This is part two of the series called, "ROM's Glossary for the Atari." Since I will not likely have all the definitions for the Atari, I would very much appreciate hearing from whoever could add to my list.


    DCB: Acronym for Device Control Block. The DCB is used by the I/O subsystem to communicate between the device handler and SIO.
    Debug: This is where a program is tested to see if it works the way it should. If it doesn't, the programmer must go through it to find the "bug" and "de-bug it. "
    Default: A value for a parameter or variable which is supplied by a system if the user does not specify an explicit value.
    Default Mode: On the Atari, Gr. 0 is considered the default mode, since the computer returns to it after a program is finished.
    Device Handlers: Routines present in OS ROM which are called through ?hIO to communicate with particular devices. Currently supported are the display editor, the screen, the keyboard, the printer, and the cassette. More handlers can automatically boot in during power-up.
    Device Spec: A special HATABS code which specifies a particular I/O device.
    Diagonal Scrolling: This results from the combination of horizontal and vertical scrolling of the screen image. This is done by adding one to the X coordinate and one to the y coordinate or vice versa.
    Display List: The display list specifies where the screen data may be found, what display modes to use to interpret screen data, and what special display options should be implemented.
    Display List Interrupt: A special ANTIC display list instruction which interrupts the 6502 microprocessor during the drawing of the screen image, allowing the 6502 to change the screen parameters.
    DLI vector: This is a 2-byte vector (low byte, high byte) to the Display List Interrupt service routine. This vector is set by the user and is located at (512,513) decimal.
    DMA: Direct Memory Access. This happens when ANTIC halts the 6502 and takes control of the system buses to fetch an instruction or data byte from memory.
    DMACTL: The hardware register whose bit settings control the use of DMA by the ANTIC chip. This affects, among other things, player vertical resolution, and player-missile graphics enabling.
    DOS: Acronym for Disk Operating System which is an extension of the OS that allows the user to access disk drive mass storage as files.
    Double-Density: The technique used to store twice as much data on a magnetic storage medium.
    Double-Line Resolution: A unit of vertical resolution for a player in player-missile graphics. Each player byte occupies two horizontal scan lines on the screen, and each player table is 128 bytes long.
    DUP: Acronym for Disk Utility Package. DUP is a set of utilities for disk drive usage, familiarly seen as the DOS menu. DUP executes commands by calling FMS through CIO.
    Dummy Variable: A stand-in, which will be altered when replaced by an actual variable name or literal value.
    Dynamic Display List: This is an ANTIC display list which the 6502 changes during vertical blank periods, allowing for even greater flexibility in the screen display.
    Edit: To change the contents of a program or data files. Editors are classified as line-oriented editors if they work on one line at a time, and deal with text by line number.
    EOL: In BASIC, "End-of-Line," a character with value $9B or 155 decimal.
    Error Codes: A statement or code printed out or displayed on the screen by a program to let you know what is happening. Error messages tell you something about what went wrong.
    Expansion Board: A printed circuit board which accommodates components or cards that expand a computer. It is called an expansion chassis if it connects an additional cabinet to the system.
    File: In cassette I/O, this consists of a 20-second leader of the mark tone plus any number of data bytes, and end-of-file. In diskette I/O, this consists of a number of sectors linked by pointers (125 data bytes per sector).
    File Pointer: For diskette I/O, this is a value which indicates the current position in a file by specifying the Sector Number and the Byte Count. DOS keeps a file pointer for every file currently open.
    Fine Scrolling: The process of horizontally or vertically scrolling a screen image in color clock or scan line increments. The horizontal scrolling or vertical scrolling hardware registers must be used to fine scroll.
    Flag: A status indicator for a special condition. A flag is normally stored in a flip-flop or in a register. Typically, a microprocessor provides at least the following status flags: carry, zero, sign, overflow, and half-carry or auxiliary carry.
    FMS: File Manager System. FMS is a nonresident device handler which supports some special CIO functions.
    FONT: A collection of characters which constitutes a character set. These characters can be either text or graphics images.
    Foreground: Equivalent to playfield, the area of the screen which directly overlays the background of the screen. Foreground is formed by map displays and/or text.
    Format: A resident disk handler command that clears all the tracks on a diskette.
    FORTH: A programming language and operating system, FORTH is characterized by threaded code and postfix, or reverse Polish notation. FORTH is an expandable language; you can create new commands defined in terms of the existing commands, or in Machine language. One of the best Forths for the Atari is Valforth from Valpar International.
    Full Duplex: A communication technique which allows data to be transmitted and received simultaneously. Most modems require you to set your computer to half duplex.
    Function: In BASIC, a token that, when executed, returns a value to the program.
    Grid: The invisible "checkerboard" on the TV or monitor screen that's used for text or graphics. Grids are comprised of lines of boxes arranged in columns and in rows. Grids vary as to the total number of boxes in their respective columns and rows.
    Graphics Indirection: A special feature of the ATARI Computer which allows color register and character set generality by using indirect pointers to color and character set values.
    Half Duplex: A mode of communication in which data may be transmitted in only one direction at a time.
    Hard Copy: Compute output printed on paper.
    Hard Disk: A disk composed of a magnetic coating applied to a rigid substrate, such as aluminum or ceramic. The term is generally used in contrast to "soft" (floppy) disks which are flexible. Floppy disks are slower and have less storage capacity.
    High-Resolution Graphics: Resolution is the total number of pixels a computer uses to display a picture on screen. The higher the number of pixels, the higher the resolution; the higher the resolution. the more detail a picture can have.
    Horizontal Blank: This is the period during which the electron beam (as it draws the screen image) turns off and returns from the right edge of the screen to the left edge.
    Horizontal Position Register: A special register which contains a user-definable value for the horizontal position of a player in player-missile graphics. This value is measured in units of color clocks.
    Horizontal Scan Line: The fundamental unit of measurement of vertical distance on the screen. The scan line is formed by a single trace of the electron beam across the screen.
    Horizontal Scroll Enable Bit: This is bit D6 of the ANTIC display instruction, which enables horizontal scrolling through HSCROL register.
    Horizontal Scrolling: This is the process of sliding the screen window to left or right over display memory in order to display more information than could be seen with a static screen. Both coarse and fine horizontal are available.
    HSCROL: This is the horizontal fine scrolling register located at $D404, containing the number of color clocks by which a line is to be horizontally scrolled.
    HUE: The upper nibble value of a color register's color. There are 16 possible hues ($0 to $F) which in combination with a luminance value constitute distinct colors. Examples of hues are black, red, and gold.
    IC: Integrated circuit.
    Immediate Mode: In BASIC, the mode where the input line is not preceded by a line number. BASIC immediately executes the line.
    Input Baud Rate: For cassette I/O, this is assumed to be nominal 600 baud. However, this rate is adjusted by SIO to account for drive motor variations, stretched tape, etc.
    I/O: Input/Output.
    IOCB: Acronym for Input/Output Block. There are eight of these whose function is to communicate between the user program and CIO.
    IRQ: Maskable (can be enabled or disabled by the 6502) interrupts such as the Break Key IRQ.
    IRQEN: The write-only register that contains the IRQ enable/disable bits. IRQEN is shadowed at POKMSK.
    Joystick: A popular game aid with a stick, a base and fire button. Primarily used to maneuver game shapes and graphics. Long, hard use causes fatigue in the hands and wears out the joystick.
    Kernel: A primitive software/harware technique which consists of a 6502 program loop which is precisely timed to the display cycle of the television set. The kernel code monitors the VCOUNT register and consults a table of screen changes catalogued as a function of VCOUNT values so that the 6502 can arbitrarily control all graphic values for the entire screen.