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Atari 8-Bit Computers: Frequently Asked Questions
Section - 1.1) What is an Atari 8-bit computer?

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Based in Silicon Valley in the U.S.A., the company known as Atari produced
a line of home computers from 1979 to 1992 often referred to collectively as
the "Atari 8-bits," the "8-bit Ataris," the "400/800/XL/XE series," etc.

The computers included the 400, 800, 1200XL, 600XL, 800XL, 65XE, 130XE, 800XE,
and the XE video game system.

Notable home computers that were introduced before the Atari 400/800:
1977: Apple II, Tandy Radio Shack TRS-80 (Model I), Commodore PET

Notable home computers that were introduced after the Atari 400/800:
1979: Texas Instruments TI-99/4
1980: Commodore VIC-20, TRS-80 Color Computer, Osborne 1
1981: Texas Instruments TI-99/4A, IBM PC, Sinclair ZX81 / TS 1000, BBC Micro
1982: Kaypro II, Sinclair ZX Spectrum, Commodore 64
1983: Coleco Adam, MSX
1984: Apple Macintosh, Amstrad CPC
1985: Atari ST, Commodore Amiga
1987: Acorn Archimedes

In marketing their computers to the public, Atari always had to contend with
their company history and reputation as a maker of video games.  While the
8-bit Atari computers in their heyday were technically quite comparable if not
superior in the worlds of home and business personal computing, they also live
up to the name "Atari" with a huge library of video games which were often
outstanding for their time.

The 8-bit Atari computers do not use the same cartridges or floppy disks as
any other Atari platforms, such as the 2600 Video Computer System (VCS), the
5200 SuperSystem, the 7800 ProSystem, or the ST/TT/Falcon computers.  All of
these but the 5200, however, do share the same joystick/controller hardware
port.

The 5200 SuperSystem is actually nearly identical to the 8-bit computers
internally, yet cartridges for the 5200 and the 8-bit computers cannot be
exchanged, primarily due to the physically different cartridge ports.

Here are some of the performance specifications of the 8-bit Atari computers:

CPU:
      6502B (most 400/800 machines), or
      Atari SALLY 6502 (late 400/800 machines and all XL/XE machines)

CPU CLOCK RATE:
      1.7897725 MHz (NTSC machines), or
      1.7734470 MHz (PAL/SECAM machines)

FRAME REFRESH RATE:
      59.94 Hz (NTSC machines), or
      49.86 Hz (PAL/SECAM machines)

MACHINE CYCLES per FRAME:
      29859 (NTSC machines) (1.7897725 MHz / 59.94 Hz), or
      35568 (PAL/SECAM machines) (1.7734470 MHz / 49.86 Hz)

SCAN LINES per FRAME
      262 (NTSC machines), or
      312 (PAL/SECAM machines)

MACHINE CYCLES per SCAN LINE
      114     (NTSC machines: 29859 cycles/frame / 262 lines/frame)
         (PAL/SECAM machines: 35568 cycles/frame / 312 lines/frame)

COLOR CLOCKS per MACHINE CYCLE
      2

COLOR CLOCKS per SCAN LINE
      228  (2 color clocks/machine cycle * 114 machine cycles/scan line)

MAXIMUM SCAN LINE WIDTH = "WIDE PLAYFIELD"
      176 color clocks

MAXIMUM RESOLUTION = GRAPHICS PIXEL
      0.5 color clock

MAXIMUM HORIZONTAL FRAME RESOLUTION
      352 pixels (176 color clocks / 0.5 color clock)

MAXIMUM VERTICAL FRAME RESOLUTION
      240 pixels (240 scan lines per frame)

GRAPHICS MODES:
ANTIC  GTIA    CIO/BASIC     Display     Resolution        Number of
Mode # Mode #  Graphics #    Type        (full screen)     Colors/Hues
----------------------------------------------------------------------
  2               0          Char         40 x 24            1 *
  3               -          Char         40 x 19            1 *
  4              12 ++       Char         40 x 24            5
  5              13 ++       Char         40 x 12            5
  6               1          Char         20 x 24            5
  7               2          Char         20 x 12            5
  8               3          Map          40 x 24            4
  9               4          Map          80 x 48            2
  A               5          Map          80 x 48            4
  B               6          Map         160 x 96            2
  C              14 ++       Map         160 x 192           2
  D               7          Map         160 x 96            4
  E              15 ++       Map         160 x 192           4
  F               8          Map         320 x 192           1 *
 +F       1       9          Map          80 x 192           1 **
 +F       2      10          Map          80 x 192           9
 +F       3      11          Map          80 x 192           16 ***
  * 1 Hue, 2 Luminances
 ** 1 Hue, 16 Luminances (GTIA); or, 1 Hue, 8 Luminances (FGTIA)
*** 16 Hues, 1 Luminance
  + require the GTIA/FGTIA chip. (1979-1981 400/800's shipped with CTIA.)
 ++ Not available via the BASIC GRAPHICS command in 400/800 version of OS.

(See a separate section in this FAQ list for a discussion of the "missing"
ANTIC Modes 0 and 1.)

GRAPHICS INDIRECTION (COLOR REGISTERS AND CHARACTER SETS):
Nine color registers are available.  Each color register holds any of 16
luminances x 16 hues = 256 colors.  (Four registers are for player-missile
graphics.

Character sets of 128 8x8 characters, each with a normal and an inverse
video incarnation, are totally redefinable.

PLAYER-MISSILE GRAPHICS:
    Four 8-bit wide, 120 or 240 byte high single color players, and four
    2-bit wide, 120 or 240 byte high single color missiles are available.
    A mode to combine the 4 missiles into a 5th 8-bit wide player is also
    available, as is a mode to OR colors or blacken out colors when players
    overlap (good for making three colors out of two players!)  Players
    and missiles have adjustable priority and collision detection.

DISPLAY LIST:
    Screen modes can be mixed (by lines) down the screen using the Display
    List - a program which is executed by the ANTIC graphics chip every
    screen refresh.

DISPLAY LIST INTERRUPTS (DLIs):
    Other screen attributes (color, player/missile horizontal position,
    screen width, player/missile/playfield priority, etc.) can be adjusted
    at any point down the screen via DLIs.

SCROLLING:
    Fine scrolling (both vertical and horizontal) can be enabled on any
    line on the screen.

SOUND:
    Sound is monaural/monophonic (one channel output).

    Up to 4 separate simultaneous voices can be produced, configured as one of
    the following:
     - 4 voices, each with one of 256 unique frequencies/pitches
     - 2 voices, each with one of 65,536 unique frequencies/pitches
     - 1 voice with one of 65,536 pitches and 2 voices with one of 256 pitches

    Each voice may be produced with one of 8 available "noise" settings/
    polynomial-counter combinations, commonly called "distortion" settings.
      (There are actually only 6 distinct combinations of 3 poly-counters
      offered, but one of the poly-counters has 2 available settings itself,
      resulting in 2 additional noise settings for the total of 8 available.)

    Each voice may be produced at one of 16 volumes.

    Direct control of the position of the speaker cone is also available, with
    4-bit (16 position) resolution.  Known as "volume only mode" on the Atari.

    A fifth "voice" is produced as a separate signal by the internal speaker
    on the Atari 400/800.  This is typically used only for keyclick and
    buzzer.  In XL/XE systems these sounds are output as part of the normal
    monaural audio output signal.

VERTICAL BLANK INTERRUPTS (VBIs):
    A software routine may be designed to execute as a VBI.  There are two
    varieties of VBI: Immediate and Deferred.  An Immediate VBI completes 
    execution within the vertical blank time, which is the time allotted for a
    CRT display to shut the electron beam off at the lower-right of the 
    display and reposition it back on the top-left of the display to commence
    drawing of the next frame.  A Deferred VBI routine completes execution
    between the initiation of one vertical blank and the next.

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Last Update March 27 2014 @ 02:11 PM