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DAC (Digital-to-Analogue Converter) A device (usually a single chip) that converts digital data into analogue signals. Video adapters require DACs to convert digital data to analogue signals that the monitor can process. Modems require a DAC to convert data to analogue signals that can be carried by telephone wires.

Daily copy backup Backs up files that have changed during a day, but does not change the archive attributes of files; in this way, incremental and differential backups don’t "sense" the backup.

Database A file consisting of a number of records or tables, each of which is constructed of fields (in column format) of a particular type, together with a collection of operations that facilitate searching, sorting, recombination, and similar acts.

Datagrams Packets of data that travel between networks from a sender to a receiver. A datagram typically includes an IP header, address information, a checksum, and data.

Data bits The bits sent by a modem. These bits make up characters and don't include the bits that make up the communications parameters. See also bit

Data cartridge A type of tape medium typically used for backups.

Data compression Reducing the size of files by various techniques such as using a shortcut code to represent repeated data.

Data line protectors Surge protectors designed to protect the telephone line to a modem.

Data Link layer The OSI layer that disassembles packets and reassembles data in to packets.

Data path The size of a bus, such as a 32-bit-wide data path in a PCI bus.

Daughter Board A printed circuit board that plugs into another circuit board (usually the motherboard). It is similar to an expansion board, but accesses the motherboard components (memory and CPU) directly rather than through the slower expansion bus.

DC (Direct Current) An electrical current that travels in one direction and used within the computer's electronic circuits. Contrast with AC.

DCE (Data Communications Equipment) The hardware, usually a dial-up modem, that provides the connection between a data terminal and a communication line.

DDE (Dynamic Date Exchange) A mechanism used in Windows to transfer data between two applications or two separate instances of the same application. Windows itself uses DDE for a variety of purposes, from opening documents in running applications when a document icon is double-clicked in the shell to obtaining program icons for DOS applications. DDE is also used to support OLE. See also OLE.

DDR (Double Data Rate) or SDRAM II The next generation of the SDRAM. DDR finds it’s foundations on the same design core of SDRAM, yet adds advances to enhance it’s speed capabilities. As a result, DDR allows data to be read on both the rising and the falling edge of the clock, delivering twice the bandwidth of standard SDRAMs. DDR essentially doubles the memory speed from SDRAMs without increasing the clock frequency.

De facto standard A standard that does not have an official backing use and acceptance by the industry.

Default gateway The main gateway or unit that will send or receive packets addressed to other networks.

Default Windows printer The printer that Windows software will use unless the user specifies another printer.

Defragment To "optimize" or rewrite a file to a disk in one continuous chain, thus speeding up data retrieval.

Device Any piece of computer hardware.

DEL command A DOS command that deletes files (for example, DEL A:FILE.EXT deletes the file named FILE.EXT from drive A).

DELTREE command A DOS command used to delete a directory, all its subdirectories, and all files within it (for example, DELTREE DIRNAME deletes the directory named DIRNAME and everything in it).   

Demodulation When digital data that has been converted to analog data is converted back to digital data. See Modulation.

Desktop publishing Using a microcomputer for tasks once done by graphic designers, typesetters, and print shops, to produce professional-quality, camera-ready output.           

Device driver A software routine that links a peripheral device(hardware) to the operating system. It acts like a translator between a device and the applications that use it. Each device has its own set of specialised commands known only to its driver. In contrast, most applications access devices by using high-level, generic commands. The driver accepts these generic and translates them into the low-level specialised commands required by the device.

Device-level interface An interface that uses an external controller to connect the disk drives to the PC. Among its other functions, the controller converts the serial stream of data read from the drive into parallel data for the host computer's bus. ST506 and ESDI are device-level interfaces.

Device Manager A Windows (95) program that allows the user to view and set hardware configurations.

DHCP (Dynamic Host Configuration Protocol) The protocol of a server that manages dynamically assigned IP addresses. DHCP is supported by both Windows 95 and Windows NT.

Diagnostic cards Adapter cards designed to discover and report computer errors and conflicts at POST time (before the computer boots up), often by displaying a number on the card.

Diagnostic software Utility programs that help troubleshoot computer systems. Some DOS diagnostic utilities are CHKDSK and SCANDISK. PC Technician is an example of a third-party diagnostic program.

Dial-Up Networking A Windows application that allows a PC to remotely connect to a network through a phone line. Normally, Dial-Up Network icon can be found under My Computer.

Differential backup Backs up only files that have changed or have been created since the last full backup. When recovering data, only two backups are needed: the full backup and the last differential backup.

Differential SCSI device A device (with a cable up to 75 feet long) that sends signals through a pair of wires and is less vulnerable to noise than single-ended SCSI devices. See Single-ended SCSI devices.

Digital signal A signal that has only a finite number of values in the range of possible values. An example is the transmission of data over a serial cable as bits, where there are only two values: 0 and 1.

DIMM (Dual In-line Memory Module) A printed circuit board with gold or tin/lead contacts and memory devices. A DIMM is similar to a SIMM, but with this primary difference: unlike the metal leads on either side of a SIMM, which are "tied together" electrically, the leads on either side of a DIMM are electricity independent.

DIMM banks sometimes called DIMM sockets, because the physical slot and the logical unit are the same. That is, one DIMM module fits into one DIMM socket, which is capable of acting as a memory bank.

DIN Connector A German standard used mostly for audio applications. DIN connectors are used for keyboards, PS/2 style mice, MIDI, and Apple printer attachments.

DIP switch (Dual In-line Packet switch) A switch that has only two settings, and can be used to set configurations such as modem COM ports or printer setup.

Directory An area or data structure in which information is stored regarding the location and contents of files and/or file structures. Also called directory partition.

Disk A circular metal platter or mylar diskette with magnetic material on both sides that stores programs and data. Disks are rotated continuously so that read/write heads mounted on movable or fixed arms can read or write programs or data to and from the disk. See also floppy disk, hard disk

Disk cache A portion of a computer's RAM set aside for temporarily holding information read from a disk. The disk cache does not hold entire files as does a RAM disk, but information that has either been recently requested from a disk or has previously been written to a disk.

Disk compression Compressing data on a hard drive to allow more data to be written to the drive.

Disk defragmenter Defragmentation is the rewriting of all the parts of a file on contiguous sectors. When files on a hard disk drive are being updated, the information tends to be written all over the disk, causing delays in file retrieval. Defragmentation reverses this process, and is often achieved with special defragmentation programs that provide up to 75 percent improvement in the speed of disk access and retrieval.

Disk drive The motor that actually rotates the disk, plus the read/write heads and associated mechanisms, usually in a mountable housing. Sometimes used synonymously to mean the entire disk subsystem.

Disk duplexing An improvement of disk mirroring, whereby redundant data is written to two or more drives, and each hard drive has its own adapter card. This provides greater protection than disk mirroring.

Disk editor A powerful tool for editing any part of a disk, including the partition table, directory entries, DOS boot record, and FAT.

Disk mirroring A strategy whereby the same data is written to two hard drives in a computer, to safeguard against hard drive failure. Disk mirroring uses only a single adapter for two drives.

Disk striping Treating multiple hard drives as a single volume. Data is written across the multiple drives in small segments, in order to increase performance and logical disk volume, and, when parity is also used, to provide fault tolerance. RAID 5 is disk striping with an additional drive for party.

Disk format Refers to the method in which data is organized and stored on a floppy or hard disk.

Disk thrashing A condition that results when the hard drive is excessively used for virtual memory because RAM is full. It dramatically slows down processing and can cause premature hard drive failure.

DISKCOPY COMMAND A DOS command that copies the entire contents of one disk to another disk of the same type, while formatting the destination disk so that the two will be identical (for example, DISKCOPY A:A: makes a duplicate floppy disk using drive A).

Display adapter Another name for a video controller card.

Diskette See floppy disk

DLL Dynamic Link Library: a special type of Windows program containing functions that other programs can call, resources (such as icons) that other programs can use, or both. Unlike a standard programming library, whose functions are linked into an application when the application's code is compiled, an application that uses functions in a DLL links with those functions at runtime - hence the term 'dynamic'.

DMA (Direct Memory Access) DMA channels are similar to IRQs. DMA channels allow hardware devices (like sound cards or keyboards) to access the main memory without involving the CPU. This frees up CPU resources for other tasks. As with IRQs, it is vital that you do not double up devices on a single line. Plug and Play devices will take care of this for you.

DMA controller (Direct Memory Access) A chip or chip logic on the systemboard that provides channels that a device may use to send data directly to memory, bypassing the CPU.

DNS (Domain Name System or Domain Name Service) A database on a top-level domain name server that keeps tracks of assigned domain names and their corresponding IP addresses.

Docking station A system that is designed to connect to a portable or notebook computer for downloading and uploading data and for sharing local peripheral devices.

Documentation Manuals, tutorials, and help files that provide information that a user needs in order to use a computer system or software application.

Domain A logical group of networked computers, such as those on a college campus, that share a centralized directory database of user account information and security for the entire domain.

Domain name A unique, text-based name that identifies an IP (Internet address). Typically, domain names in the United States end in .edu, .gov, .com, .org, or .net. Domain names also include a country code, such as .uk for the United Kingdom.

DOS (disk operating system) A set of programs that control the communications between components of the computer. Examples of DOS functions are: displaying characters on the screen, reading and writing to a disk, printing, and accepting commands from the keyboard. DOS is a widely used operating system on IBM-compatible personal computers (PCs).

Dot matrix printer A type of printer technology using a print head with pins to poke out arrays of dots that form text and graphics.

Dot pitch A color monitor characteristic; specifically, the distance between the holes in the shadow mask. It indirectly describes how far apart the individual dots are on screen. The smaller the dot pitch, the finer the image's "grain." Some color monitors, such as the Sony Trinitron, use a slot mask (also known as an aperture grille) that is perforated by strips, not holes, in the shadow mask. In this case, the dots are arranged in a linear fashion, and their density is called striped dot pitch. (Monochrome monitors do not use a shadow mask and therefore do not have a dot pitch.)

Double conversion The process by which the in-line UPS converts the AC power to battery power in DC from and then AC power to battery power in DC from and then back to AC power.

Download To receive information from another modem and computer over the telephone lines. It is the opposite of upload.

Doze mode In Doze mode , only the CPU's speed is slowed.

Doze time The time before an Energy Star or "green" system will reduce 80 percent of its activity.

DPMS (Display Power Management Signaling) Energy Star standard specifications that allow for the video card and monitor to go into sleep mode simultaneously.

DRAM (Dynamic Random Access Memory) The most common from of system memory packaging. DRAM can hold a charge (that is, data) for only a short period of time. Therefore, to retain the data it must be refreshed periodically if the cell is not refreshed, the data is lost.

DriveSpace A utility that compresses data on a disk drive, creating a single large file on the disk to hold all the compressed files.

Drive array A storage system composed of several hard disks. Data is divided among the different drives for greater speed and higher reliability.

Drive Bay Refers to a site in a where many mass storage devices can be internally installed. Usually a 5.25in-wide 1in-tall hole in a computer case. The number of drive bays in a computer determines how many such devices can be internally installed. The other common size is 3.5in. Bays are described as either internal or exposed. An internal bay is meant for hard disk drives, an exposed bay for removable media drives, such as a CD-ROM, tape backup or floppy disk unit. Some manufacturers use the terms hidden and accessible in place of internal and exposed

Drop height The height that the manufacturer states that a drive can be dropped without making the drive unusable.

DSDD (double-sided, double-density) On PCs and laptops, DSDD means 720K 3 1/2-inch diskettes or 360K 5 1/4-inch diskettes.

DSHD (Double-Sided, High-Density) On PCs and laptops, DSHD means 1.44Mb 3 1/2-inch diskettes or 1.2Mb 5 1/4-inch diskettes.

DTR (Data Transfer Rate) The speed at which data is transferred between a host and a data recording device. Usually noted in KBps or MBps, and sometimes in MB/minute. Can mean a 'peak' rather than a 'sustained' transfer rate.

DTE (Data terminal equipment) This term refers to both the computer and a remote terminal of other computer to which it is attached.

Dual Boot Allows the computer to boot to two different operating systems (DOS & UNIX, Windows NT & Windows 95 for example). However, programs cannot be shared between two OS.

Dual ported When the video chip set (input) and the RAM DAC (output) can access video memory at the same time. A special kind of video RAM is required.

Dual voltage CPU A CPU that requires two different voltages, one for internal processing and the other for I/O processing.

DVD (Digital Video/ Versatile Disk) A faster, larger CD-ROM format that can read older CDs, store over 4 gigabytes of data, and hold full-length motion picture videos.

Dynamic IP address An assigned IP address that is used for the current session only. When the session is terminated, the IP address is returned to the list of available addresses.

Dynamic routing Routing tables that are automatically updated as new information about routes becomes known and is shared by one router with another. Compare to static routing.