"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 139 entries EOF