Glossary: D

This page contains glossary terms beginning with the letter D.
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Word or Term Description or Explanation
DAB Digital Audio Broadcasting (Comms)
Daemon Disc And Execution Monitor - an otherwise harmless Unix program that normally works out of sight of the user - on the Internet, you'll most likely encounter it only when your e-mail is not delivered to your recipient - you'll get back your original message plus an ugly message from a "mailer daemon".
DAT Digital Audio Tape - originally intended for home audio use, DAT proved suitable for serial storage of large amounts of data - aDAT drive can store Gigabytes on a single tape in a couple of hours - ideal for back-up (also referred to as R-DAT)
Data encryption
key (DEK)
used for the encryption of message text and for the computation of message integrity checks (signatures)
Data encryption
standard (DES)
a popular standard encryption scheme
Database the definition of this word has changed over the years - it used to mean a unified collection of data files but now it tends to refer to an integrated system of data plus the means of interrogating and manipulating the data - even to the extent of referring to a database language
Datagram information is passed around the Internet in blocks of data, each block carrying the IP address of the sender and of the destination - at the destination, the datagrams have to be joined together in the correct order to recreate the original data
Daughterboard see under Motherboard
Daytime Daytime Protocol (RFC 867 - http://www.faqs.org/rfcs/rfc867.html) is an older internet date and time system. It can be either TCP or UDP on port 13. Date and time is sent as an ASCII string.
DCI (Acorn's) Driver Control Interface - this is simply a standard set by Acorn for writing device drivers
DEBI DMA Extended Bus Interface - this is a means of connecting expansion cards to the RiscPC (depending on which magazine you read, it can support anything between eight and fourteen expansion cards!- currently, the maximum is eight but we think it is theoretically possible to have more)
Debug remove the bugs (errors) within a program
Debugger an application written to provide tools to aid the process of finding errors in a program
Decompression see under Compression
Default the standard setting or option which the computer selects unless the user has specifically told it to do otherwise
Device this is the technical term for any information storage medium (e.g. floppy or hard drive) or transmission system (e.g. network, modem or printer) that can be accessed through the computer - each currently available device is represented by an icon at the lefthand side of the iconbar
Device driver software which drives the actual hardware that communicates from the computer to the serial port, parallel port or network - device drivers can also be used for non-physical devices like pseudo-terminals (rlogin in Unix)
DHCP Dynamic Host Configuration Protocol - an internet protocol to allow automatic configuration of computers using TCP/IP - DHCP can be used to automatically assign IP addresses, to deliver TCP/IP stack configuration parameters and to provide other configuration information such as the addresses for printer, time and news servers
Dhrystones strictly Dhrystones/second - this is a means of indicating the speed of a processor by testing the number of iterations of a particular test program it can perform each second - for example, the RiscPC 600 with a 30MHz ARM600 processor can perform about 40,000 Dhrystones/second
Dialogue box a window in which you are expected to select various options and/or enter a filename before the computer performs a particular task or operation
Dialup a temporary connection to a computer or server via a telephone line rather than through a fixed connection
Digest some mail servers give you a compilation of the day's (or week's or month's) messages
Digitiser an electronic circuit which takes an analogue signal such as a sound of a video signal and turns it into digital information which can be stored and/or manipulated within the computer and output again as a sound or video signal
Direct account an Internet account where you can connect directly with the Internet
Direct drive
laser printer
in conventional laser printers, the computer sends information to the printer about what is to be printed, in what position and with what styles, etc. - the processor and memory in the laser printer are then used to work out what dots to place where on the paper - in a direct drive laser, by contrast, the computer works out what the pattern of dots should be and then sends the dot pattern down to the printer on a high speed ("video") parallel interface - this means that the printer needs little or no memory of its own and the process is faster because ARM processors tend to be much more powerful than the processors used in laser printers
Directory when data is stored on a storage medium such as a floppy disc, it can be grouped into directories - when the iconbar icon is clicked, a window appears showing the data in the root (i.e. main) directory - this may be items of data (stored as files), applications or other directories - these directories in turn can contain more information and/or further sub-directories - this system of directories within directories is called a hierarchical data structure
Distribution a way to limit where your Usenet postings go - handy for such things as "for sale" messages or discussions of regional politics - however, the distribution field is not normally used on Usenet, as the distribution nowadays is based upon not feeding groups that are local to the world, etc. - most NNTP-servers ignore this field
Dithering when displaying a picture on a computer screen in, say, a 256 colour mode, it is possible to give the impression of intermediate colours by mixing dots of the different colours in different ratios - this is done using a random displacement of these dots to avoid a patterned effect. This technique is called dithering
DMA Direct Memory Access - when information is being taken into a computer from an external source, the cpu normally reads a location, or block of locations, in the external device and writes the information into its own RAM memory - with DMA, by contrast, the information is written directly from the external device into the cpu's memory without the cpu being involved - this means that the process is much faster, as the cpu can be doing other things while the information is being loaded automatically into memory
DNS Domain Name System - a machine that translates the domain name of the Internet addresses from the textual form (e.g. ftp.demon.co.uk) into the numeric form (e.g. 158.53.8.13) that is actually used to send messages - there will usually be at least one name server in each domain - having more than one is a useful redundancy since you don't want the network to grind to a halt whenever your one and only name server crashes - DNS will also handle the translation from IP adresses to names if used the other way around. DNS is hierarchical in that each server only knows about a certain domain, and uses other DNS servers to find the rest of the computers on the Net
DOCSIS Data Over Cable Service Interface Specifications is a standard interface for cable modems, the devices that handle incoming and outgoing data signals between a cable TV operator and a computer or television set
Domain a group of computers sharing the same set of services, usually a company wide address - a domain is the list of hosts in one specific part of the Net, and can be as little as one host or as much as the complete Net
Domain name the textual form of the Internet address - for example, if you have an account with Paston, and your address is Paul.NCS@paston.co.uk, your domain name is paston.co.uk. (i.e. it's the bit after the @ sign)
Dongle this is a small electronic circuit, usually held in a plastic connector block, that goes on the parallel port of a computer - it forms an electronic 'key' that allows you to use a particular application (such as Impression or ArtWorks) - the application checks every now and then to see if a dongle is present and, if not, it shuts itself down and will not restart
DOS Disc Operating System - but also now used as shorthand for DR-DOS or MS-DOS - these are the operating systems most commonly used on IBM PC computers and compatibles - they can be used on Acorn computers by using the PC Emulator or a PC expansion card or, on a RiscPC, a PC processor card - they are command-line operating systems, i.e. they do not use a wimp interface (for most practical purposes, there is very little difference between DR-DOS and MS-DOS)
Dot file a file on a Unix public-access system that alters the way you or your messages interact with that system - for example, your .login file contains various parameters for such things as the text editor you get when you send a message - when you do an ls command, these files do not appear in the directory listing; do ls -a to list them
Dotted quad the group of four numbers, separated by dots (e.g. 158.53.8.13), that make up the IP address of a computer
Double-click press the (left hand) button of the mouse twice in quick succession
Double density
floppy discs
discs that can store approximately 800Kb of data when formatted
Down when a public-access site runs into technical trouble, and you can no longer gain access to it, it's 'down'
Download transfer files or messages from a distant computer to your own computer - there are several different methods, or protocols, for downloading files, most of which periodically check the file as it is being copied to ensure no information is inadvertently destroyed or damaged during the process - some, such as XMODEM, only let you download one file at a time - others, such as batch-YMODEM and ZMODEM, let you type in the names of several files at once, which are then automatically downloaded
dpi dots per inch - on a desktop printer, the text and graphics are reproduced by rows and rows of tiny dots - the smaller the dots, the better the definition of the resulting printout - this is usually specified as the number of dots per inch that the printer can lay onto the paper t he same idea is used for scanning - when scanning a picture or diagram, the grey level (see below) of rows of tiny areas of the picture are assessed by the scanner - the closeness of the areas being differentiated is measured in dots per inch
DPMS Display Power Management Signalling - when monitors are not being used, it is a waste of energy to have them on at normal brightness - RISC OS 3 supports screen blanking which helps to reduce energy consumption, but some monitors can accept special electronic signals from a computer to tell it to drop into a very low power consumption mode if it is not being used - this signalling between computer and monitor is not available in all monitors, although a lot of newer monitors are beginning to provide it
DPOB Dual Processor Open Bus - on the RiscPC, the processor(s) are held on separate cards which plug into the main PCB or motherboard - it can take two such processor cards and the dual processor open bus is a way of allowing two different (or similar) processors to share the use of the computer's memory and input/output and data storage facilities
DRAM Dynamic Random Access Memory - this is the most common form of RAM memory used in computers today - 'Dynamic' is a technical term which refers to the way in which the information is stored within the chips
DSP Digital Signal Processing - a set of techniques for modifying (sound) signals - it involves digitising the signals, performing various mathematical processes on the data and then turning the resulting data back into an analogue signal
DSTN Dual Scan Twisted Nemonic - a technology used for screen displays, typically for portable computers
DTP DeskTop Publishing - as desktop computers have increased in processing power, it has become possible for them to handle both text and graphics, to allow users to manipulate them on-screen and output them to a printer - in this way, "books" can be produced on a desktop computer - hence the term 'desktop publishing'
Dual port RAM see VRAM
Duplex see Full Duplex and Half Duplex
DVD Digital Versatile Disc, or Digital Video Disc, or Digital Video Display
DVD-RW Digital Versatile Disc-ReWritable

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