D

D

DA n. See desk accessory.

DAC n. See digital-to-analog converter.

DACL n. See discretionary access control list.

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 chain¹ 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. SCSI (Small Computer System Interface) and the newer USB (Universal Serial Bus) both support daisy-chained devices. See also SCSI, USB.

daisy chain² vb. To connect a series of devices, one to another, like daisies in a chain of flowers.

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 Daisy-wheel printers were standard for high-quality printing until being superseded by laser printers.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.

damping n. A technique for preventing overshoot (exceeding the desired limit) in the response of a circuit or device.

D-AMPS n. Acronym for Digital Advanced Mobile Phone Service. The digital form of the analog AMPS cellular phone service. D-AMPS, sometimes spelled DAMPS, differs from AMPS in being digital and in tripling the number of available channels by using time division multiple access (TDMA) to divide each of the 30 AMPS channels into three separate channels. See also AMPS, FDMA, TDMA.

DAO n. See Data Access Objects.

DAP n. See Directory Access Protocol.

DaratechSUMMIT n. Conference on emerging engineering and technology developments in the information technology industry. The DaratechSUMMIT focuses on how information technology affects business practices and assists in manufacturing and production.

dark fiber n. Unused capacity in fiber-optic communications.

Darlington circuit n. An amplifier circuit made of two transistors, often mounted in the same housing. The collectors of the two transistors are connected, and the emitter of the first is connected to the base of the second. Darlington circuits provide high-gain amplification. Also called: Darlington pair.

Darlington pair n. See Darlington circuit.

DARPA n. See Defense Advanced Research Projects Agency.

DARPANET n. Short for Defense Advanced Research Projects Agency NETwork. See ARPANET.

Darwin n. Apple Computer s open-source operating system, which forms the core of Mac OS X. Darwin is a processor-independent BSD UNIX operating system based on FreeBSD and Mach 3.0 technologies. Darwin offers advanced networking, protected memory, preemptive multitasking, and support for Macintosh and UNIX file systems. Darwin can be run on both the Power PC Macintosh and Intel processor based computers. See also Mac OS X.

DAS n. See dual attachment station.

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.

.dat n. A generic file extension for a data file.

DAT n. See digital audio tape, dynamic address translation.

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. See also datum. Compare information.

Data Access Objects n. A data access interface that communicates with Microsoft Jet and ODBC-compliant data sources to connect to, retrieve, manipulate, and update data and the database structure. Acronym: DAO.

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

data aggregate n. A collection of data records. It usually includes a description of the placement of the data blocks and their relation to the entire set.

data attribute n. Structural information about data that describes its context and meaning.

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. Acronym: DB.

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. Acronym: DBA. Also called: database manager.

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 machine n. 1. A peripheral that executes database tasks, thereby relieving the main computer from performing them. 2. A database server that performs only database tasks.

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. Acronym: DBMS. Also called: database manager. See also database engine.

database manager n. See database administrator, database management system.

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

database server n. A network node, or station, dedicated to storing and providing access to a shared database. Also called: database machine.

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 buffer n. An area in memory where data is temporarily stored while being moved from one location to another. See also buffer¹.

data bus n. See bus.

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

data capture n. 1. The collection of information at the time of a transaction. 2. The process of saving on a storage medium a record of interchanges between a user and a remote information utility.

data carrier n. See carrier (definition 1).

Data Carrier Detected n. See DCD (definition 1).

data chaining n. The process of storing segments of data in noncontiguous locations while retaining the ability to reconnect them in the proper sequence.

data channel n. See channel (definition 1).

data closet n. See wiring closet.

data collection n. 1. The process of acquiring source documents or data. 2. The grouping of data by means of classification, sorting, ordering, and other organizing methods.

datacom n. See communications. Short for data communications.

data communications n. See communications.

data compaction n. See data compression.

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, file storage and transfer, and CD-ROM publishing. Also called: data compaction.

data conferencing n. Simultaneous data communication among geographically separated participants in a meeting. Data conferencing involves whiteboards and other software that enable a single set of files at one location to be accessed and modified by all participants. See the illustration. See also desktop conferencing, whiteboard. Compare video conferencing.

Data conferencing.

data control n. The aspect of data management concerned with tracking how and by whom data is used, accessed, altered, owned, and reported on.

data conversion n. Changing the way information is represented in a document or file for instance, changing binary representation to decimal or hexadecimal.

data corruption n. See corruption.

data declaration n. A statement in a program that specifies the characteristics of a variable. The requirements for data declarations vary among different programming languages but can include such values as variable name, data type, initial value, and size specification. See also array, data type, record, variable.

data definition language n. A language that defines all attributes and properties of a database, especially record layouts, field definitions, key fields, file locations, and storage strategy. Acronym: DDL.

data description language n. A language designed specifically for declaring data structures and files. See also data definition language.

data dictionary n. A database containing data about all the databases in a database system. Data dictionaries store all the various schema and file specifications and their locations. They also contain information about which programs use which data and which users are interested in which reports.

data directory n. See catalog, data dictionary.

data-driven attack n. A form of attack in which malicious code is hidden in a program or other innocuous data. When the data is executed, the virus or other destructive code is activated. A data-driven attack is typically used to bypass a firewall or other security measures.

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. Compare demand-driven processing. .

data element n. A single unit of data. Also called: data item. See also data field.

data encapsulation n. A method of dealing with computers with Year 2000 problems that entailed modifying the input and output logic of a program, leaving the actual data unchanged as it was processed. The input logic was modified to reflect a date in the past that the computer could handle that paralleled the current calendar. When output was generated, the output logic changed the data to reflect the correct date.

data encryption n. See encryption.

data encryption key n. A sequence of secret information, such as a string of decimal numbers or binary digits, that is used to encrypt and decrypt data. Acronym: DEK. See also decryption, encryption, key (definition 3).

data encryption standard n. See DES.

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 field masking n. The process of filtering or selecting part of a data field to control the way it is returned and displayed.

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 flow or dataflow n. 1. The movement of data through a system, from entry to destination. 2. In parallel processing, a design in which a calculation is made either when all necessary data is available (data-driven processing) or when other processors request the data (demand-driven processing). See also parallel processing.

data fork n. In Macintosh files, the part of a stored document that contains user-supplied information, such as the text of a word-processing document. A Macintosh file can have a data fork, a resource fork (which contains information such as program code, font data, digitized sound, or icons), and a header. All three parts are used by the operating system in file management and storage. See also resource (definition 2), resource fork.

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

data frame n. A packet of information transmitted as a unit on a network. Data frames are defined by the network s data-link layer and exist only on the wire between network nodes. See also data-link layer, frame (definition 2).

data glove n. A data input device or controller in the form of a glove fitted with sensors that convert movement of the hand and fingers into commands. See also virtual reality.

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 independence n. The separation of data in a database from the programs that manipulate it. Data independence makes stored data as accessible as possible.

data integrity n. The accuracy of data and its conformity to its expected value, especially after being transmitted or processed.

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. Acronym: DIF. See also ASCII.

data item n. See data element.

data library n. A cataloged collection of data files on disk or in another storage medium.

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 (definition 1), DTE.

Data Link Connection Identifier n. A virtual circuit on frame relay networks that permanently identifies the path to a particular destination. See also frame relay, virtual circuit.

Data Link Control n. See DLC.

data link escape n. In data transmission, a control character that changes the meaning of the characters immediately following it.

data-link layer n. The second of seven layers in the ISO/OSI reference 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 the illustration. See also ISO/OSI reference model.

Data-link layer on ISO/OSI reference model.

data management n. The control of data from acquisition and input through processing, output, and storage. In microcomputers, hardware manages data by gathering it, moving it, and following instructions to process it. The operating system manages the hardware and ensures that the parts of the system work in harmony so that data is stored safely and accurately. Application programs manage data by receiving and processing input according to the user s commands, and sending results to an output device or to disk storage. The user also is responsible for data management by acquiring data, labeling and organizing disks, backing up data, archiving files, and removing unneeded material from the hard disk.

data manipulation n. The processing of data by means of programs that accept user commands, offer ways to handle data, and tell the hardware what to do with the data.

data manipulation language n. In database management systems, a language that is used to insert data in, update, and query a database. Data manipulation languages are often capable of performing mathematical and statistical calculations that facilitate generating reports. Acronym: DML. See also structured query language.

data mart n. A scaled-down version of a data warehouse that is tailored to contain only information likely to be used by the target group. See also data warehouse.

data medium n. The physical material on which computer data is stored.

data migration n. 1. The process of moving data from one repository or source, such as a database, to another, usually via automated scripts or programs. Often data migration involves transferring data from one type of computer system to another. 2. In supercomputing applications, the process of storing large amounts of data off line while making them appear to be on line as disk-resident files.

data mining n. The process of identifying commercially useful patterns, problems, or relationships in a database, a Web server, or other computer repository through the use of advanced statistical tools. Some Web sites use data mining to monitor the efficiency of site navigation and to determine changes in the Web site s design based on how consumers are using the site.

data model n. A collection of related object types, operators, and integrity rules that form the abstract entity supported by a database management system (DBMS). Thus, one speaks of a relational DBMS, a network DBMS, and so on, depending on the type of data model a DBMS supports. In general, a DBMS supports only one data model as a practical rather than a theoretical restriction.

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

Data Over Cable Service Interface Specification n. See DOCSIS.

data-overrun error n. An error that occurs when more data is being acquired than can be processed. See also bps.

data packet n. See packet.

data path n. The route that a signal follows as it travels through a computer network.

data point n. Any pair of numeric values plotted on a chart.

data processing n. 1. The general work performed by computers. 2. More specifically, the manipulation of data to transform it into some desired result. Acronym: DP. Also called: ADP, automatic data processing, EDP, electronic data processing. See also centralized processing, decentralized processing, distributed processing.

Data Processing Management Association n. See DPMA.

data projector n. A device, similar to a slide projector, that projects the video monitor output of a computer onto a screen.

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

data rate n. The speed at which a circuit or communications line can transfer information, usually measured in bits per second (bps).

data record n. See record¹.

data reduction n. The process of converting raw data to a more useful form by scaling, smoothing, ordering, or other editing procedures.

data segment n. The portion of memory or auxiliary storage that contains the data used by a program.

Data Service Unit n. See DDS.

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 Set Ready n. See DSR.

data sharing n. The use of a single file by more than one person or computer. Data sharing can be done by physically transferring a file from one computer to another, or, more commonly, by networking and computer-to-computer communications.

data signal n. The information transmitted over a line or circuit. It consists of binary digits and can include actual information or messages and other elements such as control characters or error-checking codes.

data sink n. 1. Any recording medium where data can be stored until needed. 2. In communications, the portion of a Data Terminal Equipment (DTE) device that receives transmitted data.

data source n. 1. The originator of computer data, frequently an analog or digital data collection device. 2. In communications, the portion of a Data Terminal Equipment (DTE) device that sends data.

data stream n. An undifferentiated, byte-by-byte flow of data.

data structure n. An organizational scheme, such as a record or array, that can be applied to data to facilitate interpreting the data or performing operations on it.

data switch n. A device in a computer system that routes incoming data to various locations.

Data Terminal Equipment n. See DTE.

Data Terminal Ready n. See DTR.

data traffic n. The exchange of electronic messages control and data across a network. Traffic capacity is measured in bandwidth; traffic speed is measured in bits per unit of time.

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 transfer rate n. See data rate.

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 validation n. The process of testing the accuracy of data.

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 warehouse¹ 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).

data warehouse² vb. To acquire, collect, manage, and disseminate information gathered from various sources into a single location; or to implement an informational database used to store sharable data. Data warehousing is a four-step process: gathering data; managing the data in a centralized location; providing access to the data along with tools for interpreting, analyzing, and reporting on the data; and producing reports on the data to be used for decision making. See also downflow, inflow, metaflow, upflow.

date and time stamp n. See time stamp.

date counter overflow n. A problem that may occur in systems or programs when the value in a date variable exceeds allowable values. A date counter overflow can occur when an incremental date produces a number that the system interprets as zero or a negative number. This is likely to cause the system or program to post an error message in turn or to revert to the original starting point. Although this was largely considered a Year 2000 problem, such an error is not necessarily confined to the year 2000.

date dependency n. In terms of the Year 2000 problem, the need many programs have for date-related input or output data and the way dates are represented in that data. This dependency affects whether the program can run correctly when the turn of the century is reached.

date expansion n. A method of dealing with programs with Year 2000 problems that entails changing data, data descriptions, and (if necessary) program logic that pertains to dates by expanding date fields from two digits to four digits for example, from DDMMYY to DDMMYYYY.

date format n. The manner in which dates are formatted in a computer system or program. While some organizations require that the same format be used throughout their systems and programs, many organizations have not, which can make tracking down potential date problems, such as the Year 2000 problem, difficult. In addition, date formats can vary widely from organization to organization, although many have opted to standardize on formats specified in ANSI X3.30-1997 or ISO8601:1988.

date horizon n. A period of time that a program uses to determine the beginning or ending point in performing its functions. A program that tracks inventory may have one date horizon that trails the current date by two months (a trailing date horizon) to process returned merchandise and another that precedes it by another two months (a leading date horizon) for planning purposes. If the program logic doesn t account for any date horizons it may have, for example, if the year was 1999, the program could experience Year 2000 problems when the leading date horizon enters January 1, 2000. See also event horizon.

date-in-key problem n. A potential problem in computer systems that depend on indexed files using a two-digit date as part of the key, such as certain databases. If the files need to be in chronological order, the files beginning with the year 2000 will be out of sequence for example, (19)99 would be interpreted as more recent than (20)00.

date rollover n. See Year 2000 rollover.

date stamp n. See time stamp.

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

datum n. Singular of data; a single item of information. See also data.

daughterboard n. A circuit board that attaches to another, such as the main system board (motherboard), to add extra capabilities. See also motherboard.

DAV connector n. See digital audio/video connector.

day-of-the-week problem n. A reference to an inaccuracy that may occur after the Year 2000 in computers that calculate the day of the week based on the last two digits of the year, assuming that the dates they calculate fall in the 1900s. Because January 1, 1900 was a Monday, but January 1, 2000 will be a Saturday, those computers may not be able to correctly determine the day of the week. This is particularly problematic in computers that regulate timed systems based on the business week, such as a door or vault that unlocks during business hours.

DB n. See database.

dB n. See decibel.

DBA n. See database administrator.

DB connector n. Any of various connectors that facilitate parallel input and output. The initials DB (for data bus) are followed by a number that indicates the number of lines (wires) within the connector. For example, a DB-9 connector has nine pins and supports up to nine lines, each of which can connect to a pin on the connector.

.dbf n. A file extension for a dBASE database file.

DBMS n. See database management system.

DBS n. See direct broadcast satellite.

dbXML n. Acronym for database XML. A native XML database server designed to manage large collections of XML documents. dbXML may be embedded in custom applications or run as a stand-alone database.

DC n. See direct current.

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. 1. 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. See also RS-232-C standard. 2. Acronym for Document Content Description. A specification governing the rules for defining the structure and content of XML documents. The specification was created by IBM and Microsoft in 1998 and was submitted to the World Wide Web Consortium for approval. See also XML.

DCE n. 1. Acronym for Data Communications Equipment. The term used in RS-232 and X.25 specifications for a device, such as a modem, that provides another device (known as the Data Terminal Equipment or DTE) with access to a communications line. A DCE is an intermediary device that often transforms input from a DTE before sending it to a recipient. See also RS-232-C standard, X series. Compare DTE. 2. See Distributed Computing Environment.

D channel n. Short for data channel. In the ISDN communications architecture, the channel dedicated to carrying control signals, such as packet-switching information; and user-related data, such as phone numbers. The basic ISDN connection, called the Basic Rate Interface (BRI), is composed of two B (bearer) channels, which carry as much as 64 Kbps of actual data each, and one D channel, which transmits at either 16 Kbps or 64 Kbps. The faster Primary Rate Interface (PRI) is composed of one 64-Kbps D channel and either 23 or 30 B channels operating at 64 Kbps. See also B channel, BRI, ISDN.

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).

DCS n. Acronym for Desktop Color Separation. The primary format for preparing digital publication text and graphics for printing. DCS layouts consist of five files, one for each of the CMYK colors, and a master file which, includes the display version of the page and information on the other four files. See also OPI.

DCTL n. See direct-coupled transistor logic.

DDBMS n. See distributed database management system.

DDC n. Acronym for Display Data Channel. A VESA standard that allows software control of graphical computer monitors. Under DDC, monitor characteristics are provided to the graphics subsystem, which uses the data to configure the display and provide a bidirectional communication channel between the monitor and computer. Also called: VESA DDC. See also VESA².

DDCP n. See direct digital color proof.

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.

DDK n. Acronym for Driver Development Kit. A set of tools used to create software that enables an operating system to work with hardware devices. With a DDK, a software developer can build drivers to support network, storage, print, sound, video, input, and other devices. Also called: Device Driver Kit, Device Driver Developer Kit. See also driver.

DDL n. See data definition language.

DDoS n. Acronym for distributed denial of service attack. A form of denial of service attack (DoS) originating from several computers that seeks to disrupt Web access by overwhelming a target with connection requests that cannot be completed. A DDoS attack involves cracking into a number of computers and planting programs that lie dormant until sent a signal to attack. At that point the computers send a steady stream of data packets to the targeted Web site, overwhelming the ability of the Web server to respond. Because the attack is coming from many computers, security features that might otherwise recognize the attack and stop accepting data packets from a single source are unable to shut down connections to all the attackers. See also DoS, packet, zombie.

DDR SDRAM n. Short for Double Data Rate Synchronous Dynamic RAM (SDRAM). A form of SDRAM that essentially doubles memory throughput to 200 megahertz or better. DDR SDRAM gets a boost in data transfer rates by producing output on both the rising and falling of the system clock that is, twice for each clock cycle. See also SDRAM.

DDS n. Acronym for digital data service, a dedicated communications line that provides transmission at speeds up to 56 Kbps. DDS lines use a device known as a CSU/DSU rather than a modem for connecting two networks. The CSU, or Channel Service Unit, connects the network to the transmission line; the DSU, or Data Service Unit, converts data for transmission by the CSU and controls data flow.

dead code n. Program code that never gets executed, possibly because the programmer has eliminated all references to it, or possibly because the program is written in such a way that the instruction(s) will never be needed for example, an ELSE statement would never be needed in an IF condition that always proved to be true. Dead code can slow program execution and increase the size of the program in memory. Also called: grunge, software rot.

dead halt n. A machine stop with no hope of recovery by either the program or the operating system. The only choice after a dead halt is to reboot. Also called: drop-dead halt. See also hang. Compare reboot.

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).

dead-letter box n. In e-mail or message systems, a file to which undeliverable messages are sent.

deadlock n. 1. A situation that occurs when two programs or devices are each waiting for a response from the other before continuing. Also called: deadly embrace. 2. In operating systems, a situation in which two or more processes are prevented from continuing while each waits for resources to be freed by the continuation of the other. 3. In computer games, a deadlock occurs when the resources needed to continue the game become unavailable to the player. The deadlock condition could be intentional, such as a loss condition, or a design error on the part of the game developer. See also computer games.

deadly embrace n. See deadlock.

deallocate vb. To free previously allocated memory. See also pointer. Compare allocate.

deblock vb. To remove one or more logical records (units of stored information) from a block. Application or database systems must often deblock information to make specific units of information available for processing. Compare block² (definition 1).

debounce algorithm n. A set of instructions that makes an assumption about how fast a user can press and release a switch and then ensures that only one press is registered in the time specified.

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.

deca- prefix Metric prefix meaning 10 that is, 10 to the first power, or 10¹.

decay n. A decrease in the amplitude of a signal over time.

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.

deceleration time n. The time required for an access arm to come to a stop as it approaches the desired portion of a disk. The faster the arm moves, the more momentum it gains and the greater the deceleration time.

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.

deci- prefix Metric prefix meaning 10 ¹ (one-tenth).

decibel n. 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: dB = n log (x/r) where x is the measured quantity, r is the reference quantity, and n is 10 for voltage and current measurements and 20 for power measurements.Abbreviation: dB.

decimal n. The base-10 numbering system. See also base (definition 2).

decision box n. A diamond-shaped flowchart symbol denoting a decision that results in a branching in the process being considered. See the illustration.

Decision box.

decision support system n. A set of programs and related data designed to help with analysis and decision making. A decision support system provides more help in formulating decisions than a management information system (MIS) or an executive information system (EIS). It includes a database, a body of knowledge about the subject area, a language used to formulate problems and questions, and a modeling program for testing alternative decisions. Acronym: DSS. Compare executive information system, management information system.

decision table n. A tabular listing of possible conditions (inputs) and the desired result (output) corresponding to each condition. A decision table may be used in the preliminary analysis of program flow, or it may be converted and incorporated into the program itself.

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 the illustration. See also branch, tree structure.

Decision tree.

deck n. A storage device, such as a tape deck, or a group of such devices.

declaration n. A binding of an identifier to the information that relates to it. For example, to make a declaration of a constant means to bind the name of the constant with its value. Declaration usually occurs in a program s source code; the actual binding can take place at compile time or run time. See also bind, constant, data declaration, data type, identifier, instruction, routine, type declaration, variable.

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. Acronym: DML. Also called: data manipulation language. See also HTML, SGML.

declare vb. To specify the name and type of a variable that will be used in a program. In most high-level programming languages, variables are declared at the beginning of sections of code. See also variable.

DECnet n. A hardware, software, and protocol stack designed by Digital Equipment Corporation for its Digital Network Architecture (DNA).

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.

decollate vb. To separate copies in a multipart continuous paper form.

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).

decompress vb. See uncompress.

decrement¹ n. The amount by which a number is decreased. Compare increment¹.

decrement² vb. To decrease a number by a given amount. Compare increment².

decryption n. The process of restoring encrypted data to its original form. See also data encryption key. Compare encryption.

deCSS n. Decrypt CSS. A utility capable of cracking the CSS encryption system used on DVD discs. By decrypting the CSS code, DVD movies and other copyrighted material can be used with any DVD playback device without regard to license or region coding. The origin of deCSS can be traced to a number of individuals interested in creating a DVD player for the Linux OS. The term deCSS is sometimes used generically for any software capable of defeating CSS technology. See also CSS, region code.

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

dedicated adj. Of, pertaining to, or being a device, program, or procedure devoted to a single task or function.

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

dedicated circuit n. See dedicated line.

dedicated connection n. See dedicated line.

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: dedicated connection, 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.

dedicated server n. A computer usually quite powerful that is used solely as a network server. See also server. Compare nondedicated server.

deep copy n. A copy of the contents of a data structure, including all its substructures.

deep hack n. A state of total concentration on and preoccupation with a programming effort. Also called: deep hack mode.

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. Acronym: DARPA. See also ARPANET.

deferral time n. The length of time that nodes on a CSMA/CD network wait before trying to retransmit after a collision. See also CSMA/CD.

deferred address n. An indirect address (memory location) whose calculation is delayed until a program is run. See also relative address.

deferred processing n. Processing of data after it has been received and stored in blocks. Compare direct processing.

deflection coils n. See yoke.

deflection routing n. See hot potato routing.

deformation n. In multimedia and computer-aided design applications, the process of altering a model via certain tools, such as stretch, shatter, bend, and twist. See also CAD, multimedia.

defrag vb. Slang for defragment. To rearrange data on a disk drive so that whole files are stored in contiguous sectors and the drive heads do not have to travel to scattered locations on the disk in order to read or write portions of a particular file. See also defragmentation.

defragger n. A software utility for reuniting parts of a file that have become fragmented through rewriting and updating. A defragger physically restores the file to contiguous sectors on a hard disk to speed up access as much as 75 percent. See also defragmentation, fragmentation, optimizer.

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. See also optimization (definition 1). Compare fragmentation.

degausser n. A device used to remove magnetization from a video monitor or tape recorder head and to erase information from magnetic storage media, such as tapes and disks.

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.

deinstall vb. See uninstall.

deinterlace n. To combine two interlaced fields into a single frame that is not interlaced. Deinterlacing is done to remove artifacts and improve the quality of encoded video.

dejagging n. Smoothing of the jagged, stairstep appearance of diagonal lines and curves in graphical images. Also called: anti-aliasing. Compare aliasing.

de jure standard n. A standard for hardware or software development that has been issued or approved through a formal process by a standards organization. See also standard. Compare de facto standard.

DEK n. See data encryption key.

deka- prefix See deca-.

delay distortion n. See envelope delay.

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. See the illustration. Also called: Del key. 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.

Delete key.

deletia n. Omitted material. The term is used in responses to Usenet or mailing list messages to indicate that some unnecessary material has been excluded from the incorporated message being answered.

delimit vb. To set the limits of some entity, generally by using a special symbol called a delimiter. Programming languages typically delimit such variable-length elements as comments, strings, and program blocks. See also delimiter.

delimiter n. A special character that sets off, or separates, individual items in a program or set of data. Special characters often used include commas, semi-colons, tabs, and paragraph marks. See also delimit, field (definition 1), record¹.

Del key n. See Delete key.

delta channel n. See ISDN.

demand-driven processing n. The processing of data immediately as it becomes available or ready. Such real-time processing avoids the need to store data that has not been processed. Compare data-driven processing.

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.

demand priority n. A network access method in which hubs control network access; a feature of 100Base-VG Ethernet networks. With demand priority, nodes send requests to hubs and the hubs give permission to transmit based on priority levels assigned to the requests by the nodes. See also 100Base-VG.

demand publishing n. Producing print copies of publications on an as-needed basis rather than in a single long press run. Demand publishing is a by-product of desktop publishing and advancements in printer capabilities.

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. Data is converted to the digital form needed by a computer to which the modem is attached, with as little distortion as possible. Compare modulation (definition 1).

demon dialer n. See war dialer.

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.

denial of service attack n. See DoS.

denizen n. A participant in a Usenet newsgroup.

dense wavelength division multiplexing n. A data transmission technique in which multiple optical signals, each assigned to a separate color (wavelength frequency), are multiplexed onto a single strand of optical fiber. Because each signal travels separately in its own color band on the fiber, dense wavelength division multiplexing allows for the simultaneous transmission of different types of signals, such as SONET and ATM, each traveling at its own rate of speed. Dense wavelength division multiplexing can greatly increase the carrying capacity of a single optical fiber. Depending on the number, type, and rate of the signals involved, bandwidth can range from more than 40 Gbps to projected highs of 200 Gbps or more. Acronym: DWDM. Also called: wave divisision multiplexing, WDM. Compare time division multiple access.

dependence n. The state in which one entity relies upon specific hardware, software, or specific events for its own definition or functionality. See also context-dependent, dependent variable, device dependence, hardware-dependent, software-dependent.

dependent variable n. A variable in a program whose value relies on the outcome of another operation.

deployment descriptor n. In the Java J2EE network platform, a deployment descriptor is an XML file provided for each module or application describing how it should be deployed. The deployment descriptor directs a deployment tool to deploy a module or application with specific container options. It also describes the specific configuration requirements that an administrator must resolve when installing modules and J2EE applications into an operational environment. See also container, J2EE, module, XML.

depth queuing vb. 1. In computer graphics and modeling, giving a two-dimensional object a three-dimensional appearance through such techniques as shading and hidden-line removal. 2. Drawing objects from background to foreground to ease in the task of hidden-line removal.

deque n. Short for double-ended que. A form of the queue data structure that can have elements added to or removed from either end of the list. See also queue.

dequeue vb. To remove from a queue. See also queue.

dereference vb. In programming, to access information at the address contained by a pointer. The syntax for dereferencing varies among computer languages. See also double-dereference, handle (definition 1), pointer.

derived class n. In object-oriented programming, a class created from another class, called the base class. A derived class inherits all the features of its base class. It can then add data elements and routines, redefine routines from the base class, and restrict access to base-class features. See also base class, class, inheritance (definition 1), object-oriented programming.

derived font n. A font that has been scaled or modified from a previously existing font. For example, the Macintosh operating system can generate characters in font sizes other than the installed range of sizes. See also font. Compare intrinsic font.

derived relation n. A relation produced as the result of one or more relational-algebra operations on other relations. See also relational algebra, view(definition 2).

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).

descendant n. 1. In object-oriented programming, a class (group) that is a more specialized form of another, higher-level class. See also class, object-oriented programming. 2. In computing, a process (roughly, a program or task) that is called by another process and inherits certain of the originator s properties, such as open files. See also child (definition 1), inheritance (definition 2). Compare client (definition 2).

descendent key n. All the subkeys that appear when a key in the registry is expanded. A descendent key is the same as a subkey. Also called: descendant key. See also key, subkey.

descender n. The portion of a lowercase letter that falls below the baseline. See the illustration. See also baseline, x-height. Compare ascender.

Descender.

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.

descriptor n. 1. In information retrieval, a word, similar to an index entry in a book, that identifies a significant topic or element in a stored document or group of documents. It is used as a key in rapid search and retrieval of information. See also keyword (definition 1). 2. In programming, a piece of stored information used to describe something else, often in terms of structure, content, or some other property. Compare identifier.

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

deserialize vb. To change from serial (by bit) to parallel (by byte); to convert a single (serial) stream of bits to parallel streams representing the same information. Compare serialize.

Design by Contract n. An approach to building reusable systems where a software system is viewed as a set of communicating components whose interaction is based on precisely defined specifications of the mutual obligations, also known as contracts.

design cycle n. All the phases involved in developing and producing new hardware or software, including product specification, creation of prototypes, testing, debugging, and documentation.

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. Acronym: DA. Also called: desktop accessory. See also control panel.

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 accessory n. See desk accessory.

Desktop Color Separation n. See DCS.

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 enhancer n. Software that adds functionality to a windows-based operating system such as Microsoft Windows or Mac OS for example, an enhanced file browser, clipboard, or multimedia player.

desktop environment n. The appearance and user interface of a computer operating system (OS). An OS may offer the user opportunities to customize the desktop environment, or sometimes a choice of alternate desktop environments, with the OS underneath remaining the same.

Desktop file n. A hidden file maintained on a particular volume (roughly equivalent to a disk) by the Macintosh operating system for storing information about the files on it, such as version data, lists of icons, and file references.

Desktop Management Interface n. See DMI.

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.

destructive read n. An attribute of certain memory systems, notably core systems. In a destructive read of a memory location, the data is passed on to the processor, but the copy in memory is destroyed by the process of reading. Destructive memory systems require special logic to rewrite data back to a memory location after it is read. Also called: destructive readout. See also core. Compare nondestructive readout.

detail file n. See transaction file.

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

determinant n. In database design theory, any attribute or combination of attributes on which any other attribute or combination of attributes is functionally dependent.

determinism n. In computing, the ability to predict an outcome or to know in advance how data will be manipulated by a processing system. A deterministic simulation, for example, is one in which a certain input always produces the same output.

developer n. 1. One who designs and develops software. 2. See programmer.

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 control character n. See control character.

device controller n. See input/output controller.

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 Driver Developer Kit n. See DDK.

Device Driver Kit n. See DDK.

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-independent bitmap n. See DIB.

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.

device partnership n. A registry key, stored on the Windows CE device, that a desktop computer uses to identify that Windows CE device when it is connected to the desktop. The key defines values for synchronization, file conversions, and backup and restore information, which enable multiple Windows CE devices to connect to the same desktop computer. A device partnership is created the first time you connect a Windows CE device to a desktop computer.

device resolution n. See resolution (definition 1).

DFP n. See digital flat panel port.

DFS n. See distributed file system.

DGIS n. Acronym for Direct Graphics Interface Specification. An interface developed by Graphics Software Systems. DGIS is firmware (generally implemented in ROM on a video adapter) that allows a program to display graphics on a video display through an extension to the IBM BIOS Interrupt 10H interface.

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. See dynamic HTML.

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.

diacritical mark n. An accent mark above, below, or through a written character for example, the acute ( ) and grave (`) accents.

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 boot loader n. A tool for upgrading a version of an operating system on a target device. Acronym: DUB.

dial-up networking n. Connection to a remote network through use of a modem. Dial-up networking is typically used in reference to telecommuting, although the term is equally applicable to connecting to the Internet.

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 bitmap. 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).

dibit n. A set of 2 bits representing one of four possible combinations: 00, 01, 10, and 11. In communications, a dibit is a kind of transmission unit made possible by the modulation technique known as differential phase-shift keying, which encodes data by using four different states (phase shifts) in the transmission line to represent each of the four dibit combinations. See also phase-shift keying.

dichotomizing search n. See binary search.

dictation software n. Computer programs that can recognize spoken words as input. Used as an alternative to keyboard input, dictation software cannot comprehend the spoken language; it can only convert and transmit the sounds to the computer. Speaker-dependent dictation software requires the user to train the computer to become familiar with his or her voice patterns and accent. First-generation discrete speech systems require the user to speak slowly and distinctly, with pauses between words. Next-generation continuous speech systems can interpret natural speech patterns and speeds. See also voice recognition.

dictionary attack n. Originally a method of guessing a user s password or PIN by trying every word in the dictionary until successful. Currently used to identify any attack that tries known words or alphanumeric character strings to break a simple password.

dielectric n. Insulating material, such as rubber or plastic, that does not conduct electricity.

DIF n. See data interchange format.

difference n. 1. The amount by which two values differ. In electronics, differences in physical elements, such as waveforms or voltages, are used in the operation of circuits, amplifiers, multiplexers, communications equipment, and so on. 2. In database management, it is an operator in relational algebra that is used in sorting record sets (tuples). For example, given two relational tables, A and B, that are union-compatible (contain the same number of fields, with corresponding fields containing the same types of values), the statement DIFFERENCE A, B builds a third relation containing all those records that appear in A but not in B. See also relational algebra, tuple. Compare intersect, union.

Difference Engine n. An early computerlike mechanical device designed by British mathematician and scientist Charles Babbage in the early 1820s. The Difference Engine was intended to be a machine with a 20-decimal capacity capable of solving mathematical problems. The concept of the Difference Engine was enhanced by Babbage in the 1830s in the design of his more famous Analytical Engine, a mechanical precursor of the electronic computer. See also Analytical Engine.

differential adj. In electronics, a reference to a type of circuit that makes use of the difference between two signals rather than the difference between one signal and some reference voltage.

differential backup n. A backup that copies files created or changed since the last normal or incremental backup. It does not mark files as having been backed up (in other words, the archive attribute is not cleared). If you are performing a combination of normal and differential backups, restoring files and folders requires that you have the last normal, as well as the last differential, backup.

differential phase-shift keying n. See phase-shift keying.

differentiator n. A circuit whose output is the differential (first derivative) of the input signal. The differential measures how fast a value is changing, so the output of a differentiator is proportional to the instantaneous rate of change of the input signal. See the illustration. Compare integrator.

Differentiator.

Diffie-Hellman n. Diffie-Hellman key agreement protocol. A public-key cryptography method that allows two hosts to create and share a secret key. Diffie-Hellman is used for key management by virtual private networks (VPNs) operating on the IPSec standard. See also IPSec.

digerati n. Cyberspace populace that can be roughly compared to literati. Digerati are people renowned as or claiming to be knowledgeable about topics and issues related to the digital revolution; more specifically, they are people in the know about the Internet and online activities. See also guru, techie, wizard (definition 1).

digest n. 1. An article in a moderated newsgroup that summarizes multiple posts submitted to the moderator. See also moderator, newsgroup. 2. A message in a mailing list that is sent to subscribers in place of the multiple individual posts that the digest contains. If the mailing list is moderated, the digest may be edited. See also moderated.

digicash n. See e-money.

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. A reference to something based on digits (numbers) or their representation. 2. In computing, analogous in use, though not in meaning, to binary because the computers familiar to most people process information coded as different combinations of the binary digits (bits) 0 and 1. Compare analog.

Digital Advanced Mobile Phone Service n. See D-AMPS.

digital audio disc n. An optical storage medium for recording digitally encoded audio information. See also compact disc (definition 1).

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 broadcast satellite n. See direct broadcast satellite.

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 photography.

digital cash n. See e-money.

digital certificate n. 1. An assurance that software downloaded from the Internet comes from a reputable source. A digital certificate provides information about the software such as the identity of the author and the date on which the software was registered with a certificate authority (CA), as well as a measure of tamper-resistance. 2. A user identity card or driver s license for cyberspace. Issued by a certificate authority (CA), a digital certificate is an electronic credential that authenticates a user on the Internet and intranets. Digital certificates ensure the legitimate online transfer of confidential information, money, or other sensitive materials by means of public encryption technology. A digital certificate holder has two keys (strings of numbers): a private key held only by the user, for signing outgoing messages and decrypting incoming messages; and a public key, for use by anyone, for encrypting data to send to a specific user. See also certificate authority, encryption, private key, public key.

digital communications n. Exchange of communications in which all information is transmitted in binary-encoded (digital) form.

digital computer n. A computer in which operations are based on two or more discrete states. Binary digital computers are based on two states, logical on and off, represented by two voltage levels, arrangements of which are used to represent all types of information numbers, letters, graphics symbols, and program instructions. Within such a computer, the states of various circuit components change continuously to move, operate on, and save this information. Compare analog computer.

Digital Darkroom n. A Macintosh program developed by Silicon Beach Software for enhancement of black-and-white photographs or scanned images.

digital data service n. See DDS.

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 divide n. The gap between those who have the opportunity to take advantage of the Internet and related information resources, and those who do not. Differences in income, education, and comfort levels with technology are contributing factors to the separation between those with access to technological resources and those without.

digital DNA n. 1. Broadly, a reference to the bits that comprise digital information. 2. In the gaming world, a technology called Cyberlife that mimics biological DNA in the creation and development of trainable creatures known as Norns. Like real DNA, digital DNA is passed from parent to offspring and determines the artificial creature s characteristics and adaptability.

digital fingerprinting n. See digital watermark.

digital flat panel port n. An interface designed to allow direct connection between a flat panel monitor and a computer without requiring an analog-to-digital conversion. Acronym: DFP.

digital home n. See smart home.

digital light processing projector n. See DLP.

digital line n. A communications line that carries information only in binary-encoded (digital) form. To minimize distortion and noise interference, a digital line uses repeaters to regenerate the signal periodically during transmission. See also repeater. Compare analog line.

digital linear tape n. A magnetic storage medium used to back up data. Digital linear tape allows for faster transfer of data compared with other tape technologies. Acronym: DLT.

Digital Micromirror Device n. The circuit technology behind Texas Instruments Digital Light Processing, used in image projectors. A Digital Micromirror Device, or DMD, consists of an array of individually addressable, hinged mirrors on a chip. Each chip, which is less than 0.002 mm wide, rotates in response to a digital signal to reflect light into the lens of the projection system and thus create a bright, full-color display. Displays can be combined to create high-definition systems of 1920 1035 (1,987,200) pixels with 64 million colors. Acronym: DMD.

digital modem n. 1. A communications device that acts as the intermediary between a digital device such as a computer or terminal and a digital communications channel, such as a high-speed network line, an ISDN circuit, or a cable TV system. Although a digital modem supports standard (analog) modem protocols, it is not a typical modem in the sense of being a device whose primary function is to modulate (convert digital to analog) before transmission and demodulate (convert analog to digital) after transmission. It uses advanced digital modulation techniques for changing data frames into a format suitable for transmission over a digital line. See also terminal adapter. Compare modem. 2. A 56 Kbps modem. Such a modem is not purely digital but does eliminate the traditional digital-to-analog conversion for downstream transmissions that is, transmissions moving from the Internet to the end user. A 56 Kbps modem is also digital in that it requires a digital connection, such as T1, between the telephone company and the user s Internet Service Provider (ISP) in order to achieve its highest speed. See also 56-Kbps modem. 3. A term used to distinguish all-digital communications devices, such as ISDN and cable modems from the more traditional analog-to-digital, phone-based modems.

Digital Network Architecture n. A multilayered architecture and set of protocol specifications for networks. Designed by the Digital Equipment Corporation, Digital Network Architecture is implemented in the set of products known by the name DECnet. Acronym: DNA. See also DECnet.

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 picture frame n. Electronic device used in displaying digital photos and graphics while giving the outward appearance of a traditional picture frame. Digital picture frames allow users to rotate photos within the frame at specified intervals, display a series of photos as a slide show, or use an Internet connection to download photos, order prints, or send customized photo sets to others.

Digital Print Order Format n. See DPOF.

digital proof n. See direct digital color proof.

digital recording n. The storage of information in binary-encoded (digital) format. Digital recording converts information text, graphics, sound, or pictures to strings of 1s and 0s that can be physically represented on a storage medium. Digital recording media include computer disks and tapes, optical (or compact) discs, and ROM cartridges of the type used for some software and many computer games.

Digital Rights Management n. See DRM.

digital satellite system n. A high-powered satellite system with the capability to deliver high-quality transmissions of hundreds of channels directly to television receivers. A DSS broadcast begins as a digital signal sent from a service provider s station to a satellite. From there, it is directed to a satellite dish (typically 18 inches) at the user s premises. The dish next transmits the signal to a converter box, which changes it to an analog signal before sending it to the television set. Acronym: DSS.

Digital Services n. See DS.

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 n. See DS.

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 security mechanism used on the Internet that relies on two keys, one public and one private, that are used to encrypt messages before transmission and to decrypt them on receipt.

Digital Signature Algorithm n. The U.S. government standard for digital signatures, as specified by the National Institute of Standards and Technology, in FIPS 186, Digital Signature Standard. DSA is based on signature encryption based on a public and a private key. Acronym: DSA. See also digital signature.

Digital Signature Standard n. A public key cryptographic standard issued in 1994 by the United States National Institute of Standards and Technology (NIST) to authenticate electronic documents. The DSS uses a Digital Signature Algorithm (DSA) to generate and verify digital signatures based on a public key, which is not secret, and a private key, which is known or held only by the person generating the signature. A digital signature serves to authenticate both the identity of the signer and the integrity of the transmitted information. Acronym: DSS. See also public key encryption.

Digital Simultaneous Voice and Data n. A modem technology 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 sort n. A type of ordering process in which record numbers or their key values are sorted digit by digit, beginning with the least significant (rightmost) digit. Also called: radix sort.

digital speech n. See speech synthesis.

digital subscriber line or Digital Subscriber Line n. See DSL.

Digital Subscriber Line Access Multiplexer or Digital Subscriber Line Multiplexer n. See DSLAM.

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. See the illustration. Acronym: DAC. Compare analog-to-digital converter.

Digital-to-analog converter.

digital versatile disc n. See digital video disc.

digital video n. Video images and sound stored in a digital format. Acronym: DV.

digital TV or digital television n. The transmission of television signals using digital rather than the conventional analog signals. A digital TV standard for the United States was approved by the FCC in 1996. Digital TV provides a better television experience and new information services. Digital signals produce higher quality pictures and CD-quality sound, compared to the analog signals used with today"s television. Digital TV can support interactive television, electronic program guides, and a variety of digital services, such as Internet channel broadcasting and data services. Acronym: DTV. Compare HDTV.

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. Acronym: DVD. Also called: digital versatile disc. See also digital video disc ROM.

digital video disc erasable n. A proposed extension to the digital video disc recording format to allow multiple re-recording by a consumer. Acronym: DVD-E. Also called: digital video disc ROM.

digital video disc recordable n. A proposed extension to the digital video disc recording format to allow one-time recording by a consumer. Acronym: DVD-R.

digital video disc ROM n. A computer-readable version of a digital video disc containing either 4.7 or 8.5 GB of storage per side, the larger if 3M s dual-layer 2P technology is used. Acronym: DVD-ROM. Also called: digital video disc erasable. See also digital video disc.

Digital Video Interactive n. A hardware/software system developed by RCA, General Electric, and Intel that implements compression of digital video and audio for microcomputer applications. Acronym: DV-I.

Digital Video Interface n. See DVI.

digital video recording n. See DVR.

digital watermark n. A unique identifier embedded in a file to deter piracy and prove file ownership and quality. Digital watermarking is often used with graphics and audio files to identify the owner s rights to these works. See also fingerprint (definition 2).

digiterati n. See digerati.

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.

digitizing tablet n. See graphics tablet.

DikuMUD n. 1. Multiuser 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.

dimensioning n. In CAD programs, a means of specifying and possibly controlling the measurements and spatial relationships of elements in a modeled object, such as using lines, arrows, and text (that is, measurements) to indicate the length, height, and thickness of each of the walls in a modeled room or house. See also CAD.

DIMM n. Acronym for dual inline memory module. A type of memory board comprised of RAM chips mounted on a circuit board, similar to the more commonly used SIMM (Single Inline Memory Module). DIMMs are characterized by a 64-bit data path and pins (connectors) on each side that are on different circuits and that respond to different signals. SIMMs, in contrast, have a 32-bit data path, and their connectors are on the same circuit and respond to the same signal. While SIMMs must be added in pairs, DIMMs can be added to a computer one at a time. See also memory chip. Compare SIMM.

dimmed adj. Shown on the screen in gray characters instead of black characters on white or white characters on black. Menu options appear 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 the illustration. See also semiconductor.

Diode. The drawings (top) show two of the many types of diode packages. The band on the right end of each indicates polarity. At bottom is a schematic representation of a diode.

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 inline 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. See document image processing. Compare leadless chip carrier, pin grid array, SIP, surface-mount technology.

dipole n. A pair of opposite electric charges or magnetic poles of opposite sign separated by a small distance.

DIP switch n. Short for Dual Inline Package switch. One or more small rocker- or sliding-type toggle switches contained in the plastic or ceramic housing of a dual inline 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).

Direct3D n. See DirectX.

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 access storage device n. See DASD.

direct address n. See absolute address.

DirectAnimation n. See DirectX.

direct broadcast satellite n. A digital telecommunications service that delivers television programming via the Digital Satellite System (DSS). Direct broadcast satellite technology uses a geostationary orbit satellite (GEO) to receive digitized signals sent by ground-based uplink centers; the satellite then beams the signal across a wide swath on Earth. Subscribers within that swath use small (18-inch) satellite dishes to bring the signal into a set-top box decoder for playback. Although primarily used for television broadcasts, the technology is seen as having potential to also deliver high-quality, digital communications and multimedia content in the future. Acronym: DBS. Also called: digital broadcast satellite. See also digital satellite system, geostationary orbit satellite, webcasting.

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 that plugs directly into a telephone jack, eliminating the need for an intermediary telephone. Compare acoustic coupler.

direct-coupled transistor logic n. A circuit design that uses transistors and resistors only, with the transistors directly connected to each other. This design was used in the earliest commercial integrated circuits. The switching speed and power consumption of such circuits are about average. Acronym: DCTL.

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. Acronym: DC. Compare alternating current.

direct digital color proof n. A test sheet produced by a lower-cost output device, such as a color laser printer, to serve as an approximation of what the final image will look like when produced on professional-quality printing equipment. A direct digital color proof does not involve color separation, as in traditional proofs. Instead, a direct digital color proof is printed in all colors at one time on a single page, resulting in somewhat lower quality compared with traditional separation methods but having the advantages of increased speed and reduced cost. Acronym: DDCP. Also called: digital proof. See also color separation (definition 1).

DirectDraw n. See DirectX.

Direct Graphics Interface Specification n. See DGIS.

DirectInput n. An API (application programming interface) developed by Microsoft for joysticks and similar pointing devices in Windows 9x. See DirectX.

direction key n. See arrow key.

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. Compare PIO.

DirectMusic n. See DirectX.

directory n. 1. 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 uppermost 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 9x operating systems, directories are called folders. 2. On a network, an index of names and pertinent information related to authorized users and network resources.

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

Directory Client Agent n. See DUA.

Directory Information Base n. See DIB (definition 2).

Directory Mozilla n. See Open Directory Project.

directory path n. See pathname.

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 Server Agent n. See DSA.

directory service n. A service on a network that returns mail addresses of other users or enables a user to locate hosts and services.

Directory System Agent n. See DSA.

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.

Directory User Agent n. See DUA.

DirectPlay n. See DirectX.

direct processing n. Processing of data as it is received by the system, as opposed to deferred processing, in which data is stored in blocks before processing. Compare deferred processing.

direct read after write n. See DRAW.

direct read during write n. See DRDW.

direct sequence n. In spread spectrum communication, a form of modulation in which a carrier is modulated by a series of binary pulses. See also modulation (definition 1), spread spectrum.

DirectShow n. See DirectX.

DirectSound n. See DirectX.

direct view storage tube n. A type of cathode-ray tube (CRT) in which the screen can retain images for a long time and in which a beam of electrons from an electron gun can be moved arbitrarily across the screen surface (as opposed to a standard cathode-ray tube, in which the electron beam is moved in a specific pattern). This type of CRT is capable of displaying a precise, detailed image without requiring any screen refresh. However, once the image is drawn, it cannot be changed without a complete erasing of the screen. Acronym: DVST. Also called: storage tube. Compare CRT.

DirectX n. A set of Microsoft technologies that provide developers with the tools needed to create sophisticated multimedia applications on Windows-based computers. DirectX consists of components making up two integrated layers. The Foundation layer provides low-level functions, such as support for input devices, designed to ensure that applications can run on and take full advantage of Windows-based hardware. The Media layer, above the Foundation layer, provides high-level services, such as support for media streaming and animation, that are needed in creating applications incorporating such features as surround sound, video, and 3-D animation. DirectAnimation, DirectSound, and other similarly named application programming interfaces (APIs) are members of the DirectX family. See the table. See also application programming interface.

Table D.1 ATA Specifications.

DirectX Component	Part Of	Supports
Direct3D Immediate Mode	Foundation layer	Access to 3-D video hardware
Direct3D Retained Mode	Media layer	Creation and animation of onscreen 3-D worlds
Direct Animation	Media layer	Interactive animation and integration of different multimedia types
DirectDraw	Foundation layer	Access to display memory and hardware capabilities
DirectInput	Foundation layer	Direct access to various input devices, including force-feedback joysticks
DirectMusic	Foundation layer	Real-time music composition
DirectPlay	Foundation layer	Multiplayer online gaming and other networked applications
DirectShow	Media layer	Capture and playback of streaming multimedia
DirectSound	Foundation layer	Direct access to sound cards; wave sound capture and playback
Direct Sound3D	Foundation layer	3-D sound positioning
DirectX Transform	Media layer	Extensibility of the DirectX platform to include value-added products

DirectX Transform n. See DirectX.

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.

dirty power n. A power source that can cause damage to electronic components, due to noise, voltage spikes, or incorrect voltage levels.

dirty ROM n. Short for dirty read-only memory. In the earlier versions of the Macintosh (Mac II, IIx, SE/30, and IIcx), a memory system that simulates a 32-bit system but is not a true (clean) 32-bit system. Among other flaws, a dirty ROM machine can access only 8 megabytes of memory under Mac OS System 7. System extensions such as MODE32 and the 32-bit enabler are available to allow a dirty ROM machine to function like a true, 32-bit clean machine.

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.

disabled folders n. In the Mac OS, several folders in the System folder that contain system extensions, control panels, and other items that have been removed from the system by the extension manager. Items currently in disabled folders will not be installed upon system startup; they may, however, later be moved back to their regular folders automatically by the extension manager. See also extension manager, System folder.

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

disaster dump n. A dump (transfer of memory contents to a printer or other output device) made when a program fails without hope of recovery.

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.

discrete adj. Separate; individual; identifiable as a unit. For example, bits are discrete elements of data processed by a computer.

discrete multitone n. In telecommunications, a technology that uses digital signal processors to split available bandwidth into a number of subchannels, allowing over 6 Mbps of data to be carried over one copper twisted-pair wire. Acronym: DMT.

discrete speech recognition n. Computer speech recognition format in which each word is recognized as a distinct individual unit, requiring a pause between each word spoken.

discretionary access control list n. The part of an object s security descriptor that grants or denies specific users and groups permission to access the object. Only the owner of an object can change permissions granted or denied in a DACL; thus, access to the object is at the owner s discretion. Acronym: DACL. See also distribution group.

discretionary hyphen n. See hyphen.

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.

dish n. See satellite dish.

disk n. 1. A round, flat piece of flexible plastic coated with a magnetic material that can be electrically influenced to hold information recorded in digital (binary) form and encased in a protective plastic jacket to protect the disk from damage and contamination. Also called: floppy, floppy disk, microfloppy disk. Compare compact disc, disc. 2. See hard drive.

disk access time n. See access time (definition 2).

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 cartridge n. A removable disk enclosed in a protective case. A disk cartridge can be used by certain types of hard disk drives and related devices, such as the external data storage units known as Bernoulli boxes.

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 crash.

disk directory n. An index of the files on a disk, analogous to a card catalog. A disk directory includes information about the files, such as their names, sizes, dates of creation, and physical locations on the disk. See also directory.

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 heads 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 duplexing n. See disk mirroring.

disk envelope n. The paper container that holds a 5.25-inch floppy disk and its attached jacket. The disk envelope protects exposed surfaces of the disk from dust and other foreign material that can scratch and otherwise damage the surface, resulting in the loss of recorded data. See also disk jacket.

diskette n. See floppy disk.

disk farm n. A number of disk drives in a single location used together to store or process vast quantities of information, such as scientific data, years worth of corporate sales figures, large numbers of graphic images, or telephone company billing records. Current disk farms consist of magnetic or optical disks and can hold terabytes of information. In older usage, disk farms were sometimes known as Laundromats because they contained large drives referred to in jargon as washing machines. See also server farm.

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

disk jacket n. The protective plastic sheath that covers a floppy disk.

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 memory n. See virtual memory.

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 operating system n. See DOS.

disk pack n. A collection of disks in a protective container. Used primarily with minicomputers and mainframe computers, a disk pack is a removable medium, generally a stack of 14-inch disks in a plastic housing.

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 server n. A node on a local area network that acts as a remote disk drive shared by network users. Unlike a file server, which performs the more sophisticated tasks of managing network requests for files, a disk server functions as a storage medium on which users can read and write files. A disk server can be divided into sections (volumes), each of which appears to be a separate disk. Compare file server.

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 re-created using the information stored across the remaining partitions in the disk stripe. See also disk striping, fault tolerance, parity.

disk unit n. A disk drive or its housing.

dispatcher n. In some multitasking operating systems, the set of routines responsible for allocating CPU (central processing unit) time to various applications.

dispatch table n. A table of identifiers and addresses for a certain class of routines such as interrupt handlers (routines carried out in response to certain signals or conditions). Also called: interrupt vector table, jump table, vector table. See also interrupt handler.

disperse vb. To break up and place in more than one location for example, to disperse results among several sets of data or to disperse items (such as fields in records) so that they appear in more than one place in the output. Compare distribute.

dispersion n. The degree to which, at any given time, data in a distributed (interconnected) system of computers is stored at different locations or on different devices.

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 adapter n. See video adapter.

display attribute n. A quality assigned to a character or an image displayed on the screen. Display attributes include such features as color, intensity, and blinking. Users of applications can control display attributes when programs allow them to change color and other screen elements.

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 board n. See video adapter.

display card n. See video adapter.

display cycle n. The complete set of events that must occur in order for a computer image to be displayed on the screen, including both the software creation of an image in a computer s video memory and the hardware operations required for accurate on-screen display. See also refresh cycle.

Display Data Channel n. See DDC.

display device n. See display.

display element n. See graphics primitive.

display entity n. See entity, graphics primitive.

display face n. A typeface suitable for headings and titles in documents, distinguished by its ability to stand out from other text on the page. Sans serif faces such as Helvetica and Avant Garde often work well as display faces. See also sans serif. Compare body face.

display frame n. One image in an animation sequence. See also frame (definition 3).

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

display page n. One screenful of display information stored in a computer s video memory. Computers can have enough video memory to hold more than one display page at a time. In such instances, programmers, especially those concerned with creating animation sequences, can update the screen rapidly by creating or modifying one display page while another is being viewed by the user. See also animation.

display port n. An output port on a computer that provides a signal for a display device such as a video monitor. See the illustration. Also called: monitor port.

Display port.

Display PostScript n. An extended version of the PostScript language intended to provide a device-independent language for displaying images and text on bitmapped displays. See also PostScript.

Display Power Management Signaling n. See DPMS.

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

display terminal n. See terminal (definition 1).

distance learning n. Broadly, any educational or learning process or system in which the teacher/instructor is separated geographically or in time from his or her students, or in which students are separated from other students or educational resources. Contemporary distance learning is effected through the implementation of computer and electronics technology to connect teacher and student in either real or delayed time or on an as-needed basis. Content delivery may be achieved through a variety of technologies, including satellites, computers, cable television, interactive video, electronic transmissions via telephone lines, the World Wide Web and other Internet technology, and others. Distance learning does not preclude traditional learning processes; frequently it is used in conjunction with in-person classroom or professional training procedures and practices.

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.

distance-vector routing algorithm n. See Bellman-Ford distance-vector routing algorithm.

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.

distribute vb. To allocate among locations or facilities, as in a data-processing function that is performed by a collection of computers and other devices linked together by a network.

distributed bulletin board n. A collection of newsgroups distributed to all computers in a wide area network. See also newsgroup, Usenet.

Distributed COM n. See DCOM.

Distributed Component Object Model n. See DCOM.

distributed computing n. See distributed processing.

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. Acronym: DCE. See also distributed processing.

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 database management system n. A database management system capable of managing a distributed database. Acronym: DDBMS. See also distributed database.

distributed denial of service attack n. See DDoS.

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. Acronym: DFS.

distributed intelligence n. A system in which processing ability (intelligence) is distributed among multiple computers and other devices, each of which can work independently to some degree but can also communicate with the other devices to function as part of the larger system. See also distributed processing.

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 services n. See BISDN.

distributed system n. A noncentralized network consisting of numerous computers that can communicate with one another and that appear to users as parts of a single, large, accessible storehouse of shared hardware, software, and data.

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. Acronym: DSOM. See also SOM (definition 1).

distributed transaction processing n. Transaction processing that is shared by one or more computers communicating over a network. Acronym: DTP. See also distributed processing, transaction processing.

distributed workplace n. An environment other than the traditional office or factory, in which work is carried out on a regular basis. The flexibility afforded by the combination of communications and computing technologies enables many workers to conduct business anywhere the appropriate computer and data communications infrastructure has been set up. See also SOHO, telecommute.

distribution group n. A group that is used solely for e-mail distribution and that is not security-enabled. Distribution groups cannot be listed in discretionary access control lists (DACLs) used to define permissions on resources and objects. Distribution groups can be used only with e-mail applications (such as Microsoft Exchange) to send e-mail messages to collections of users. If you do not need a group for security purposes, create a distribution group instead of a security group. See also discretionary access control list, security group.

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.

distribution services n. See BISDN.

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.

distro¹ n. 1. A distribution of software (usually a version of Linux), digital music, or an online magazine or e-zine. See also e-zine, Linux. 2. A company or individual that sells items, typically software, music CDs, or books, via the Web.

distro² vb. To distribute or sell software releases, digital music, or text items via the Web.

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 the illustration. See also aliasing, halftone.

Dithering. A halftone image (left) and a dithered image (right) both at 72 cells per inch.

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.

divide overflow n. See overflow error.

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.

DIX n. Acronym for Digital Intel Xerox, the companies that developed the AUI connector for thicknet Ethernet cable. See also AUI.

DJGPP n. A compiler and a set of tools used by some game programmers to produce 32-bit protected-mode programs that run on Windows operating systems. DJGPP is a complete 32-bit C/C++ development system for PCs running MS-DOS; it includes ports of many GNU development utilities. In most cases, programs produced using DJGPP can be sold commercially without license or royalties. See also 32-bit, Allegro, GNU.

djinn n. A group of devices, resources, and users joined by Sun Microsystem s JINI technology. The group, controlled by the JINI technology infrastructure, agrees on basic specifications for administration, trust, identification, and policy. See also JINI.

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. Supported by Microsoft Windows NT and Windows 2000, DLC is designed to provide access to IBM mainframe computers and to Hewlett-Packard printers connected to the network. See also HDLC, SNA.

DLCI n. See Data Link Connection Identifier.

.dll n. A file extension for a dynamic-link library. See also dynamic-link library.

DLL n. See dynamic-link library.

DLL hell n. A problem occurring in versions of Microsoft Windows prior to Windows Me and Windows 2000 in which a newly installed application overwrites shared dynamic-link library (DLL) files with the (older or newer) versions it needs in order to run. If the replaced files are incompatible with those needed by other applications, those applications may exhibit buggy behavior or crash when they access the incompatible DLL files. The latest versions of the Windows operating system, Windows 2000 and Windows XP, incorporate a feature called Windows File Protection that eliminates this situation by monitoring and correcting installation and replacement of DLL files. See also dynamic-link library.

DLP n. Short for Digital Light Processing. a digital projection technology developed by Texas Instruments in which a signal sent from a computer to a DLP projector is projected onto a screen by means of light reflected from a Digital Micromirror Device, or DMD, that consists of thousands of tiny hinged mirrors, each representing one pixel, attached to a chip. The chip acts as a bank of switches, one switch per mirror. These switches, in turn, rotate the mirrors in response to the digital signal to reflect light through a projection lens to create the image. DLP projectors represent a newer technology than the LCD projectors also used to display images on screen. See also Digital Micromirror Device.

DLS n. See Downloadable Sounds.

DLT n. See digital linear tape.

DMA n. See direct memory access, document management system.

DMD n. See Digital Micromirror Device.

DMI n. Acronym for Desktop Management Interface. A system for managing the configurations and status of PCs on a network from a central computer. In DMI an agent program runs in the background on each machine and returns information or performs some action (as specified by a file on that machine) in response to a query received from the central computer. The actions to be performed by the agent might include watching for errors and reporting them to the central computer as they occur; for example, a printer might be set up to report to the central computer when paper runs out or jams. DMI was developed by the DMTF (Desktop Management Task Force), a consortium of computer equipment manufacturers, and competes with SNMP (although the two can coexist on the same system). See also agent (definition 1), DMTF. Compare SNMP.

DML n. See data manipulation language, declarative markup language.

DMOZ n. See Open Directory Project.

DMQL n. Acronym for Data Mining Query Language. Any query language developed and used for data mining relational databases. DMQLs provide a syntax for specifying the kind of knowledge to be mined, pattern presentation and visualization, conceptual hierarchies, and task relevant data. See also data mining. Compare structured query language (SQL).

DMS n. See document management system.

DMT n. See discrete multitone.

DMTF n. Acronym for Desktop Management Task Force. A consortium formed in 1992 to develop standards for PC-based stand-alone and networked systems based on user and industry needs.

DNA n. See digital DNA, Digital Network Architecture, distributed network, Windows DNA.

DNS n. 1. Acronym for Domain Name System. The hierarchical 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. DNS names consist of a top-level domain (such as .com, .org, and .net), a second-level domain (the site name of a business, an organization, or an individual), and possibly one or more subdomains (servers within a second-level domain). See also domain name address, IP address. 2. Acronym for Domain Name Service. The Internet utility that implements the Domain Name System. DNS servers, also called name servers, maintain databases containing the addresses and are accessed transparently to the user. See also Domain Name System (definition 1), DNS server.

DNS name server n. See DNS server.

DNS server n. Short for Domain Name System server. a computer that can answer Domain Name System (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. Also called: name server. See also DNS (definition 2), IP address.

DNS zone transfer n. See zone transfer.

.doc n. A file extension that identifies document files formatted for a word processor. This is the default file extension for Microsoft Word document files.

dock vb. 1. To connect a laptop or notebook computer to a docking station. See also docking station, laptop, portable computer. 2. To move a toolbar to the edge of an application window so that it attaches to and becomes a feature of the application window.

Dock n. An organizational feature of Mac OS X that keeps track of frequently used applications, documents, and windows. Users can drag icons to the dock for easy access or can minimize an active window to the Dock and still see the application running while working with other windows. The Dock can run along the bottom or either side of the screen. See also Mac OS X.

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 the illustration. See also expansion slot, laptop, peripheral, portable computer.

Docking station.

DOCSIS n. Acronym for Data Over Cable Service Interface Specification. The International Telecommunications Union standard (ITU Recommendation J.112) that specifies functions and internal and external interfaces for high-speed, bidirectional transfer of digital data between cable television networks and subscribers. DOCSIS-compliant equipment ensures interoperability between cable modems and the cable television infrastructure, regardless of manufacturer or provider. Initially developed by a group of cable television providers, including Time Warner and TCI, DOCSIS was designed to support data, video, and rapid Internet access. Data rates are 27 Mbps to 36 Mbps downstream (from the cable network) and 320 Kbps to 10 Mbps upstream (to the cable network). See also cable modem. Compare IEEE 802.14.

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 Content Architecture n. See DCA (definition 1).

Document Content Description n. See DCD (definition 2).

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 Interchange Architecture n. See DIA.

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

document management system n. A server-based network facility designed for the storage and handling of an organization s documents. A document management system, or DMS, is built around a central library known as a repository and typically supports controlled access, version tracking, cataloging, search capabilities, and the ability to check documents in and out electronically. The open interface specification known as ODMA (Open Document Management API) enables desktop applications that support ODMA to interface with a DMS so that users can access and manage documents from within their client applications. Acronym: DMS. Also called: EDMS, electronic document management system.

Document Object Model n. A World Wide Web Consortium specification that describes the structure of dynamic HTML and XML documents in a way that allows them to be manipulated through a Web browser. In the Document Object Model, or DOM, a document is presented as a logical structure rather than as a collection of tagged words. In essence, DOM is a means of defining a document as a treelike hierarchy of nodes in which the document is an object containing other objects, such as images and forms. Through DOM, programs and scripts can access these objects in order to change aspects such as their appearance or behavior. DOM is a vehicle for adding depth and interactivity to what would otherwise be a static Web page. Acronym: DOM.

document processing n. The act of retrieving and manipulating a document. In terms of the way a computer works, document processing involves three main steps: creating or retrieving a data file, using a program to manipulate the data in some way, and storing the modified file.

document reader n. A device that scans printed text and uses character recognition to convert it to computer text files. See also character recognition.

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: source, source document. See also HTML.

Document Style Semantics and Specification Language n. An ISO standard derived from SGML that addresses the semantics of high-quality composition in a manner independent of particular formatting systems or processes. Like CSS and XSL, it can be used to format XML documents. Acronym: DSSSL. See also ISO, SGML.

document type definition n. See DTD.

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.

DoD n. See U.S. Department of Defense.

do-gooder virus n. A virus or worm that has been released with the intention of correcting problems caused by other, more malicious viruses. The do-gooder virus typically looks for computers that have been compromised and then infects the system and fixes back doors and other vulnerabilities left behind by the malicious program. The do-gooder virus may then use the repaired computer as a platform to infect other computers. See also anti-worm, automatic patching.

DO loop n. A control statement used in programs that executes a section of code a number of times until a specified condition is met. DO loops are found in FORTRAN and Basic, among other languages. See also iterative statement. Compare FOR loop.

DOM n. See Document Object Model.

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 controller n. In Windows NT, the master server that holds the directory services database that identifies all network users and resources.

domain name n. An address of a network connection 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.

Domain Name Server n. See DNS server.

Domain Name Service n. See DNS (definition 2).

Domain Name System n. See DNS (definition 1).

Domain Naming System n. See DNS (definition 1).

domain slamming n. The practice of transferring ownership of domain names from one customer to another without the permission of the first customer.

Domino n. See Lotus Domino.

dongle n. 1. See hardware key. 2. An adapter device or cable enabling a nonstandard interface between a computer and a peripheral device or between two disparate items of computer hardware.

do-nothing instruction n. See no-operation instruction.

doorway page n. A Web page that functions as a doorway into a Web site. Usually a doorway page contains keywords, which Internet search engines seek when they scan the Internet. Placing the correct keywords on a doorway page can increase the number of viewers visiting a site.

dopant n. An impurity that is added in small quantities to semiconductor material during the manufacture of diodes, transistors, and integrated circuits. The resistance of a semiconductor falls between the resistance of a conductor and the resistance of an insulator (hence its name); dopants are added to the semiconductor to increase its conductivity. The type and amount of dopant determine whether the semiconductor will be N-type (in which current is conducted by free electrons) or P-type (in which current is conducted by electron vacancies, called holes). Common dopants include arsenic, antimony, bismuth, and phosphorus. See also N-type semiconductor, P-type semiconductor.

DoS n. Acronym for denial of service attack. A computerized assault, usually planned, that seeks to disrupt Web access. A denial of service attack can occur in a number of forms. The most common form of attack is to overwhelm an Internet server with connection requests that cannot be completed. This causes the server to become so busy attempting to respond to the attack that it ignores legitimate requests for connections. One example of this type of attack, known as a SYN flood, inundates the server s entry ports with false connection messages. Another, known as the Ping of Death, sends a ping command with an oversized IP packet that causes the server to freeze, crash, or restart. Other forms of denial of service attacks include the destruction or alteration of a server s configuration data, such as router information; unauthorized access to physical components of a system; and the sending of large or invalid data that causes a system to crash or freeze. See also packet, Ping of Death, SYN flood.

DOS n. 1. 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. 2. See MS-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.

DOS Protected Mode Interface n. A software interface, originally developed for Microsoft Windows version 3, that enables MS-DOS-based application programs to run in the protected mode built into 80286 and later microprocessors. In protected mode, the microprocessor can support multitasking and use of memory beyond 1 MB capabilities otherwise unavailable to programs designed to run under MS-DOS. See also protected mode, real mode, Virtual Control Program Interface.

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-bomb n. An Internet-based company or organization that has failed or downsized significantly. See also dot-commed.

dot-com n. A company doing business primarily or entirely on the Internet. The term is derived from the top-level domain, .com, at the end of the Web addresses of commercial Web sites.

dot command n. A formatting command typed into a document and preceded by a period (dot) to distinguish it from printable text. Text formatting programs such as the XENIX nroff editor and word processing programs such as WordStar use dot commands for formatting.

dot-commed adj. Losing a job because of the downsizing or failure of an Internet-based company or organization. See also dot-bomb.

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.

dot pitch n. 1. In printers, the distance between dots in a dot-matrix. See also dot matrix. 2. In video displays or CRTs, a measure of image clarity. A video display s dot pitch is the vertical distance, expressed in millimeters, between like-colored pixels. A smaller dot pitch generally means a crisper image, although the difference between two displays can vary because some manufacturers use different methods to determine the dot pitch of their products. A display s dot pitch is an integral part of the product and so cannot be altered. See also CRT, display.

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.

dotted decimal notation n. The process of formatting an IP address as a 32-bit identifier made up of four groups of numbers, with each group separated by a period. For example, 123.432.154.12.

double buffering n. The use of two temporary storage areas (buffers) rather than one to hold information coming from and going to a particular input/output device. Because one buffer can be filled while the other is being emptied, double buffering increases transfer speed. Also called: ping-pong buffer.

double-byte characters n. A set of characters in which each character is represented by two bytes. Some languages, such as Japanese, Chinese, and Korean, require double-byte character sets.

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 dabble n. A method of converting binary numbers to decimals by a process of doubling sums and adding successive bits: doubling the bit farthest to the left, adding the next bit and doubling the sum, adding the next bit and doubling the sum, and so on until the rightmost bit has been included in the total.

Double Data Rate SDRAM n. See DDR SDRAM.

Double Data Rate Synchronous Dynamic RAM n. See DDR SDRAM.

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-dereference vb. To dereference a pointer that is pointed to by another pointer; in other words, to access the information pointed to by a handle. See also dereference, handle (definition 1), pointer (definition 1).

double leap year n. The mistaken idea that the year 2000 would have two leap days February 29 and February 30 instead of one. In actuality, there was a potential leap year problem in 2000, but it was based on three rules for calculating leap years: (1) A year is a leap year if it is divisible by 4, but (2) not if it is divisible by 100, unless (3) it is also divisible by 400. Thus, 1900 was not a leap year, but 2000 is, although systems based on incorrect algorithms may not recognize it as a leap year and so may have difficulties functioning correctly after February 28, 2000.

double posting n. In newsgroup discussions, the practice of replying to one s own posts. Because it may be seen as the digital equivalent to talking to one s self, double posting is considered an undesirable practice.

double-precision adj. Of, pertaining to, or characteristic of a number stored in twice the amount (two words typically 8 bytes) of computer memory that is required for storing a less precise (single-precision) number. Double-precision numbers are commonly handled by a computer in floating-point form. See also floating-point number. Compare single-precision.

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

double slash n. See //.

double-strike n. On an impact printer, such as a daisy-wheel printer, the process of printing twice over a word, producing text that appears darker and heavier, or bolder, than it normally appears. On dot-matrix printers, double striking with a slight offset can be used to fill in the space between the dots, producing smoother and darker characters.

double supertwist nematic display n. See supertwist display.

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.

doubly linked list n. A series of nodes (items representing discrete segments of information) in which each node refers to both the next node and the preceding node. Because of these two-way references, a doubly linked list can be traversed both forward and backward, rather than in a forward direction only, as with a singly linked list.

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

downflow n. One of the four stages of the data warehousing process, during which stored information is delivered and archived. See also data warehouse. Compare inflow, metaflow, upflow.

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.

Downloadable Sounds n. A standard for synthesizing wave sounds from digital samples stored in software. The DLS level 1 and level 2 standards are published by the MIDI Manufacturers Association. Acronym: DLS.

downsample n. To decrease the number of audio samples or pixels, by applying an operation such as averaging. Popular internet music formats, such as MP3, use downsampling to reduce file size.

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.

downstream¹ n. The direction in which information, such as a news feed for a newsgroup or data from an http (Web) server, is passed from one server to the next. See also news feed, newsgroup, server.

downstream² adv. 1. The location of a client computer in relation to a server. 2. The direction in which data moves from the server to the client.

downstream³ adj. Refers to data that moves from a remote network to an individual computer. In some Internet-related communications technologies, data flows more quickly downstream than upstream; cable modems, for example, can transfer data as fast as 30 Mbps downstream but support much slower rates, from 128 Kbps to around 2 Mbps, upstream. Compare upstream.

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.

DP n. See data processing.

dpi n. See dots per inch.

DPMA n. Acronym for Data Processing Management Association. A trade organization of information systems (IS) professionals. DPMA was founded in 1951 as the National Machine Accountants Association.

DPMI n. See DOS Protected Mode Interface.

DPMS n. Acronym for VESA Display Power Management Signaling. A VESA standard for signals that put a video monitor into standby or suspend mode to reduce power consumption. See also green PC, VESA.

DPOF n. Short for Digital Print Order Format. A printing specification developed by Canon Computer Systems, Inc., Eastman Kodak Company, Fuji Photo Film Co., Ltd., and Matsushita Electric Industrial Co., Ltd. DPOF is intended to ease the process of printing images stored on digital camera memory cards by enabling users to select the images to print, as well as specify the number of copies desired, on the card. The images ordered can then be printed by a professional photofinishing service or on a home printer.

DPSK n. Acronym for differential phase-shift keying. See phase-shift keying.

draft mode n. A high-speed, relatively low-quality print mode offered by most dot-matrix printers. See also dot-matrix printer, draft quality, print quality.

draft quality n. A low grade of printing generated by the draft mode on dot-matrix printers. Draft quality varies among printers, ranging from suitable for most purposes to nearly useless. See also draft mode, print quality.

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. 1. In general, to delve into something in increasing detail. 2. More specifically, to perform operations in a graphical user interface by dragging objects on the screen with the mouse. For example, to delete a document, a user can drag the document icon across the screen and drop it on the trashcan icon (Macintosh OS) or in the Recycle Bin (Windows). See also drag, graphical user interface.

drain n. 1. In an FET, the electrode toward which charge carriers (electrons or holes) move from the source under control of the gate. See also FET, gate (definition 2), MOSFET, source (definition 2). 2. See current drain.

DRAM n. See dynamic RAM.

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.

drawer n. In the Mac OS X Aqua interface, small child windows containing extra information that slide out of the side of main parent windows. Drawers are intended to reduce clutter on the computer desktop by enabling more information to be displayed without opening additional full-sized windows. Also called: Drop Drawer.

drawing interchange format n. See DXF.

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.

Dreamcast n. A console game system designed by the Sega corporation. It features a Hitachi 128-bit graphics engine with an on-board SH-4 RISC processor (operating frequency of 200 MHz 360 MIPS/1.4 GFLOPS) and a customized OS using Windows CE as its base (supporting DirectX). Game developers for the Dreamcast platform use an environment supported by Microsoft Visual Studio and refined Visual C++. See also computer game, console game, DirectX, gigaflops, MIPS, OS, RISC, Visual C++. Compare GameCube, PlayStation, Xbox.

dribbleware n. Updates, patches, and new drivers for a software product that are released one at a time, as they become available, rather than being issued together in a new version of the product. A company using the dribbleware technique might distribute new and replacement files on diskette or CD-ROM, or make them available for download through the Internet or a private network. See also driver, patch¹.

drift n. The movement of charge carriers in a semiconductor caused by an applied voltage. The term is also used to refer to any slow, unwanted change in a parameter; for example, the value of a resistor might change, or drift, slightly as the resistor warms or cools.

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 n. See disk drive.

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 software 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. Because the driver handles device-specific features, the operating system is freed from the burden of having to understand and support the needs of individual hardware devices. See also device driver.

Driver Development Kit n. See DDK.

DRM n. Acronym for Digital Rights Management. A group of technologies developed to protect intellectual property from online piracy by controlling who can view protected content and in what form. A DRM package may allow the purchaser to view protected content, but prevent printing or forwarding. Content may also be set to expire after a set amount of time or if distributed to multiple users. DRM technology is meant to protect multiple forms of digital and analog content, and includes encryption, digital watermarking, and content tracking software.

DRO n. Acronym for destructive read out. See destructive read.

drop cable n. A cable, also known as a transceiver cable, that is used to connect a network interface card (NIC) to a Thick Ethernet network.

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. See the illustration.

Drop cap.

drop-dead halt n. See dead halt.

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.

drop in vb. To read a spurious signal during a data read/write operation, producing erroneous data.

droplet n. 1. An extension for Quark XPress that allows files to be dragged onto a page from the finder. 2. A feature from Frontier that allows scripts to be embedded within an application and run when the application is double-clicked. 3. A general name for any AppleScript program that allows files to be dragged and dropped into it for processing. See also AppleScript.

drop out vb. To lose the signal momentarily during a data read/write operation, thus producing erroneous data.

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.

drum plotter n. A plotter in which paper is wrapped around a large revolving drum, with a pen that moves back and forth at the uppermost point on the drum. The paper is rolled with the drum to align the correct point on the paper with the pen. Drums take up a fraction of the space required by flatbed plotters that can handle the same paper size. They also effectively have no limit on the length of the paper they can handle, which can be an advantage in some applications. See also plotter. Compare flatbed plotter, pinch-roller plotter.

drum scanner n. A type of scanner where the medium being scanned, such as a sheet of paper, is rotated around a stationary scan head. See also scanner. Compare feed scanner, flatbed scanner, handheld scanner.

.drv n. The file extension for a driver file. See also driver.

dry run n. Running a program intended to have a dramatic effect, such as formatting a disk or printing a book, with the effect disabled, thus avoiding formatting a disk with data on it or wasting paper.

DS n. Acronym for Digital Services or Digital Signal, a category used in referencing the speed, number of channels, and transmission characteristics of T1, T2, T3, and T4 communications lines. The basic DS unit, or level, is known as DS-0, which corresponds to the 64 Kbps speed of a single T1 channel. Higher levels are made up of multiple DS-0 levels. DS-1 represents a single T1 line that transmits at 1.544 Mbps. For higher rates, T1 lines are multiplexed to create DS-2 (a T2 line consisting of four T1 channels that transmits at 6.312 Mbps), DS-3 (a T3 line consisting of 28 T1 channels that transmits at 44.736 Mbps), and DS-4 (a T4 line consisting of 168 T1 channels that transmits at 274.176 Mbps).

DSA n. 1. Acronym for Directory System Agent or Directory Server Agent. An X.500 server program that looks up the address of a user on the network when requested by a DUA (Directory User Agent). See also agent (definition 3), CCITT X series, DUA. 2. See Digital Signature Algorithm.

DSL n. Acronym for Digital Subscriber Line, a recently developed (late 1990s) digital communications technology that can provide high-speed transmissions over standard copper telephone wiring. DSL is often referred to as xDSL, where the x stands for one or two characters that define variations of the basic DSL technology. Currently, ADSL (Asymmetric DSL) is the form most likely to be provided, but even it is, as yet, available only to limited groups of subscribers. See also ADSL, DSL Lite, HDSL, RADSL, SDSL, VDSL.

DSLAM n. Acronym for Digital Subscriber Line Access Multiplexer. A device in a telephone company central office that splits DSL subscriber lines and connects them to Internet network hosts and to the public telephone network. The use of a DSLAM makes it possible to provide both voice and data service through a single pair of copper wires.

DSL Lite n. Short for Digital Subscriber Line Lite. A variation of ADSL currently under development that simplifies installation but transmits more slowly, at 1.544 Mbps. See also ADSL, DSL.

DSO n. Acronym for Dynamic Shared Object. An Apache HTTP server module that supports all UNIX-based platforms. DSO uses a dynamically linked shared library of resources that are loaded and executed only at run time when necessary. DSO is most commonly used with Linux and is included in most Linux distributions.

DSOM n. See Distributed System Object Model.

DSP n. See digital signal processor.

DSR n. Acronym for Data Set Ready. A signal used in serial communications sent, for example, by a modem to the computer to which it is attached, to indicate that it is ready to operate. DSR is a hardware signal sent over line 6 in RS-232-C connections. See also RS-232-C standard. Compare CTS.

DSS n. See decision support system, digital satellite system, Digital Signature Standard.

DSSSL n. See Document Style Semantics and Specification Language.

DSTN display n. Acronym for double supertwist nematic display. See supertwist display.

DSU n. See DDS.

DSVD n. See Digital Simultaneous Voice and Data.

DTD n. Acronym for document type definition. A separate document that contains formal definitions of all of the data elements in a particular type of HTML, SGML, or XML document, such as a report or a book. By consulting the DTD for a document, a program called a parser can work with the markup codes that the document contains. See also HTML, SGML.

DTE n. Acronym for Data Terminal Equipment. In the RS-232-C and X.25 specifications, a device, such as a PC, that has the ability to transmit information in digital form over a cable or a communications line to a mediating device (known as the DCE). See also RS-232-C standard. Compare DCE (definition 1).

DTL n. See diode-transistor logic.

DTMF n. Acronym for Dual Tone Multiple Frequency. See touch tone dialing.

DTP n. See desktop publishing, distributed transaction processing.

DTR n. Acronym for Data Terminal Ready. A signal used in serial communications sent, for example, by a computer to its modem to indicate that the computer is ready to accept an incoming transmission. See also RS-232-C standard.

DTV n. Acronym for desk top video. The use of digital cameras over a network for video conferencing. See also video conferencing.

DUA n. Acronym for Directory User Agent. An X.500 client program that sends a request to a DSA for the address of a user on the network. Also called: DCA, Directory Client Agent. See also agent (definition 3), DSA.

dual attachment station n. An FDDI node with two connections to the network either through a node and a concentrator or through two concentrators. Compare single attachment station.

dual-band phone n. Wireless phone that broadcasts and receives signals on both 800-MHz (digital cellular) and 1900-MHz (personal communications service, or PCS) networks.

dual boot n. A computer configuration in which two different operating systems are installed and either can be loaded at start-up. A user might set up a dual boot system to take advantage of specific applications and functions in each operating system. A dual boot system might also be set up with each operating system in a different language. A dual boot system is not limited to only two operating systems, and when more than two are installed, it may be called a multi-boot system. See also boot¹.

dual channel controller n. A circuit or device that governs signal access to two pathways.

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 n. A computer that has two floppy disk drives.

dual homing n. A form of fault tolerance used with critical network devices on FDDI networks, in which such devices are attached to both the primary and secondary (backup) rings through two concentrators to provide the maximum possible security in case the primary ring fails.

dual inline memory module n. See DIMM.

dual inline package or dual in-line package n. See DIP.

dual-mode phone n. Wireless phone that broadcasts and receives signals on both analog and digital networks. Dual-mode phones allow wireless phone users with digital service to send and receive calls on analog networks in areas where wireless carriers do not provide digital service.

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.

dual-ring topology n. A token-passing ring topology implemented in FDDI networks that consists of two rings in which information travels in opposite directions. One ring, the primary ring, carries information; the second ring is used for backup. See also FDDI.

dual-scan display n. A passive matrix LCD-type display used in laptop computers. The screen refresh rate is twice as fast in dual-scan displays as in standard passive matrix displays. Compared with active matrix displays, dual-scan displays are more economical in terms of power consumption but have less clarity and a smaller viewing angle. See also passive matrix display.

dual-sided disk drive n. A disk drive that can read or write information to both the top and bottom sides of a double-sided disk. Dual-sided disk drives have two read/write heads, one for each disk surface.

Dual Tone Multiple Frequency n. See touch tone dialing.

DUB n. See dial-up boot loader.

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.

dummy n. A placeholder, usually a character, a record, or a variable, that is used to reserve space until the intended item is available. See also stub.

dummy argument n. In programming, an argument that does not convey any information into or out of the called routine and is usually used to hold a place for an argument that will be used in a future revision of the routine. See also argument.

dummy instruction n. See no-operation instruction.

dummy module n. A module, or group of routines, that performs no function but will do so in some future revision essentially, a collection of dummy routines. See also dummy routine.

dummy routine n. A routine that performs no action but that can be rewritten to do so at some future time. Top-down program development usually involves the creation of dummy routines that are turned into functional routines as development proceeds. Also called: stub. See also dummy argument, dummy module, top-down programming.

DUN n. See dial-up networking.

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 channel n. A communications link that allows for duplex (two-way) transmission.

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.

duplex transmission n. See duplex² (definition 1).

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.

duplication check n. 1. A survey made to determine whether duplicate records or keys exist in a file. See also key. 2. The use of separate independent calculations to establish the accuracy of a result.

DV n. See digital video.

DVD n. See digital video disc.

DVD decoder n. A hardware or software component that allows a digital video disc (DVD) drive to display movies on your computer screen. See also digital video disc.

DVD-E n. See digital video disc erasable.

DVD-R n. See digital video disc recordable.

DVD-ROM n. See digital video disc ROM.

DVI n. Acronym for Digital Video Interface. A hardware-based compression/decompression technique for storing full-motion video, audio, graphics, and other data on a computer or on a CD-ROM. DVI technology was developed by RCA in 1987 and acquired by Intel in 1988. Intel has since developed a software version of DVI, called Indeo. Also called: digital video interactive.

DV-I n. See digital video interactive.

DVMRP n. See Distance Vector Multicast Routing Protocol.

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 the illustration. See also ergonomic keyboard, keyboard. Compare QWERTY keyboard.

Dvorak keyboard.

DVR n. Acronym for Digital Video Recording. Technology allowing broadcast television programming to be digitized and played back immediately. Television signals are routed through a hard drive, converted to a digital format and displayed in real-time or, at the viewer s option, on a delayed basis. DVR technology can be used like a VCR to record favorite programs in advance, with the user picking the programs to be recorded from an online programming guide. DVR capabilities can also be added to products that have related digital technologies and components, such as set-top boxes and digital TV converters.

DVST n. See direct view storage tube.

DWDM n. See dense wavelength division multiplexing.

DXF n. Short for drawing interchange format. A computer-aided design file format originally developed by Autodesk; for use with the AutoCAD program to facilitate transfer of graphics files between different applications

dyadic adj. Of, pertaining to, or characteristic of a pair for example, a dyadic processor, which contains two processors controlled by the same operating system. The term is usually limited to describing a system with two microprocessors. Dyadic Boolean operations are those such as AND and OR in which the outcome depends on both values. See also Boolean algebra, operand. Compare unary.

dye-diffusion printer n. See continuous-tone printer.

dye-polymer recording n. A recording technology used with optical discs in which dye embedded in a plastic polymer coating on an optical disc is used to create minute bumps on the surface that can be read by a laser. Dye-polymer bumps can be flattened and re-created, thus making an optical disc rewritable.

dye-sublimation printer n. See continuous-tone printer.

Dylan n. Short for Dynamic Language. An object-oriented programming language developed by Apple Computer in the mid-1990s for application and systems development. It includes garbage collection, type-safety, error recovery, a module system, and programmer control over runtime extensibility of programs.

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

Dynaload drivers n. Device drivers that are supported by Dynaload. Dynaload is a command that can be run from a DOS prompt under IBM s PC DOS 7 and will load compliant device drivers without modification of the CONFIG.SYS file. See also CONFIG.SYS.

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 allocation n. The allocation of memory during program execution according to current needs. Dynamic allocation almost always implies that dynamic deallocation is possible too, so data structures can be created and destroyed as required. See also allocate, deallocate. Compare static allocation.

dynamic binding n. Binding (converting symbolic addresses in the program to storage-related addresses) that occurs during program execution. The term often refers to object-oriented applications that determine, during run time, which software routines to call for particular data objects. Also called: late binding. Compare static binding.

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 Data Exchange n. See DDE.

dynamic dump n. A listing, either stored on disk or sent to a printer, of memory contents generated at the time of a break in the execution of a program a useful tool for programmers interested in knowing what is happening at a certain point in the execution of a program.

Dynamic Host Configuration Protocol n. See DHCP.

dynamic HTML n. A technology designed to add richness, interactivity, and graphical interest to Web pages by providing those pages with the ability to change and update themselves dynamically that is, in response to user actions, without the need for repeated downloads from a server. This is done by enabling the interaction of HTML, cascading style sheets (CSS), and JavaScript. Examples of dynamic HTML actions include moving graphics on the page and displaying information, such as menus or tables, in response to mouse movements or clicks. Interoperability is governed by the World Wide Web Consortium (W3C) Document Object Model (DOM) specification, a platform- and language-neutral interface to ensure that programs and scripts can dynamically access and update the content, structure, and style of documents. Acronym: DHTML.

dynamic keys n. An encryption technique in which messages are encrypted differently for each transmission based on different keys so that if a key is captured and decrypted, it would never be useful again. See also encryption, key (definition 3).

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 page n. An HTML document that contains animated GIFs, Java applets, or ActiveX controls. See also ActiveX control, GIF, HTML, Java applet.

dynamic RAM n. A form of semiconductor random access memory (RAM). Dynamic RAM stores 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 RAM is more commonly used than RAM because its circuitry is simpler and because it can hold up to four times as much data. Acronym: DRAM. See also RAM. Compare static RAM.

dynamic random access memory n. See dynamic RAM.

dynamic relocation n. The relocation in memory of data or of the code of a currently running program by an internal system routine. Dynamic relocation helps a computer use memory efficiently.

dynamic routing n. Routing that adjusts automatically to the current conditions of a network. Dynamic routing typically uses one of several dynamic-routing protocols such as Routing Information Protocol (RIP) and Border Gateway Protocol (BGP). Compare static routing.

dynamic scheduling n. The management of concurrently running processes (programs), usually by the operating system.

Dynamic Shared Object n. See DSO.

dynamic SLIP n. Short for dynamic Serial Line Internet Protocol. Internet access under SLIP in which the user s IP address is not permanent but is reassigned from a pool each time the user connects. The number of IP addresses an Internet service provider needs to offer is reduced to the number of connections that can be in use at once, rather than the total number of subscribers. See also IP address, ISP, SLIP. Compare DHCP.

dynamic storage n. 1. Information storage systems whose contents will be lost if power is removed from the system. RAM (random access memory) systems are the most common form of dynamic storage, and both dynamic RAM (DRAM) and static RAM (SRAM) are considered forms of dynamic storage. See also dynamic RAM, static RAM. Compare permanent storage. 2. In programming, blocks of memory that can be allocated, deallocated, or freely changed in size.

dynamic Web page n. A Web page that has fixed form but variable content, allowing it to be tailored to a customer s search criteria.