.

display

n. The visual output device of a computer, which is commonly a CRT-based video display. With portable and notebook computers, the display is usually an LCD-based or a gas plasma-based flat-panel display. See also flat-panel display, liquid crystal display, video adapter, video display.

display background

n. In computer graphics, the portion of an on-screen image that remains static while other elements change; for example, window borders on a screen, or a palette of shapes or patterns in a drawing program.

display image

n. The collection of elements displayed together at a single time on a computer screen.

display screen

n. The part of a video unit on which images are shown. See also CRT.

Distance Vector Multicast Routing Protocol

n. An Internet routing protocol that provides an efficient mechanism for connectionless datagram delivery to a group of hosts across an Internet network. It is a distributed protocol that dynamically generates IP multicast delivery trees using a technique called Reverse Path Multicasting (RPM). Acronym: DVMRP.

distortion

n. An undesirable change in the waveform of a signal. Distortion can occur during signal transmission, as when a radio broadcast becomes garbled, or when a signal passes through a circuit, as when a stereo system is turned up too loud. Distortion often results in loss of information. It is mainly a problem in analog signals; digital signals are not affected by moderate distortion.

Distributed Computing Environment

n. A set of standards from the Open Group (formerly the Open Software Foundation) for development of distributed applications that can operate on more than one platform. See also distributed processing. Acronym: DCE.

distributed database

n. A database implemented on a network. The component partitions are distributed over various nodes (stations) of the network. Depending on the specific update and retrieval traffic, distributing the database can significantly enhance overall performance. See also partition (definition 2).

distributed file system

n. A file management system in which files may be located on multiple computers connected over a local or wide area network.

distributed network

n. A network in which processing, storage, and other functions are handled by separate units (nodes) rather than by a single main computer.

distributed processing

n. A form of information processing in which work is performed by separate computers linked through a communications network. Distributed processing is usually categorized as either plain distributed processing or true distributed processing. Plain distributed processing shares the workload among computers that can communicate with one another. True distributed processing has separate computers perform different tasks in such a way that their combined work can contribute to a larger goal. The latter type of processing requires a highly structured environment that allows hardware and software to communicate, share resources, and exchange information freely.

Distributed System Object Model

n. IBM's System Object Model (SOM) in a shared environment, where binary class libraries can be shared between applications on networked computers or between applications on a given system. The Distributed System Object Model complements existing object-oriented languages by allowing SOM class libraries to be shared among applications written in different languages. See also SOM. Acronym: DSOM.

distribution list

n. A list of recipients on an e-mail mailing list. This can be in the form of either a mailing list program, such as LISTSERV, or an alias in an e-mail program for all recipients of an e-mail message. See also alias (definition 2), LISTSERV, mailing list.

distributive sort

n. An ordering process in which a list is separated into parts and then reassembled in a particular order. See also sort algorithm. Compare bubble sort, insertion sort, merge sort, quicksort.

dithering

n. A technique used in computer graphics to create the illusion of varying shades of gray on a monochrome display or printer, or additional colors on a color display or printer. Dithering relies on treating areas of an image as groups of dots that are colored in different patterns. Akin to the print images called halftones, dithering takes advantage of the eye's tendency to blur spots of different colors by averaging their effects and merging them into a single perceived shade or color. Depending on the ratio of black dots to white dots within a given area, the overall effect is of a particular shade of gray. Dithering is used to add realism to computer graphics and to soften jagged edges in curves and diagonal lines at low resolutions. See also aliasing, halftone.

divergence

n. A moving apart or separation. On computer displays, divergence occurs when the red, green, and blue electron beams in a color monitor do not collectively light the same spot on the screen. Within a program, such as a spreadsheet, divergence can occur when a circular set of formulas is repeatedly recalculated (iterated), with the results of each iteration moving further from a stable solution. Compare convergence.

division by zero

n. An error condition caused by an attempt to divide a number by zero, which is mathematically undefined, or by a number that is sufficiently near to zero that the result is too large to be expressed by the machine. Computers do not allow division by zero, and software must provide some means of protecting the user from program failure on such attempts.

DLC

n. Acronym for Data Link Control. An error-correction protocol in the Systems Network Architecture (SNA) responsible for transmission of data between two nodes over a physical link. See also HDLC, SNA.

DNS

n. 1. Acronym for Domain Name System. The system by which hosts on the Internet have both domain name addresses (such as bluestem.prairienet.org) and IP addresses (such as 192.17.3.4). The domain name address is used by human users and is automatically translated into the numerical IP address, which is used by the packet-routing software. See also domain name address, IP address. 2. Acronym for Domain Name Service. The Internet utility that implements the Domain Name System (see definition 1). DNS servers, also called name servers, maintain databases containing the addresses and are accessed transparently to the user.

DNS server

n. A computer that can answer Domain Name Service (DNS) queries. The DNS server keeps a database of host computers and their corresponding IP addresses. Presented with the name apex.com, for example, the DNS server would return the IP address of the hypothetical company Apex. See also DNS (definition 2), IP address.

dock

vb. To connect a laptop or notebook computer to a docking station. See also docking station, laptop, portable computer.

docking mechanism

n. The portion of a docking station that physically connects the portable computer with the station. See also docking station.

docking station

n. A unit for housing a laptop or notebook computer that contains a power connection, expansion slots, and connections to peripherals, such as a monitor, printer, full-sized keyboard, and mouse. The purpose of a docking station is to turn the laptop or notebook computer into a desktop machine and allow users the convenience of using such peripherals as a monitor and a full-sized keyboard. See also expansion slot, laptop, peripheral, portable computer.

doctype

n. A declaration at the beginning of an SGML document that gives a public or system identifier for the document type definition (DTD) of the document. See also SGML.

document¹

n. Any self-contained piece of work created with an application program and, if saved on disk, given a unique filename by which it can be retrieved. Documents are generally thought of as word-processed materials only. To a computer, however, data is nothing more than a collection of characters, so a spreadsheet or a graphic is as much a document as is a letter or report. In the Macintosh environment in particular, a document is any user-created work named and saved as a separate file.

document²

vb. To explain or annotate something, such as a program or a procedure.

documentation

n. The set of instructions shipped with a program or a piece of hardware. Documentation usually includes necessary information about the type of computer system required, setup instructions, and instructions on the use and maintenance of the product.

document-centric

adj. Of, pertaining to, or characteristic of an operating system in which the user opens document files and thus automatically invokes the applications (such as word processors or spreadsheet programs) that process them. Many graphical user interfaces, such as the Macintosh Finder, as well as the World Wide Web, are document-centric. Compare application-centric.

document file

n. A user-created file that represents the output of a program. Also called data file. Compare program file.

document image processing

n. A system for storing and retrieving information for an enterprise in the form of bitmapped images of paper documents input with a scanner rather than in the form of text and numeric files. Document image processing takes more memory than purely electronic data processing, but it more readily incorporates signatures, drawings, and photographs and can be more familiar to users without computer training. See also paperless office.

document management

n. The full spectrum of electronic document creation and distribution within an organization.

document retrieval

n. A capability built into some application programs that enables the user to search for specific documents by specifying items of information, such as date, author, or previously assigned keywords. Document retrieval depends on an indexing scheme that the program maintains and uses. Depending on the program's capabilities, document retrieval might allow the user to specify more than one condition to refine a search.

document source

n. The plain-text HTML form of a World Wide Web document, with all tags and other markup displayed as such rather than being formatted. Also called document source, source. See also HTML.

Document Style Semantics and Specification Language

n. An ISO standard under preparation addressing the semantics of high-quality composition in a manner independent of particular formatting systems or processes. It is intended as a complementary standard to SGML for the specification of semantics. See also ISO, SGML. Acronym: DSSSL.

document window

n. In windowing environments, such as the Apple Macintosh and Microsoft Windows, an on-screen window (enclosed work area) in which the user can create, view, or work on a document.

domain

n. 1. In database design and management, the set of valid values for a given attribute. For example, the domain for the attribute AREA-CODE might be the list of all valid three-digit numeric telephone area codes in the United States. See also attribute (definition 1). 2. For Windows NT Advanced Server, a collection of computers that share a common domain database and security policy. Each domain has a unique name. 3. In the Internet and other networks, the highest subdivision of a domain name in a network address, which identifies the type of entity owning the address (for example, .com for commercial users or .edu for educational institutions) or the geographical location of the address (for example, .fr for France or .sg for Singapore). The domain is the last part of the address (for example, www.acm.org). See also domain name.

domain name

n. An address of a network connection in the format that identifies the owner of that address in a hierarchical format: server.organization.type. For example, www.whitehouse.gov identifies the Web server at the White House, which is part of the U.S. government.

domain name address

n. The address of a device connected to the Internet or any other TCP/IP network, in the hierarchical system that uses words to identify servers, organizations, and types, such as www.logos.net. See also TCP/IP.

DOS

n. Acronym for disk operating system. A generic term describing any operating system that is loaded from disk devices when the system is started or rebooted. The term originally differentiated between disk-based systems and primitive microcomputer operating systems that were memory-based or that supported only magnetic or paper tape. See also MS-DOS, PC-DOS.

DOS box

n. 1. An OS/2 process that supports the execution of MS-DOS programs. Also called compatibility box. 2. A computer that uses the MS-DOS or PC-DOS operating system, as opposed to one that runs some other operating system, such as UNIX.

DOS extender

n. A program designed to extend the 640 KB of conventional memory available for use by DOS and DOS-based applications. A DOS extender works by claiming a portion of reserved memory (memory used by other parts of the system, such as the video adapter, the ROM BIOS, and the I/O ports).

DOS prompt

n. The visual indication from the MS-DOS command processor that the operating system is ready to accept a new command. The default DOS prompt is a path followed by a greater-than sign (for example, C:>); the user can also design a custom prompt with the PROMPT command.

dot

n. 1. In the UNIX, MS-DOS, OS/2, and other operating systems, the character that separates a filename from an extension as in TEXT.DOC (pronounced "text-dot-doc"). 2. In computer graphics and printing, a small spot combined with others in a matrix of rows and columns to form a character or a graphic element in a drawing or design. The dots forming an image on the screen are called pixels. The resolution of a display or printing device is often expressed in dots per inch (dpi). Dots are not the same as spots, which are groups of dots used in the halftoning process. See also pixel, resolution (definition 1). Compare spot. 3. In an Internet address, the character that separates the different parts of the domain name, such as the entity name from the domain. See also domain (definition 3), domain name.

dot address

n. An IP address in dotted quad form. See also IP address.

dot-addressable mode

n. A mode of operation in which a computer program can address ("point to") individual dots on the screen or in a printed character. See also all points addressable.

dot file

n. A file under UNIX whose name begins with a period. Dot files do not appear in ordinary listings of the files in a directory. Dot files are often used to store program setup information for the particular user; for example, .newsrc in a user's account indicates to a newsreader which newsgroups the user subscribes to.

dot-matrix¹

adj. Referring to video and print hardware that forms character and graphic images as patterns of dots.

dot matrix²

n. The rectangular grid, or matrix, of tiny "cells" in which dots are displayed or printed in the patterns required to form text characters, circles, squares, and other graphical images. Depending on the frame of reference, the size of a dot matrix varies from a few rows and columns to an invisible grid covering an entire display screen or printed page. See also dot-matrix printer, raster.

dot-matrix printer

n. Any printer that produces characters made up of dots using a wire-pin print head. The quality of output from a dot-matrix printer depends largely on the number of dots in the matrix, which might be low enough to show individual dots or might be high enough to approach the look of fully formed characters. Dot-matrix printers are often categorized by the number of pins in the print head--typically 9, 18, or 24. Compare daisy-wheel printer, laser printer.

dots per inch

n. A measure of screen and printer resolution that is expressed as the number of dots that a device can print or display per linear inch. Acronym: dpi.

double-click

vb. To press and release a mouse button twice without moving the mouse. Double-clicking is a means of rapidly selecting and activating a program or program feature. Compare click, drag.

double-density disk

n. A disk created to hold data at twice the density (bits per inch) of a previous generation of disks. Early IBM PC floppy disks held 180 KB of data. Double-density disks increased that capacity to 360 KB. Double-density disks use modified frequency modulation encoding for storing data. See also floppy disk, microfloppy disk, modified frequency modulation encoding. Compare high-density disk.

double-sided disk

n. A floppy disk that can hold data on both its top and bottom surfaces.

double word

n. A unit of data consisting of two contiguous words (connected bytes, not text) that are handled together by a computer's microprocessor.

down

adj. Not functioning, in reference to computers, printers, communications lines on networks, and other such hardware.

downlink

n. The transmission of data from a communications satellite to an earth station.

download

vb. 1. In communications, to transfer a copy of a file from a remote computer to the requesting computer by means of a modem or network. 2. To send a block of data, such as a PostScript file, to a dependent device, such as a PostScript printer. Compare upload.

downloadable font

n. A set of characters stored on disk and sent (downloaded) to a printer's memory when needed for printing a document. Downloadable fonts are most commonly used with laser printers and other page printers, although many dot-matrix printers can accept some of them. Also called soft font.

downsizing

n. In computing, the practice of moving from larger computer systems, such as mainframes and minicomputers, to smaller systems in an organization, generally to save costs and to update to newer software. The smaller systems are usually client/server systems composed of a combination of PCs, workstations, and some legacy system such as a mainframe, connected in one or more local area networks or wide area networks. See also client/server architecture, legacy system.

downstream

n. The direction in which a news feed for a newsgroup is passed from one news server to the next. See also news feed, newsgroup, news server.

downtime

n. The amount or percentage of time a computer system or associated hardware remains nonfunctional. Although downtime can occur because hardware fails unexpectedly, it can also be a scheduled event, as when a network is shut down to allow time for maintenance.

downward compatibility

n. The capability of source code or programs developed on a more advanced system or compiler version to be executed or compiled by a less advanced (older) version. Compare upward-compatible.

drag

vb. In graphical user interface environments, to move an image or a window from one place on the screen to another by "grabbing" it and pulling it to its new location using the mouse. The mouse pointer is positioned over the object, and the mouse button is pressed and held while the mouse is moved to the new location.

drag-and-drop

vb. To perform operations in a graphical user interface by dragging objects on the screen with the mouse. For example, to delete a document in the Mac OS, a user can drag the document icon across the screen and drop it on the trashcan icon. See also drag, graphical user interface.

DRAW

n. Acronym for direct read after write. A technique used with optical discs to verify the accuracy of information immediately after it has been recorded (written) on the disc. Compare DRDW.

drawing program

n. A program for manipulating object-oriented graphics, as opposed to manipulating pixel images. In a drawing program, for example, the user can manipulate an element, such as a line, a circle, or a block of text, as an independent object simply by selecting the object and moving it. See also object-oriented graphics, pixel image, vector graphics.

DRDW

n. Acronym for direct read during write. A technique used with optical discs to verify the accuracy of information at the time it is being recorded on the disc. Compare DRAW.

drill down

vb. To start at a top-level menu, directory, or Web page and pass through several intermediate menus, directories, or linked pages, until the file, page, menu command, or other item being sought is reached. Drilling down is common practice in searching for files or information on the Internet, where high-level Gopher menus and World Wide Web pages are frequently very general and become more specific at each lower level. See also Gopher, menu, Web page.

drive bay

n. A hollow, rectangular area in a computer chassis designed to hold a disk drive. A drive bay always has side walls, usually made of metal, that generally contain holes to facilitate installation of a disk drive. Some drive bays, such as those intended to hold hard disks, are not visible to the user. Most drives are located on the front of the chassis so that the user can interact with the drive.

drive letter

n. The naming convention for disk drives on IBM and compatible computers. Drives are named by letter, beginning with A, followed by a colon.

drive mapping

n. The assignment of a letter or name to a disk drive so that the operating system or network server can identify and locate it. For example, in PCs, the primary drive mappings are A: and B: for floppy disk drives and C: for the hard disk. See also A:, disk drive, hard disk.

drive number

n. The naming convention for Macintosh disk drives. For example, a two-drive system calls its drives 0 and 1.

driver

n. A hardware device or a program that controls or regulates another device. A line driver, for example, boosts signals transmitted over a communications line. A device driver is a device-specific control program that enables a computer to work with a particular device, such as a printer or a disk drive. See also device driver.

drop cap

n. A large capital letter at the beginning of a text block that occupies the vertical depth of two or more lines of regular text.

drop-down menu

n. A menu that drops from the menu bar when requested and remains open without further action until the user closes it or chooses a menu item. Compare pull-down menu.

drum

n. A rotating cylinder used with some printers and plotters and (in the early days of mainframe computing) as a magnetic storage medium for data. In laser printers, a rotating drum is coated with a photoelectric material that retains a charge when struck by a laser beam. The electrically charged spots on the drum then attract toner particles that the drum transfers to the paper as the paper passes by.

dual boot

n. A computer configuration that allows a user to boot one of a choice of two operating systems on a PC. Some possible dual boot combinations include Windows 95/Windows NT, Windows NT/OS/2, and Windows 95/Linux. Some operating systems, such as Windows 95 and OS/2, include a multiple boot option. Older operating systems, such as Windows 3.X and DOS, require the use of a boot utility to perform a dual boot. See also boot¹.

dual density

adj. Of, pertaining to, or characteristic of floppy disk drives that can read from and write to disks in more than one density format.

dual disk drive

adj. Of, pertaining to, or characteristic of a computer that has two floppy disk drives.

dual processors

n. Two processors used in a computer to speed its operation--one processor to control memory and the bus, and another to manage input/output. Many personal computers use a second processor to perform floating-point mathematical operations. See also coprocessor, floating-point notation.

dumb quotes

n. Quotation marks that have the same appearance (usually upright like the apostrophe ' and quotation marks " on a typewriter) whether they stand before or after the material being quoted. Compare smart quotes.

dumb terminal

n. A terminal that does not contain an internal microprocessor. Dumb terminals are typically capable of displaying only characters and numbers and responding to simple control codes. Compare smart terminal.

duplex¹

adj. Capable of carrying information in both directions over a communications channel. A system is full-duplex if it can carry information in both directions at once; it is half-duplex if it can carry information in only one direction at a time.

duplex²

n. 1. Simultaneous communications, in both directions, between the sender and receiver. Also called duplex transmission, full-duplex transmission. See also half-duplex transmission. 2. Photographic paper on which an image can be printed on both sides.

duplex printer

n. A printer capable of printing on both sides of the page.

duplex system

n. A system of two computers, one of which is active while the other remains on standby, ready to take over processing if the active machine malfunctions.

duplicate key

n. A value assigned to an indexed field in one record in a database that duplicates a value assigned to the same field in another record in the database. For example, a key (or index) composed of ZIP-CODE would necessarily contain duplicate values if the file contained a number of addresses from a single ZIP code. A field in which duplicate values are permitted cannot serve as a primary key because the primary key must be unique, but it can serve as a component of a composite primary key. See also field (definition 1), key (definition 2), primary key.

Dvorak keyboard

n. A keyboard layout developed by August Dvorak and William L. Dealey in 1936 as an alternative to the overwhelmingly popular QWERTY keyboard. The Dvorak keyboard was designed to speed typing by placing the characters on the keyboard for easiest access to the most frequently typed letters. In addition, pairs of letters that often occur sequentially were separated so that the hands could alternate typing them. See also ergonomic keyboard, keyboard. Compare QWERTY keyboard.

dynalink

n. Short for dynamic link. See dynamic-link library.

dynamic

adj. Occurring immediately and concurrently. The term is used in describing both hardware and software; in both cases it describes some action or event that occurs when and as needed. In dynamic memory management, a program is able to negotiate with the operating system when it needs more memory.

dynamic address translation

n. On-the-fly conversion of memory-location references from relative addresses (such as "three units from the beginning of X") to absolute addresses (such as "location number 123") when a program is run. Acronym: DAT.

dynamic caching

n. A technique for storing recently used data in memory where cache size is based on how much memory is available rather than how much memory is assigned to the application currently running.

Dynamic HyperText Markup Language

n. A way of providing dynamically changing content and special effects on a Web page (HTML document) without relying on server-side programs.

dynamic-link library

n. A feature of the Microsoft Windows family of operating systems and OS/2 that allows executable routines to be stored separately as files with DLL extensions and to be loaded only when needed by a program. A dynamic-link library has several advantages. First, it does not consume any memory until it is used. Second, because a dynamic-link library is a separate file, a programmer can make corrections or improvements to only that module without affecting the operation of the calling program or any other dynamic-link library. Finally, a programmer can use the same dynamic-link library with other programs. Acronym: DLL.

dynamic memory allocation

n. The allocation of memory to a process or program at run time. Dynamic memory is allocated from the system heap by the operating system upon request from the program.

dynamic RAM

n. A form of semiconductor random access memory (RAM). Dynamic RAMs store information in integrated circuits containing capacitors. Because capacitors lose their charge over time, dynamic RAM boards must include logic to refresh (recharge) the RAM chips continuously. While a dynamic RAM is being refreshed, it cannot be read by the processor; if the processor must read the RAM while it is being refreshed, one or more wait states occur. Despite being slower, dynamic RAMs are more commonly used than RAMs because their circuitry is simpler and because they can hold up to four times as much data. See also RAM. Compare static RAM. Acronym: DRAM.

dynamic Web page