| 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 |