"The Atari A to Z"
by Mark S Baines
Copyright (c) 1998 Mark S Baines
All Rights Reserved
YOU MUST READ "READ_ME.NOW" BEFORE YOU LOOK AT ANY OF THIS FILE
*****************************************************************************
D
DA
Desk Accessory. See Accessories.
DAC
Digital to Analogue Convertor. The process of converting digital form data
into analogue output signals. For instance, converting the digital data stored
on a CD to the analogue voltage signals which are converted into sound waves
at the loudspeaker. The Falcon's stereo 16-bit DACs ensure excellent sound
reproduction through external speakers.
Daemon
Pronounced 'demon'. A program that works automatically in the background
usually on a UNIX system without user intervention.
Daisywheel printer
A printer that uses preformed metal character shapes arranged on a rotating
wheel striking against a ribbon onto paper. It produces a high quality text
only printout but is very slow and noisy.
DARPANET
Defense Advanced Research Projects Agency NETwork. In 1969, the Advanced
Research Projects Agency (ARPA) with some funds from the US Defense (sic)
Department connected four widely dispersed US supercomputer sites together to
form a network called the ARPANET. ARPANET was originally devised so that all
the researchers on the project could share all the resources. However, the US
government could not approve of defence budget money being used on computer
science projects without some military applications. Legislation was quickly
passed forcing the ARPANET project to show that they were doing research
relevant to defence. The name was changed to DARPANET (Defense Advanced
Research Projects Agency NETwork) to reflect this.
The aim of this project was to design a communications network that could
withstand practically any sort of attack, especially in a nuclear war. This
required the network to have no central computer or authority which would be
an obvious target of attack. In 1964 the RAND Corporation presented a paper
based on the principle of operating "while in tatters", that is, it would be
assumed at all times that the network is unreliable and that it should have
the ability to transcend this imprecision. Therefore, all nodes would be equal
in status - each would have the authority to originate, pass and receive
messages. A message is split into packets, each one separately addressed. Each
packet starts at a specified source node and arrives at a specified
destination node, the route being variable depending on the state of the
network. The route any one packet takes is not important. What matters is that
each packet is tossed from node to node until it gets closer to its
destination and finally arrives. If any one part of the network is destroyed,
the packets will find alternative routes.
DARPANET very rapidly grew as other networks and sites (mostly academic,
military and defence contractors) joined it but in time the military, being
concerned about security of data, used it less and less until in 1983 they
left DARPANET to form the MILNET. DARPANET became once again ARPANET which
itself officially closed in 1989. However, this network of networks continues
to grow and now consists of nearly four million hosts with something like 30
million people having access in over 80 countries and is known as the
Internet. See Internet.
DAT
Digital Audio Tape. This form of magnetic tape offers computer users cheap
data storage with over a gigabyte of data being stored on one small cassette.
Data
The basic facts and figures and other items on which a program works and
exists in a form without meaning. See Information.
Database
A pool of data held in storage and generally handled by programs that help to
create, display and manipulate it. The term is often confused with the
programs that manage this data.
Database key
A unique value associated with a record in a database and used to locate and
distinguish it from other records, such as a record number or stock item
number.
Database management system - DBMS
A set of software utilities, often integrated into one program on micros, for
controlling and accessing data in a database. These programs act as an
interface between the user and the data, a good example being Superbase.
Data channel
A channel used to carry data as opposed to one used to carry speech or
supervisory signals on a telephone line. Also, any path for data between any
functional units.
Data compression
See Compression.
Data file
A file used by an application to store its data.
Data network
An organization of computers and communication facilities that provide for the
transmission of data between geographically separated locations, sometimes
providing a data processing service for users.
Data sections
One of the three sections of an executable program as produced by an assembler
or compiler and linker. It contains the program's initialized variables. See
Basepage, Text section, BSS section.
Data signalling rate
The rate at which data is sent on a communications channel, measured in bits
per second. Also called the bit rate.
Data stream
The flow of data from one functional unit to another, whether from a computer
to a peripheral or via a data channel on a telephone line from one computer to
another.
Data transfer
The movement of data between different functional units, such as from a
computer to disk or via a communications link.
Data transfer rate
The average number of data units, usually characters, passed between two
functional units per unit time, usually seconds. A modem capable of 2,400 bps
data signalling rate could have a typical data transfer rate of 230 cps.
Daughter board
A printed circuit board that plugs into the main circuit board, referred to as
the 'motherboard'.
DBMS
DataBase Management System.
DCD
Data Carrier Detect. A communications system interface signal from a modem to
its computer indicating that a carrier from the other, distant modem has been
received.
DCE
Data Communications Equipment. A device in a data communications system that
performs the line interfacing functions between a unit of data terminal
equipment (DTE) and a telephone line. The term is practically synonymous with
'modem'.
DD
Double Density. A method of encoding data on a disk, usually used when
referring to floppy disks. All STs and STEs have DD floppy disks and high
density disks were introduced to Mega STEs and TTs in 1992 with TOS 2.06 and
TOS 3.06.
Deallocate
To withdraw a resource, such as a memory block or printer, from an entity,
such as a program or user, to which it had been previously allocated.
Debug
To detect and eliminate bugs in a program's source code, sometimes using a
debugger program.
Debugger
A program specifically written to enable the elimination of bugs from program
source code.
Decimal
The positional representational number system with a radix or base of ten, the
'normal' and assumed counting system.
Decimal point
The radix point in the decimal system denoted usually by a full stop, such as
1.2 meaning one whole and two tenths.
Decode
To reverse the effects of encoding.
Decrement
To decrease a value by the successive removal of equal-sized units, usually
one.
Dedicated
Reserved for a particular purpose or user.
Dedicated line
A permanently connected telephone line between two computer systems. The speed
may be anything between 56 K a second to 45 Mb a second and beyond although 64
K to 2 Mb a second is more usual.
Default
A value, option or setting that a program automatically selects where no
alternative has been specified by the user.
DEL
DELete. ASCII control character 127 that normally causes the deletion of the
character at the cursor position.
Delete
The operation of removing a selected item, such as a file or text.
Delete key
The key that normally deletes the character at the cursor position or can be
used with a secondary shift key to perform similar operations, such as [Cntl-
Delete] which may delete a line of text.
Demodulation
To reverse the effects of modulation, to reconstruct transmitted data from
signal patterns received on a modulated carrier wave.
Denary
Decimal.
Density
The number of elements per unit length, area or volume. Also, the contrast
between a printed image and its background surface.
Descender
That part of a character in type that falls below the base line, such as the
lower part of the letter 'y'.
DESKCICN.RSC
A standard GEM resource file containing colour Desktop icons for TOS 4. This
file is searched for at boot-up before the DESKICON.RSC file. The position of
the icons in the file's object tree is the index number as referred to in the
NEWDESK.INF file.
DESKICON.RSC
A standard GEM resource file containing Desktop icons for TOS 2 and above. The
position of the icons in the file's object tree is the index number as
referred to in the NEWDESK.INF file.
Desk menu
The first, leftmost GEM menu from which accessories are run on the Desktop.
When a program is loaded it will also contain details of that program and
should have the name of the program installed as its title. This is important
in a multitasking situation so that the owner of a menu bar can be recognized.
Desktop
That part of GEMDOS that appears after the ST-Falcon range boots up and allows
disk and file manipulation with the use of icons, windows and the mouse. The
Desktop was extensively updated in TOS 2 and 3 being referred to as NewDesk.
DESKTOP.INF
A file that holds the customized Desktop data for TOS 1 versions created by
selecting the "Save Desktop" menu option and read at boot-up from the root
directory. It contains the settings as made with the Control Panel accessory
as well as the default window and icon positions, icon labels and the
recognized executable file extenders. The maximum size of the file is 1,024
bytes for TOS 1.0-1.2 and 4,192 bytes for TOS 1.4.
Table D1: A Typical DESKTOP.INF File
#a060001
#b001100
#c7770007000600070055200505552220770557075055507702011501
#d
#Z 01 C:\STARTPRG.PRG@
#E 98 13
#W 00 1E 00 04 26 14 01 C:\NEODESK4\*.*@
#W 00 00 28 04 26 11 00 @
#W 00 00 10 07 28 10 00 @
#W 00 00 0F 0A 2A 0B 00 @
#M 01 00 00 FF I RAM Disk@ @
#M 07 00 00 FF H Graphics@ @
#M 06 00 00 FF G Comms@ @
#M 05 00 00 FF F Databases@ @
#M 04 00 00 FF E Languages@ @
#M 03 00 00 FF D Text@ @
#M 00 00 00 FF A Floppy Disk@ @
#M 02 00 00 FF C Utilities@ @
#C 06 07 00 FF c Back Pack@ @
#T 07 07 02 FF Rubbish!@ @
#F FF 04 @ *.*@
#D FF 01 @ *.*@
#G 03 FF *.APP@ @
#G 03 FF *.PRG@ @
#F 03 04 *.TOS@ @
#P 03 04 *.TTP@ @
#G 03 04 E:\LC5\LC5.PRG@ *.C@
DESKTOP.INF breakdown
These settings are made from the CONTROL.ACC control panel or from the
"Desktop" menu items. The XCONTROL control panel does not save its settings to
the DESKTOP.INF file unlike CONTROL.ACC. If you want to alter those values
stored in the first three lines of the DESKTOP.INF file and make them
permanent then you must install ONTROL.ACC to make the change and then save
the Desktop.
The DESKTOP.INF file is in ASCII format and can be edited with an editor or
with a word processor in ASCII or program mode.
Note that @ characters are the end-of-field and end-of-line delimiters.
Table D2: DESKTOP.INF Breakdown
#a060001
RS-232 Configuration
Format: #a123456
Item Setting Value meaning
1 Duplex 0 = full duplex
1 = half duplex
2 Baud rate 0 = 9600 bps
1 = 4800 bps
2 = 1200 bps
3 = 300 bps
4 = 19200 bps
5 = 3600 bps
6 = 2400 bps
7 = 2000 bps
8 = 1800 bps
9 = 600 bps
: = 200 bps
; = 150 bps
< = 134 bps
= = 110 bps
> = 75 bps
? = 50 bps
3 Parity 0 = none
1 = odd
2 = even
4 Word length 0 = 8 bits
1 = 7 bits
2 = 6 bits
3 = 5 bits
5 Protocol 0 = none
1 = XON/XOFF
2 = RTS/CTS
3 = XON/XOFF & RTS/CTS
6 Strip bit 0 = yes
1 = no
#b001100
Printer configuration
Format: #b123456
Item Setting Value meaning
1 Printer type 0 = dot matrix
1 = daisywheel
2 Colour 0 = black and white
1 = colour
3 Dot density 0 = 1280 dpi
1 = 960 dpi
4 Quality 0 = draft
1 = NLQ
5 Output port 0 = printer port
1 = modem port
6 Paper type 0 = continuous
1 = single sheet
#c7770007000600070055200505552220770557075055507702011501
Palette, mouse, keyboard
Format: #c111...1112345566
Item Setting Value meaning
111...111 16 colour values Three numbers, one for each RGB value
in range 000-??? where the range
includes 000-999 and 10 = ':' 11 = ';'
12 = '<' 13 = '=' 14 = '>' and 15 =
'?'
2 Double click speed 0-4
3 Keyboard click 0 = off
1 = on
4 Warning bell 0 = off
1 = on
55 Auto-repeat delay 1 (low)-46 (high)
66 Auto-repeat speed 1 (low)-21 (high)
#d
Not used. May be omitted or used for comments but must contain 46 spaces or
no more than 46 characters for comments.
#Z 01 C:\STARTPRG.PRG@
GEM Auto-booting program (.PRG or .APP) for TOS 1.4 and later
Format: #Z 11 filespec
Item Setting Value meaning
11 Program type 00 = .TOS or .TTP
01 = GEM (.PRG or .APP)
filespec Auto-boot program Path and file name of GEM program to
auto-load at boot-up
#E 98 13
Directory window display and miscellaneous
Format: #E 11 23
Item Bit Setting Value meaning
11 0 Confirm overwrite 0 = yes
1 = no
1, 2 Not used 0
3 Confirm copy 0 = no
1 = yes
4 Confirm deletes 0 = no
1 = yes
5, 6 Sort by 00 = name
01 = date
10 = size
11 = type
7 Display type 0 = icons
1 = text
2 BLiTTER 0 = off
1 = on
3 Resolution 1 = low
2 = medium
3 = high
#W 00 1E 00 04 26 14 01 C:\NEODESK4\*.*@
#W 00 00 28 04 26 11 00 @
Directory window settings - four maximum
Format: #W 11 22 33 44 55 66 77 filespec
Item Setting Value meaning
11 Slider horizontal position Column number
22 Slider vertical position Row number
33 Top left corner X-position Column number
44 Top left corner Y-position Row number
55 Window width In columns
66 Window height In rows
77 Window ID 00 = closed
08 = boot drive, numbering continues
in hex to include all drives. If boot
drive is C: then this is 08 and drive
A: is 09, B: is 0A and D: is 0B.
filespec File select mask Any length to accommodate filespec
e.g. all files in C:\NEODESK4 folder
#M 01 00 00 FF I RAM Disk@ @
#M 00 00 00 FF A Floppy Disk@ @
#M 02 00 00 FF C Utilities@ @
Desktop disk drive icons
Format: #M 11 22 33 44 5 label
Item Setting Value meaning
11 Column position 0-7
22 Row position 0-7
33 Icon type 0 = disk drive
1 = folder
2 = trash can
3 = program
4 = document
7 = invisible
44 Not known Always FF?
5 Disk drive ID A:-P:
label Disk drive label Up to 12 characters
#C 06 07 00 FF c Back Pack@ @
Cartridge port icon
Same format as for #M but 'c' used as drive ID
#T 07 07 02 FF Rubbish!@ @
Trash can icon
Same format as for #M but no 'drive' ID
#F FF 04 @ *.*@
Document file default icons
Format: #F 11 22 filespec
Item Setting Value meaning
11 Not known Always FF?
22 Icon type 0-4 as above
filespec Displayed filespec Any file name or wildcards e.g. the
example above is the default for all
document files but #F FF 02 @ *.BAK@
will display all *.BAK files as trash
cans.
By default and initially, displays ALL files as documents icons. This is
modified with the following entries. In practice, it sets all document files
as document icons. See other #F entry below.
#D FF 01 @ *.*@
Directory (folder) icon - display folders, general or specific
Format: #D 11 22 filespec
Item Setting Value meaning
11 Not known Always FF?
22 Icon type 0-4 as above
filespec Displayed folder name Any file name or wildcards e.g. the
example above is the default for
folders, but #D FF 02 @ SCRAP.*@ will
display the SCRAP folder as a trash
can.
By default, sets all directory icons to the same folder icon.
#G 03 FF *.APP@ @
#G 03 FF *.PRG@ @
#G 03 04 E:\LC5\LC5.PRG@ *.C@
GEM program (.PRG or .APP) installed application
Format: #G 11 22 filespec1 filespec2
Item Setting Value meaning
11 Application icon type 0-4 as above
22 Parameters icon type 0-4 as above
FF no icon
filespec1 Application file name Full path name
filespec2 Parameter file name Extension of file passed to
application
Installed applications are placed here so that the example above will give
LC5.PRG a program icon, all *.C files will have document icons and double
clicking on a *.C file starts up LC5.PRG and loads that file into it.
By default, displays all files with .APP and .PRG extenders as program icons
and files are not passed to them, hence the FF.
#F 03 04 *.TOS@ @
#F 03 04 E:\EDITOR\ED.TOS@ *.TXT@
.TOS file installed application
Same format as for GEM program installed application
If 11 is FF then it is the same as for #D as shown above. Installed
applications are placed here so that the example above will give ED.TOS a
program icon, and all *.TXT files a document icon. Double clicking on a
*.TXT will start up ED.TOS and load that file into it.
By default, displays all files with .TOS extenders as program icons and all
files passed to them as documents.
#P 03 04 *.TTP@ @
.TTP file installed application
Same format as for GEM program installed application
By default, displays all files with .TTP extenders as program icons and all
files passed to them as documents.
Desktop replacement
A program which replaces the normal GEM Desktop with an alternative, usually
with improved functionality. Such a program is normally run at boot-up. An
excellent example is Gribnif's very successful NeoDesk which inspired many of
the changes to the GEM Desktop in TOS 2 and 3, known as NewDesk. Other
alternative Desktops include TeraDesk, Gemini and Ease.
Destination disk
The disk that data is written to during a copy or diskcopy operation.
Device
A unit of hardware, such as a peripheral or electronic component.
Dialog box
An interactive message box containing text, buttons and/or icons that a
program uses to communicate with a user. They normally require input from a
user and/or acknowledgement in the form of a mouse click on the "OK" button.
Dialog boxes are modal, requiring input from the user before processing can
continue. Because of this, they should not be used for ongoing informational
or status output and should not form the basis of a program as this would
severely interfere with the multitasking capabilities of MultiTOS or MagiC,
blocking out the use of menus from other programs. Many modern GEM programs
place dialogs in windows making them amodal.
Dialogue
The interchange between a user and a 'conversational' computer program.
Dial-up
A connection to a remote computer made using a modem and telephone line. This
computer is usually a service provider connecting you to the Internet.
Diagnose
To identify and locate the cause of hardware and software faults.
Diagnostic cartridge
A ROM cartridge which, when inserted into the cartridge port of the ST-Falcon
range, will analyse the computer's hardware systems for faults.
Diagnostic program
A program that aids the user to identify and locate software errors or
hardware faults.
Dialect
A version of a programming language that deviates from a basic standard and
particularly caters for a specific computer system. The use of such dialects
prevent the easy portability of program source code from one computer system
or operating system to another.
Dibit
Two bits that are represented on a communications line as a single line
transition.
Digit
A graphic representation of an integer, such as the character 2 or 7.
Digital
Using binary digits to represent data in storage or transfer, that data being
therefore represented as discrete values and operated on in steps in contrast
to analogue data which exists as a continuous and infinite number of values
between two extremes. See Analogue.
Digital Research Inc (DRI)
The US company that designed and wrote the GEM user interface. Although they
continued to revise GEM, on the PC for instance, the contract with Atari and
an agreement with Apple forbad any update to it on the ST. See GEM.
Digitizer
A device that converts analogue signals (usually sound or light, such as
video) to digital code for computer processing.
DIL
Dual In-Line pin. A type of integrated circuit in which the pins are in two
parallel rows on each side of the device.
DIN
Deutsche Industrie-Norm (Deutsches Institut fr Normung). The German
equivalent of the BSI or ANSI. They are particularly noted for their standard
for DIN plugs and sockets widely used on electrical devices.
DIP
Dual In-line Pin. Synonymous with DIL.
Dip switch
Dual In-line Pin switch. Tiny sliding switches used to select a particular
hardware configuration and common on printers and computer motherboards.
On the Mega STE motherboard there is a dip switch with the following
functions:
Switch on Function
8 DMA sound hardware disabled
7 AJAX floppy disk controller fitted
6 2.88 Mb ED floppy disk drive fitted
On the Falcon there are two dip switches at locations U56 and U57 on the
motherboard.
U56
Switch Function
1 On = 1 Wait State DRAMs
Off = 0 Wait State DRAMs
2 On = 32-bit Video Bus
Off = 16-bit Video Bus
3-4 Off Off = 0 Wait State ROMs
Off On = 1 Wait State ROMs
On Off = 2 Wait State ROMs
On On = Reserved
5-6 Not used
7-8 Not used
U57
Switch Function
1-4 Not used
5 Off = 2.88 Mb ED floppy disk drive fitted
On = Any floppy disk other than ED fitted
6 Off = AJAX floppy disk controller fitted
On = WD1772 floppy disk drive fitted
7 Off = DMA sound hardware disabled
On = DMA sound hardware enabled
Direct access
An access to data on magnetic disk or RAM or any other fast access storage
device as opposed to tape access.
Directory
A list of a backup storage device's files and subdirectories (folders). Such a
list will appear in a directory window under the GEM Desktop when a disk icon
has been double-clicked on. That list of files may be represented by icons or
text. On some command line interface operating systems, a command such as dir
or ls will achieve a similar result.
Directory sectors
The sectors on a disk that contain the list of files and subdirectories in the
root directory of that disk. Each list entry contains the name of the file,
its date and time of last change, its size in bytes and the number of the
first disk cluster of data for that file.
Direct to disk
A sound recording or sampling technique where the input data is, after perhaps
being processed or filtered, stored directly to a hard disk for later playback
or processing. The Falcon DSP audio subsystem allows for this with ease with
third party software providing the interface and control. A basic direct to
disk program is shipped with the Falcon030 with 16-bit stereo 50 kHz sampling
rate recording and editing capabilities.
Disable
To prevent an event from taking place.
Disabled
An unselectable icon, text or button under GEM shown as being greyed out. This
usually signifies that that function is unavailable.
Disassembler
A program to help convert machine code into assembler language.
Disc
See Disk.
Disconnect
To terminate a communications link by going 'on-hook', the electronic
equivalent of putting the phone down. Also, to remove a device from a system
or to remove its power.
Discrete
Capable of being separately identified or handled.
Disk
A storage medium consisting of a rotating disc with a magnetizable surface
onto which data is recorded by means of a read/write head. Floppy disks are
made of a thin sheet of Mylar. Hard disks or Winchesters are made of aluminium
in a hermetically sealed container and are capable of rotating faster with the
head being closer to the surface thus enabling a higher storage capacity and a
faster data transfer. See Format.
Diskcopy
The operation of exactly duplicating a disk's contents onto another. Under the
GEM Desktop it is achieved by dragging a source disk icon on top of the
destination disk icon. All data previously held on the destination disk is
erased during the operation. If the destination disk is not formatted, it will
be formatted to the same specifications as the source disk at the same time.
However, the destination disk must be of the same type. A double-sided disk
cannot be disk-copied onto a single-sided one, for instance.
Disk drive
A long-term storage device that writes and reads data to and from a disk.
Components consist of a motor to rotate the disk at a constant speed and
read/write heads which are stepped across the surface of the disk and which
transfer the data to that surface or read from it.
Diskette
A floppy disk.
Disk identifier
A letter associated with a disk drive or disk partition normally followed by a
colon (:). Floppy disk drives use A: and B:, hard disk drives and RAM drives
C: to P: and the cartridge port c:. MiNT and MultiTOS introduce other letters,
such as U:. A disk identifier is seen in the move bar of a directory window on
the Desktop when the root directory is displayed. The presence of a backslash
(\) after the colon denotes the root directory of that drive.
Display
An electronic presentation of a meaningful arrangement of visually contrasting
elements, such as that produced by a VDU, LEDs, LCDs etc.
Display device
A device onto which data from a computer is displayed. This usually means a
screen-based device such as a VDU, but could mean a plotter or conceivably any
printer.
Display driver
A program that interprets the generic, often simple, output commands from an
operating system into the signals required by a specific display device.
Display element
The basic unit of a display, such as a pixel or LCD segment.
Display memory
That part of RAM which holds the bit-pattern representation of the data shown
on a display screen.
Dithering
The technique of representing grey scales in a black and white only image as
patterns of black and white pixels. Various arrangements and groupings of the
black pixels against a white background provide different densities thus
giving the illusion of grey tones. The ST and TT high resolution monochrome
displays use dithering.
DMA
Direct Memory Access. An input/output system in which data is transferred
directly between memory and backing storage without going through the CPU.
Also, an Atari custom chip for high-speed (8-bit wide bidirectional) data
transfer bypassing the CPU in a 40-pin 0.6 inch DIL package on the ST. The
chip takes in the processor's sixteen data lines producing an 8-bit tristate
bus which controls the ACSI or DMA port and the floppy disk controller. It is
used with floppy and hard disk drives and the Atari laser printers and another
is used for manipulating the large amount of data associated with the stereo
sound reproduction in STEs, TTs and Falcons.
The data transfer rate to and from the floppy disk is slow enough for the CPU
to handle directly, but with hard disks the MC68000 would have to pause the
disk drive to keep up because it has to go through a lot of cycles for each
word of data transferred. After the MMU has set up some registers with the
start address of the memory to be transferred, a counter for the number of
bytes to be transferred and the type of transfer required, the DMA is given
control by the CPU taking over the bus and transfers the data at high-speed,
faster than the CPU could.
The DMA is also used to transfer data within RAM with the aid of the BLiTTER,
if fitted. DMA on the TT is revised to handle the 32-bit address and data
range of the 68030. On the TT there are three independent DMA channels, the
low-speed network port on SCC Port A, the SCSI port and the ACSI/Floppy DMA as
with the ST. A DMA controller exists for each channel which is physically
implemented in two chips, one for the system bus interface (DMAC) and the
other for peripheral interface and FIFO (DAC). The DMAC is a 32-bit address
bus interface holding the address transfer registers and data count. Its data
lines are only 8-bit wide and so needs to use dynamic bus sizing to access its
registers a byte at a time. The DAC has the FIFO memory portion of the DMA. It
has 32-bit data input lines interfacing these with 8-bit output to the SCSI
and SCC chips. The DMA channel for the ACSI/Floppy subsystem is shared with
the floppy disk controller. DMA between RAM and ACSI peripherals and between
RAM and floppy, can only be performed using dual purpose ST RAM. If a transfer
is required from such a device into alternative TT RAM, a two stage transfer
is required, using the ST RAM as an intermediate buffer. See ST RAM, Fast RAM.
DMAC
Direct Memory Access Controller. An alternative name for a DMA processor.
DMA port
The ACSI port on the ST-TT range of computers consisting of a 19-pin D male
socket which connects to the DMA bus. This port is controlled by a subset of
the SCSI command set called the ACSI and can be connected to up to seven other
devices including hard disks, laser printers, tape drives, CD-ROMs. The port
can run at up to 10 Mbits per second. See ACSI, SCSI.
DMA sound
Introduced on the Atari STE where an 8-bit stereo PCM sound system is coupled
to a National LMC1992 volume and tone controller chip. This is, for all
intents and purposes, a sound sampler which plays samples but cannot record
them. Playing these samples through the ST's Programmable Sound Generator
(PSG) chip was always processor intensive, but the STE uses a DMA interface
which basically means that you tell the system where the sound sample data is
and the DMA will access it and play it without intervention from the CPU. The
digitized sound data is stored as numbers from -128 to +128 which represents
the speaker displacement from the neutral or centre position. The DMA fetches
the data during horizontal blanking and feeds it into a Digital to Analogue
Convertor (DAC) at one of several programmable frequencies of 50 kHz, 25 kHz,
12.5 kHz or 6.25 kHz. This is the sampling frequency. The data is fetched in
word size, one byte for each of the two channels. The signal is then filtered
to a frequency equal to 40% of the sample frequency by a four-pole switched
low-pass filter which anti-aliases the sound. Then the signal is filtered by a
fixed-frequency 16 kHz low-pass filter, fed to the LMC1992 volume and tone
controller and output to the two RCA-style phono audio sockets for connection
to an amplifier, headphones or tape recorder. Both channels behave as
described and are intended to be used as separate left and right stereo
channels. Sound samples are grouped together in frames where each frame can be
played once or repeated forever automatically. Frame repetition is seamless as
the registers holding the start and end addresses of the sample are buffered.
This means that the registers can be written to whilst a sample is being
played. A monophonic mode is available which will send the same sample data to
each channel output. The stereo sound output is also mixed onto the ST PSG
output to the monitor's speaker. Output from the ST's PSG sound chip can be
mixed to the monitor and to both stereo output sockets.
The Falcon030 also utilizes the DMA with its stereo eight channel 16-bit audio
input and output. See PCM, LMC1992, Sampling, MICROWIRE.
Document
One or more sheets of paper on which information is represented and used as
the source data to be inputted into a computer. Under GEM it is a data file
produced by any program, but more usually, a data file produced by a word
processor or DTP program.
Document processor
Traditionally, any program that can process text (and image) material and
output the result to a printer. It is increasingly taken to mean a word
processor that uses fonts and styles not normally available to a printer in
text mode, and creates a page image which is output to the printer in graphics
mode. Such a program is therefore using techniques traditionally associated
with DTP but falls short of being a full-blown DTP program by not having all
of its versatility and typesetting control, such as perhaps, text rotation or
columns.
Document reader
A device capable of interpreting the marks on a source document and inputting
that data into a computer.
Documentation
The written record containing notes on the rationale and history of a software
program used as an aid to the programmers maintaining it. Also, the
information that is included with a commercial program informing the user on
how to use that program which may be either in paper form or contained within
a disk file.
Dongle
A hardware device attached to a port which permits access to some proprietary
software thus preventing its use if absent. They are used to prevent
unauthorized copying of software.
DOS
Disk Operating System. That part of an operating system concerned with disk
and file management, such as GEMDOS on the ST-Falcon range and MS-DOS on PCs
which, incidentally, share many characteristics.
Dot matrix
A grid in which marks can be placed to construct graphic characters. It is the
usual method of character formation on a VDU and dot matrix printers.
Dot matrix printer
A printer that forms its characters and graphics by placing ink onto paper as
small dots within a matrix of possible positions. Some will strike small pins
through a ribbon onto paper whilst others may blow ink as tiny droplets at the
paper. These printers are quick, very versatile, cheap but awkward to use,
sometimes messy and noisy.
Dot pitch
On CRT screens, the size of the separation between the red, green and blue
elements that go to make up a single pixel. The smaller the dot pitch, the
finer the resolution and sharper the display. A dot pitch of 0.30 mm or less
is most desirable and one of 0.28 mm equates to a resolution of 89 dpi.
Double-click
Pressing the mouse button twice quickly in succession in order to activate a
function represented by an icon in a GUI or WIMP environment. For instance,
double-clicking on a .PRG file icon will load and run that program.
Double density
The method of doubling the number of bits recorded per unit of length of track
on a magnetizable disk surface and refers to the type of floppy disk giving
around 720 K of storage.
Double-sided
A disk having two sides to store data on.
Down
Out of service, inoperable due to a fault.
Down channel
A forward channel, a channel of a four-wire circuit carrying the data
transmission.
Down-load
The process of copying data or programs from one computer into another,
usually connected together via a telephone line.
Down time
The time that a system or other unit is inoperable.
DP
Data Processing.
DP8516
A National graphics processor chip in a 44-pin PLCC package used on the TT to
produce the 1,260 x 960 high resolution monochrome display. The refresh rate
is 67 Hz and the circuitry is implemented in ECL giving a stable, high quality
display.
DPI
Dots Per Inch. A measurement of resolution for printers, scanners and computer
screens, being the number of dots or pixels per inch across the page or
screen. The more dots available, the smaller they are and the 'finer' the
print quality. Normally, reference is made to the dot density (the number of
dots horizontally and vertically) as some devices, such as printers do not
have the same resolution in both directions. Most normal laser printers are
300 dpi with 600 dpi ones now freely available. The NEC P-series 24-pin
printers are capable of 360 x 360 dpi, whilst some other printers may only be
capable of 360 x 180 dpi or 240 dpi.
DRI
Digital Research Inc.
Dragging
Moving an item on the Desktop by holding down the mouse button (usually the
left) whilst, at the same time, moving the mouse pointer. When the item is
dragged, a ghost outline of it appears to show its position. The act of
selecting an icon is 'grabbing' whilst its movement across the screen is
'dragging'.
DRAM
Dynamic Random Access Memory. Semiconductor memory that stores data as
electrical charges on tiny capacitors. When charged they represent a 1-bit,
when discharged they represent a 0-bit. When one of these capacitors is read
it is also discharged and has to be recharged again. This means that a
complete memory access cycle takes almost twice as long as the access time
shown on the chip. The data is available halfway through this cycle, of
course, but the memory chip won't be ready for another read until it finishes.
All this makes DRAM much slower than static RAM, but it remains considerably
cheaper. On top of this, each capacitor is so small that the current they hold
is tiny and leaks away so that it needs to be constantly topped up or
refreshed so that data is never lost. This is done by the MMU on the ST-Falcon
range every few milliseconds transparent to the CPU. See Static RAM, Wait
states.
Drawing program
A program that uses vectors and graphic objects to draw an image to the screen
instead of the individual manipulation of pixels as in a painting program.
Such a program is also called a vector graphics program or an object-oriented
graphics program. All graphic objects are described as mathematical formulae
based on vectors, such as circles, lines, boxes etc. The resulting file does
not contain a bitmap of the image but a list of all the procedures, routines
and measurements necessary to construct it on any compatible device, such as a
printer, plotter or screen. A GEM metafile consists of such a list. When a
vector image is zoomed or magnified, the image retains its detail and does not
exhibit the jagging effect of edges common to bitmap images. A line designated
as being one pixel thick will remain so at any magnification, a pattern that
fills a square will remain the same size. The resulting file is also very much
smaller than a comparable bit-image file as the list of instructions are coded
and are mostly calls to operating system routines, such as the VDI. Under GEM,
the GEM metafile is the most common format being produced by programs, such as
Easydraw, whilst all CAD programs produce similar output but not necessarily
to the GEM metafile format.
Drive
See Disk drive.
Driver
Software that performs a control or interfacing function, normally between a
program or operating system and some hardware unit, such as a printer, hard
disk or VDU. Such programs normally install themselves at boot-up, for
instance hard disk drivers, or when a program loads, such as a printer driver
for a word processor.
Drop
To miss out or delete. Also, to break a telephone communications link.
Drop-down menu
A list of options that appear below a menu heading when touched with the mouse
pointer as in GEM. The original WIMP environments, such as on the Mac, have
pull-down menus where the mouse has to be clicked to make the menu appear. It
is said that Digital Research used the drop down menu approach so as to make
GEM sufficiently different to the Apple environment to avoid litigation over
copyright.
Drum
A selenium coated cylinder, which is electrostatically charged by a laser beam
and onto which toner adheres which is then passed onto the paper as it is
rolled across the drum's surface. See Laser printer.
DS
Double-Sided. A disk that has both sides used for data storage.
DSP
Digital Signal Processor. A microprocessor which is designed to manipulate
digital signals at very high rates. The Falcon030 is noted for its 32 MHz
Motorola 56001 DSP which forms part of the sophisticated signal processing and
audio system of the Falcon and is designed to process fast Fourier transforms
in a very short time. The DSP is designed to handle digital sound devices and
modems at extremely fast speeds (a potential of 16 MIPS) as well as low cost
laser printers, video digitizers, scanners etc. It has 1,024 x 24 bits of on-
board RAM and 512 x 24 bits of ROM. Coupled to the DSP is 96 K of zero wait
state static RAM and its 24-bit data paths yield a dynamic range of 144 dB. An
external port (a 26-pin female DB) provides an interface for external
input/output devices for a wide variety of purposes.
Useful functions of DSPs include digital filtering, signal processing, data
processing, numeric processing, modulation, spectral analysis,
telecommunications, data communications, radio communications, computer
applications, image processing, graphics, instrumentation, speech processing,
audio signal processing, high-speed control, vibration analysis, medical
electronics, digital video, radar and sonar processing, and seismic
processing. Although this may demonstrate the power and versatility of DSP
chips, don't expect to see all of these applications on the Falcon as many
will require expensive, external hardware. See MC56001.
DSR
Data Set Ready. A signal from a modem (data set) to a DTE (computer) that it
is ready to operate.
DTA
Disk Transfer Address. The DTA is the address of a 44-byte buffer required for
various disk operations, such as searching for a file in a directory with the
GEMDOS function Fsfirst(). If the file is found, its details are placed into
the current DTA buffer. The structure of a DTA is the same as the structure of
a file's information:
typedef struct
{
char d_reserved[21]; /* reserved */
char d_attrib; /* file attributes */
short d_time; /* file time */
short d_date; /* file date */
long d_length; /* file length in bytes */
char d_fname[14]; /* file name */
} DTA;
DTE
Data Terminal Equipment. Computer, terminal or printer that can originate or
receive data in a data communications system.
DTP
DeskTop Publishing. The production of printed material, usually incorporating
graphics and a variety of text fonts and styles (i.e. not word processing), on
a microcomputer rather than using the traditional 'hot metal' approach. The
aim of DTP is to produce any printed material to any design that was only
available to traditional printers a few years ago. However, many word
processors are working in graphics modes, that is, they do not just produce
files that use the printer's default and built-in fonts line by line, but use
methods akin to DTP where the whole page is translated into a graphic image
and output to the printer in graphic mode, not text mode. The boundary between
DTP and document processing or word processing is increasingly becoming
unclear as more users adopt WIMP environments which suits the use of device
independent techniques associated with true DTP.
DTR
Data Terminal Ready. A signal from the DTE (computer) which is held high while
it is ready to operate. On seeing it go low (low voltage) the modem will stop
transmitting to it.
Dual purpose RAM
See ST RAM.
DUART
Dual Universal Asynchronous Receiver/Transmitter. A semiconductor device
containing two UART devices.
Dumb
Unintelligent. The term usually refers to computer terminals without memory or
their own processing capability. Although still found, intelligent micros are
replacing dumb terminals in networks and those attached to multi-user
computers because of their cheapness and versatility.
Dummy instruction
An instruction without meaning or purpose, a 'no operation' instruction.
Dump
To write the contents of specified storage locations to another location or
printer, such as writing the contents of the RAM memory to a disk file. It is
usually performed as an aid to bug hunting in software or hardware.
Duplex
The simultaneous data transmission in both directions between two points.
Duplicate
To make a copy on the same medium, such as to copy a disk. Also, to perform
the same operation.
DVI
Digital Video Interactive. A system for storing information on CD-ROM as
implemented by Intel.
DVI
DeVice Independent.
Dyadic
An operation concerning two entities or states.
Dynamic allocation
To meet the changing needs of a program's resources, especially memory needs,
during execution.
Dynamic memory allocation
The allocation of memory to a program whilst it is executing according to its
changing needs rather than allocating a block of memory to it before it runs,
which it may or may not require.
Dynamic memory
See DRAM.
D
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