GLOSSARY
$
This symbol in front of a number indicates that the number should be
interpreted as hexadecimal.
ANTIC
This is a separate microprocessor, contained within the ATARI 400/800
Computers, which is dedicated to the television display. ANTIC is user-
programmable with an instruction set, a program (the "display list"),
and data (the "display memory").
ATTRACT MODE
This is a feature provided by the operating system which, after nine
minutes without a key being pressed, cycles the colors on the screen
through random hues at lowered luminances. This ensures that a
computer left unattended for several hours doesn't burn a static image
into the television screen.
BACKGROUND
The area of the television screen display upon which player-missile
graphics objects or playfield objects and/or text are projected.
Background has its own user-definable color.
BCD
Acronym for Binary Coded Decimal. A numbering system in which
each number is broken into a sequence of decimal digits. These decimal
digits are then coded into binary, a task which requires four bits per
digit, and stored in the resultant form. In the ATARI Computer, two
such digits are stored in each 8-bit byte.
BORDER
In BASIC Mode 0, this is the area of the television screen display which
is formed by the four edges of the screen. The border takes background
color.
BRKKEY
A flag set when the OS senses that the BREAK key is typed.
BRKKEY'S normal value is $FF -- if it changes, then the BREAK key
has been pressed.
BYTE COUNT
This is the file pointer's position within a sector on diskette.
CASSETTE BOOT FILE
A standard or user-created file which boots from cassette at power-up
or SYSTEM RESET.
CHARACTER GRAPHICS
The technique of redefining the individual characters of a character set
to form graphics images instead of text characters.
CHARACTER IMAGE
The unique 8 X 8 pixel grid which defines a particular character's shape.
CHARACTER MODE
This is a specific type of ANTIC display mode which displays screen
display memory data bytes as characters, using a character set. There
are six ANTIC character modes, three of which are accessible from
BASIC.
CHARACTER NAME BYTE
A one-byte ANTIC display memory value which selects a unique
character within the current character set using the character's
sequential position in that set.
CHARACTER SET INDIRECTION
The technique of specifying to ANTIC a particular character set to be
used by placing that set's beginning page address into CHBAS.
CHBAS
The OS shadow location which ANTIC uses to find the current
character set which is to be used for character display modes. CHBAS
is at decimal address 756.
CHECKSUM
This is a single byte sum of all the bytes in a record (either disk I/O or
cassette I/O). For cassette I/O, this includes addition of the two marker
characters, computed with end-around carry.
CIO
Acronym for Central I/O system routine. CIO routes I/O control data to
the correct device handler and then passes control to the handler. CIO is
also the common entry point for most of the OS I/O functions.
COARSE SCROLLING
The process of altering the display list LMS (Load Memory Scan)
address bytes in order to vertically or horizontally scroll the screen
image, one byte at a time. This is accomplished by adding 1 to or
subtracting 1 from the LMS address bytes.
COLDSTART
Synonym for the power-up process which performs a series of system
database initializations when the computer power switch is turned on.
After coldstart, the system surrenders control to the user.
COLLISION
This occurs when a player or missile image coincides with another
image. There are 60 possible collisions and each one has a bit assigned
to it that can be checked. These bits are mapped into 16 registers in
CTIA (with only the lower 4 bits used).
COLOR
One of 128 values obtained from a hue-luminance combination which
is stored in a color register.
COLOR CLOCK
The standard unit of horizontal distance on the television screen. There
are 228 color clocks in a horizontal scan line, but only 160 are displayed
in a normal width playfield.
COLOR REGISTER
A hardware register (with corresponding OS shadow location) used to
define the color for various portions of the screen display. There are
nine color registers available on the ATARI Home Computer.
COLOR REGISTER INDIRECTION
The technique of specifying a particular color by pointing to its color
register rather than directly specifying it.
COLOR SIGNAL
This contains the color information which is combined with the primary
signal to form the modulating television signal. The color signal
oscillates at 3.579 Mhz.
COLRSH
A zero-page location ($4F) set up and updated by the OS during vertical
blank interrupts for ATTRACT mode processing. When ATTRACT
mode is in force, COLRSH is given a new random value every 4
seconds.
COMMAND
In BASIC, this is the first executable token of a BASIC statement that
tells BASIC to interpret the tokens that follow in a particular way.
CONSTANT
In BASIC, this is a 6-byte BCD value preceded by a special token. This
value remains unchanged throughout the program execution.
CONTROL BYTE
In cassette I/O, this is part of every record. It contains one of three
possible values.
CTIA
A television interface chip which is controlled primarily by ANTIC.
CTIA converts ANTIC's digital commands into a signal that is sent to
the television.
CURRENT STATEMENT
In BASIC, this is the current token within a line of the Statement Table.
CYCLE STEALING
This occurs when ANTIC halts 6502 processing in order to perform
DMA functions for memory refresh and screen display purposes.
CYCLIC ANIMATION
The technique of repetitively flipping through colors, graphics images,
or character graphics sets to animate screen images.
DCB
Acronym for Device Control Block. The DCB is used by the I/O
subsystem to communicate between the device handier and SIO.
DEVICE HANDLERS
Routines present in OS ROM which are called through CIO (as long as
the handler has an entry in HATABS) 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.
DISPLAY LIST
ANTIC's "program" defined by the user or provided automatically
(through a GRAPHICS command) by BASIC. 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 (if any) 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.
DISPLAY MODE
Either a BASIC or ANTIC methodology for interpreting text or map
data bytes in screen memory and displaying them on the screen. ANTIC
provides 15 display modes; BASIC, through the OS, supports only 9 of
these modes.
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 occurs 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-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.
DRKMSK
A zero-page ($4E) location set up and updated by the OS during vertical
blank interrupts for ATTRACT mode process color register's value.
This ensures a low luminance for ATTRACT mode.
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.
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.
EOL
In BASIC, "End-of-Line", a character with the value $9B.
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 and
vertical scrolling hardware registers must be used to fine scroll.
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 diskette.
FUNCTION
In BASIC, a token that when executed returns a value to the program.
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.
HATABS
The device handler entry point table which is used by CIO. HATABS is
located at $031A.
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 the HSCROL register.
HORIZONTAL SCROLLING
This is the process of sliding the screen window to the 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 scrolling and
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 nybble 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.
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 a nominal 600 baud (physical bits
per second). However, this rate is adjusted by SIO to account for drive
motor variations, stretched tape, etc.
INPUT LINE BUFFER
In BASIC, from $580 to $5FF.
INTER-RECORD GAP
For cassette I/O records, this consists of the Post-record Gap of a given
record followed by the Pre-record Write Tone of the next record.
I/O
Input/Output.
IOCB
Acronym for Input/Output Control 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.
KERNEL
A primitive software/hardware 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 graphics values for the
entire screen.
LINE
In BASIC, a line consists of one or more BASIC statements preceded
either by a line number in the range of 0 to 32767, or an immediate
mode line with no line number.
LOMEM
In BASIC, this is the pointer ([80,81] decimal) to a buffer used to
tokenize one line of code. The buffer is 256 bytes long, residing at the
end of the operating system's allocated RAM.
LSI
Acronym for Large Scale Integration. This refers to a technology for
manufacturing silicon chips. LSI chips are the largest and most
powerful chips in mass production; they contain many thousands of
components.
LUMINANCE
The lower nybble of a color register's color. There are eight even-
numbered values for luminance ($0 to $F, even values only) which in
combination with hue values produce the 128 colors available on the
ATARI 400/800 Computer.
MAP MODE
This is a specific type of ANTIC display mode using simple colored
screen pixels instead of characters for the screen display. There are
eight ANTIC map modes, with varying degrees of resolution. Six of
these are callable from BASIC.
MARK
For cassette I/O, this is a 5327-Hz frequency.
MARKER CHARACTER
For cassette I/O, this is a 55 (hex) value whose purpose is for adjusting
the baud rate. Including the start and stop bits, each marker character is
10 bits long.
MEMTOP
In BASIC, a pointer ([90,91] decimal) to the top of application RAM,
the end of the user program. Program expansion can occur from this
point to the end of free RAM, which is defined by the start of the display
list. This MEMTOP is not the same as the OS variable called
MEMTOP.
MISSILE
A one-dimensional image in RAM used in player-missile graphics
which is 2 bits wide. There is a maximum of four missiles, one for each
player.
MODE LINE
A collection of horizontal scan lines for screen displays. Depending
upon the BASIC or ANTIC display mode in effect, a mode line will be
composed of varying numbers of scan lines. By the same token,
depending upon the display mode, a screen image will be composed of
varying numbers of mode lines.
MONITOR
A program in ROM that handles both the system power-up and
SYSTEM RESET sequences.
NARROW PLAYFIELD
A screen display width option equal to a width of 128 color clocks.
NMI
Non-Maskable Interrupt (i.e., cannot be disabled by the 6502). The
Display List Interrupt and the Vertical Blank Interrupt are both NMIs.
These can be disabled with the ANTIC NMIEN register.
NMIEN
The Non-Maskable Interrupt Enable register which controls enabling of
various NMI interrupts such as the Display List Interrupt (DLI).
NORMAL IRG MODE
In cassette I/O, this is a mode where the tape always comes to a stop
after each record is read. If the computer stops the tape and gets its
processing done fast enough, then the next read may occur so quickly
that the cassette deck may see only a slight dip in the control line.
NORMAL PLAYFIELD
A screen display width option equal to a width of 160 color clocks.
OPERATOR
In BASIC, any one of the 46 tokens that in some way move or modify
the values that follow them.
OPERATOR STACK
In BASIC, a software stack where operators are placed when an
arithmetic BASIC expression is being evaluated.
OVERSCAN
The "spreading out" of a television image by the raster scan method of
display so that the edges of the picture are off the edge of the television
tube. This guarantees no unsightly borders in the television picture.
PIA
Acronym for Peripheral Interface Adaptor. This is an LSI chip which
interfaces the 6502 with external devices. The joystick pins of the four
user ports are connected to a PIA inside the computer.
PIXEL
The smallest screen graphics unit addressable in a particular display
mode. It is a square whose size depends on the display mode.
PLAYER
A one-dimensional RAM image used in player-missile graphics which
can be 128 bytes (double-line resolution) or 256 bytes (single-line
resolution) long. The player appears as a vertical band 8 pixels wide
stretching from the top of the screen to the bottom. There is a maximum
of four independent players.
PLAYER COLOR
The color of a player in player-missile graphics. Each of the four
independent players has its own color stored in its associated color
register.
PLAYER-MISSILE AREA
A RAM area that contains the images of the four players and four
missiles of player- missile graphics, as well as some extra RAM. The
player-missile area must be on a 1K boundary for single-line resolution
players or a 2K boundary for double-line resolution players.
PLAYER-MISSILE GRAPHICS
Atari's solution for simplifying animation by creating an image (a
player or missile) which is one-dimensional in RAM but two-
dimensional on the screen.
PLAYFIELD
The area of the screen which directly overlays the background of the
screen. Map graphics and/or text form this playfield.
PLAYFIELD ANIMATION
The technique of animating an object by moving its image bytes to new
locations in screen memory, and then erasing the bytes of the old image
before displaying the new image.
PMBAS
A register that points to the beginning of the player-missile area.
POKEY
A digital I/O chip that handles the serial I/O bus, audio generation,
keyboard scan-, and random number generation. POKEY also digitizes
the resistive paddle inputs and controls maskable Interrupt (IRQ)
requests.
POKEY TIMERS
These are hardware timers within POKEY. Unlike System Timers,
which are maintained by the OS software and are fixed, the POKEY
chip timers are clocked by frequencies set by the user.
POST-RECORD GAP
A pure mark tone frequency used as a post-record delimiter in cassette I/O.
PRE-RECORD WRITE TONE
A pure mark tone frequency used as a pre-record delimiter in cassette I/O.
PRIMARY SIGNAL
This contains the luminance information -- brightness data, horizontal
and vertical syncs and blanks -- of the modulated television signal.
PRIORITY-CONTROL REGISTER
Also known as PRIOR, and shadowed at GPRIOR. This register
specifies which playfield, player, or background images have priority in
the case of image overlaps during the screen display process.
RAM VECTOR
Alterable system vector that contains 2-byte addresses to system
routines, handler entry pointers, or to initialization routines. RAM
vectors are initialized at power-up and SYSTEM RESET.
RASTER SCAN
A television display system that uses an electron beam generated at the
rear of the television tube. The beam sweeps across the screen in a
regular left-to-right, top-to-bottom fashion.
RECORD
For diskette I/O, a group of bytes delimited by EOLs ($9B). For cassette
I/O, this is a group of 132 bytes which is composed of two marker
characters for cassette speed measurement, a control byte, 128 data
bytes, and the checksum byte.
RESIDENT DISK HANDLER
The fundamental software in the OS ROM containing the absolutely
essential disk handler routines. This software performs five important
low- level disk I/O functions such as FORMAT, READ SECTOR,
WRITE SECTOR, WRITE/VERIFY SECTOR, and STATUS.
ROM VECTOR
Unalterable system vector that contains JMP instructions to system
routines. The ROM vector allows a programmer to write software that
uses the OS routines without running the risk of the routines being made
unworkable by new releases of the OS ROM.
RTCLOK
One of the system timers which is 3 bytes in length and is updated
during immediate VBLANK. RTCLOK can be used as a reference
clock for an application program.
RUNSTK
In BASIC, a pointer ([8E,8F] decimal) to the Run Time Stack.
RUN TIME STACK
In BASIC, a software stack that contains GOSUB and FOR/NEXT
return address entries.
SCREEN MEMORY
A RAM area used by the 6502 to store bytes of data that will be fetched
(by DMA) by ANTIC to be interpreted and eventually displayed as
images on the screen.
SECTOR
On a diskette, this is a 128-byte physical area. The diskette contains 40
tracks with. 18 sectors per track.
SECTOR NUMBER
A value from 1 to 719 that specifies the sector to which the file pointer
is currently pointing.
SETVBV
A system routine that sets the system timers and sets user-definable
interrupt vector addresses without danger of crashes due to interrupts in
mid-process.
SHADOWING
A process in which values are moved between hardware locations and
RAM locations, thereby allowing the program to monitor the contents
of write-only hardware registers or check the inputs form read-only
hardware registers.
SHORT IRG MODE
In cassette I/O, this means the tape is not stopped between records. The
BASIC commands "CSAVE" and "CLOAD" both specify this mode.
SINGLE-LINE RESOLUTION
A unit of vertical resolution for a player in player-missile graphics.
Each player byte occupies one horizontal scan line on the screen, and
each player table is 256 bytes long.
SIO
Serial I/O system routine which handles communication between the
serial device handlers in the computer and devices on the serial bus
(cassette, printer, disk drive, and RS-232).
SIO INTERRUPTS
These are three IRQ interrupts used by SIO to send and receive serial
bus communications to serial bus devices. These three are VSERIR
(Serial Input Ready), VSEROR (Serial Output Needed), and VSEROC
(Transmission Finished).
SOUND REGISTER
Audio-producing hardware in the ATARI Home Computer System
which contains frequency, volume, and distortion information, but not
duration.
SPACE
For cassette I/O, this is a 3995-Hz frequency output to the cassette tape
as a delimiter in conjunction with mark tones.
STARP
In BASIC, the pointer ([8C,8D] decimal) to the String Array Area.
STATEMENT
In BASIC, this is a complete "sentence" of tokens that causes BASIC to
perform some meaningful task. In LIST form, statements are separated
by colons.
STATEMENT TABLE
In BASIC, this is a block of data that includes all the lines of code that
have been entered by the user and tokenized by BASIC. This table also
includes the immediate mode line.
STMCUR
In BASIC, the pointer ([8A,8B] decimal) to the current BASIC
statement.
STMTAB
In BASIC, this is the pointer ([88,89] decimal) to the Statement Table.
STRING ARRAY AREA
In BASIC, this block contains all the string and array data.
SYNC MARK
This is a 3995-Hz space frequency used as a sort of "end-of-record"
marker for audio tracks on the cassette. In applications software it is
useful for synchronizing the computer screen display with cassette
audio.
SYSTEM. DATABASE
This is an area that occupies RAM pages 0 through 4, containing many
locations that store information of importance to the operating system.
SYSTEM TIMER
A timer provided by the ATARI 400/800 Computers that runs at the
frequency of the television frame which for North American televisions
(NTSC) is 59.923334 Hz. European (PAL) televisions run at 50 Hz.
There are six system timers, and they are clocked as part of the vertical
blank process.
TELEVISION ARTIFACT
A pixel on an NTSC screen, one color clock wide, that contains color
not assigned by the computer. This color is derived from internal
oddities of color television displays. Artifacting is possible in ANTIC
modes 2,3, and 15 which correspond to BASIC modes 0, no mode, and 8.
TEXT WINDOW
On a screen display, this is a two-dimensional area set aside for
character displays.
TOKEN
In BASIC, an 8-bit byte containing a particular execution code.
TOKENIZING
In BASIC, this is the process of getting a line of ATASCII character
input and creating a series of 8-bit bytes which contain tokens,
meaningful execution codes.
VARIABLE
In BASIC, a token that is an indirect pointer to an entries in variable
tables that contain the variable name and the variable value.
VARIABLE NAME TABLE
In BASIC, this is the table containing a list of all the variable names that
have been entered in a program.
VARIABLE VALUE TABLE
In BASIC, this table contains the numerical value of each variable.
VBREAK
This is the 6502 BRK instruction IRQ vector. Whenever a $00 opcode
(the software break instruction) is executed, this interrupt occurs.
VBREAK normally points to an RTI instruction.
VCOUNT REGISTER
The ANTIC register which keeps track of which horizontal scan line
ANTIC is displaying.
VDSLST
This is the Display List Interrupt NMI vector located at [$0200,$0201].
VERTICAL BLANK
The period during which the electron beam (as it draws the screen
image) returns from the bottom of the screen to the top. This period is
about 1400 microseconds in duration.
VERTICAL BLANK INTERRUPT
A non-maskable interrupt which occurs every 60th of a second during
the vertical blank time of the television display. In responding to this
interrupt, the OS performs various housekeeping functions such as
shadowing color registers.
VERTICAL SCROLL ENABLE BIT
This is bit D5 of the ANTIC display list instruction byte which enables
vertical fine scrolling through VSCROL ($D405), the vertical fine
scroll register.
VERTICAL SCROLLING
The process of vertically "rolling" the display screen "window" over a
larger amount of screen data in display memory than can be displayed
by a static screen window. Both coarse and fine vertical scrolling are
available on the ATARI 400/800 Computers.
VIMIRQ
This is the immediate IRQ vector. All IRQs vector through this location.
VIMIRQ normally points to the IRQ handier. This vector can be
"stolen" to do user IRQ processing.
VINTER
This is the Peripheral Interrupt IRQ vector. The interrupt line is also
available on the serial bus. VINTER normally points to an RTI
instruction.
VKEYBD
This is the keyboard IRQ vector which is activated by pressing any key
except BREAK. This vector normally points to the OS's own keyboard
IRQ routine.
VNTD
In BASIC, this is the pointer ([84,85] decimal) to the Variable Name
Table Dummy end. BASIC uses this pointer to indicate the end of the
name table. This pointer normally points to a dummy zero byte when
there are less than 128 variables. When 128 variables are present, this
points to the last byte of the last variable name.
VNTP
In BASIC, the pointer ([82,83] decimal) to the Variable Name Table.
VPRCED
This is the Peripheral Proceed IRQ vector. The proceed line is available
to peripherals on the serial bus. This IRQ is unused at the present and
normally points to an RTI instruction.
VSCROL
This is the vertical fine scroll register located at $D405. Into VSCROL
the user stuffs the number of scan lines by which the screen line is to be
vertically scrolled.
VSERIN
This is the POKEY serial Input Ready IRQ vector.
VSEROR
This is the POKEY serial Output Ready IRQ vector.
VTIMR1
This is the POKEY timer 1 IRQ vector.
VTIMR2
This is the POKEY timer 2 IRQ vector.
VTIMR4
This is the POKEY timer 4 IRQ vector.
VVBLKD
This is the Vertical Blank Deferred NMI interrupt vector located at
[$0224,$0225].
VVBLKI
This is the Vertical Blank Immediate NMI interrupt vector located at
[$0222,$0223].
VVTP
In BASIC, this is the pointer ([86,87] decimal) to the Variable Value
Table.
WARMSTART
Another name for SYSTEM RESET routine. The warmstart initializes
most of the system vectors but does not check RAM size.
WIDE PLAYFIELD
A screen display width option equal to a width of 192 color clocks.
WSYNC
Wait for Horizontal Sync of the electron beam which is drawing the
screen image. The WSYNC register, when written to in any way, pulls
down the RDY line on the 6502 microprocessor, freezing the 6502 until
the electron beam drawing the screen image returns to the left edge of
the screen.
ZERO-PAGE
In the ATARI Home Computer System, this is the stretch of memory
which spans locations $0000 to $00FF.
ZIOCB
Zero-page I/O Control Block is used to communicate I/O control data
between CIO and the device handlers.