1.1) What is an Atari 8-bit computer?

Based in Silicon Valley in the USA, 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 game system.

For basic marketplace context, here are the release years of the 8-bit Atari
computers alongside release years of significant competing home/personal
computers.  Note: Market dynamics varied substantially around the world.
1977: Apple II, Commodore PET, Radio Shack TRS-80
1978: ARI Cybervision 2001
1979: Atari 400/800, Apple II Plus, Texas Instruments TI-99/4
1980: Radio Shack TRS-80 Model III, TRS-80 Color Computer, Sinclair ZX80
1981: Acorn BBC Micro, Commodore VIC-20, IBM PC, Osborne 1, Sinclair ZX81,
      Texas Instruments TI-99/4A
1982: Commodore 64, Kaypro II, Sinclair ZX Spectrum
1983: Atari 1200XL/600XL/800XL, Acorn Electron, Apple IIe, Coleco Adam, MSX,
      IBM PC XT, Radio Shack TRS-80 Model 4, TRS-80 Color Computer 2
1984: Amstrad CPC, Apple IIc, Apple Macintosh, IBM PCjr, IBM PC AT 
1985: Atari 65XE/130XE, Atari 520ST, Commodore 128, Commodore Amiga
1986: Atari 1040ST, Apple Macintosh Plus, Tandy Color Computer 3
1987: Atari 800XE, Acorn Archimedes, Atari Mega ST, Apple Macintosh II,
      Apple Macintosh SE, Commodore Amiga 500, IBM PS/2

1987 also saw the release of the Atari XE game system, competing alongside
Atari's own 7800 (released in 1986) and legacy 2600 (available since 1977)
systems, and competing directly against the leading home gaming consoles of
the time, the Nintendo Entertainment System and the Sega Master System.

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 (essentially) the same joystick/
controller hardware port.

While the 5200 controller ports are different, the 5200 hardware platform is
very closely related the 8-bit Atari computers, including use of the same
custom chipset (SALLY/ANTIC/GTIA/POKEY).  Differences in the 5200 include a
fixed 16KiB of RAM, fixed 32KiB ROM cartridge size, a physically different
cartridge port, and a much smaller 2KiB operating system.

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

6502 MPU: 
      MOS Technology MCS6502A or equivalent (most NTSC 400/800 machines)
      Atari SALLY (late NTSC 400/800, all PAL 400/800, and all XL/XE)

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

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

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

SCAN LINES per FRAME
      262 (NTSC machines)
      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     AtariOS     Display     Resolution       Colors
Mode #  Mode #   Mode #      Type       (full screen)    (Color Registers)
----------------------------------------------------------------------------
  2                0         Char         40 x 24           3 *
  3                -         Char         40 x 19           3 *
  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          3 *
  F       1 +      9         Map          80 x 192          16 or 8 **
  F       2 +     10         Map          80 x 192          9
  F       3 +     11         Map          80 x 192          16 ***
  + GTIA Modes require GTIA chip or FGTIA chip
    (With CTIA these modes are considered valid by the OS but do not work.)
++ Supported by the XL OS (not supported by the 400/800 OS)
  * 2 of the 3 available color registers share the same color/hue in this mode
** GTIA: 16 luminances of one color/hue
    FGTIA: 8 luminances of one color/hue
*** 16 colors/hues all sharing the same luminance

GRAPHICS INDIRECTION (COLOR REGISTERS AND CHARACTER SETS):
Nine Color Registers are provided:
- Background         (used in all graphics modes)
- Playfield 0-3      (at least one used in all graphics modes except GTIA 1&3)
- Player-Missile 0-3 (used in GTIA Mode 2; Player-Missiles discussed below)

Each color register holds both a color/hue setting and a luminance/brightness
setting.  In most graphics modes, each of the available color registers may
hold any one of 16 colors/hues combined with any one of 8 luminance/brightness
settings, for a total palette of 128 possible colors.

In GTIA Mode 1 only, with the GTIA chip only, the 16 available colors/hues can
each be combined with 16 different luminance/brightness settings, for a total
palette of 256 possible colors.

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

PLAYER-MISSILE GRAPHICS:
    The Atari term for "sprites" where a sprite is a graphical video display
    object handled independendly from the memory bitmap of the video display.

    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:
    Up to 5 distinct sounds can be produced simultaneously: four main voices
    plus the Console Speaker.

    Sound output is monaural/monophonic/single channel, except for the
    400/800 where Console Speaker sounds are output separately to a speaker
    inside the computer.

    The four main voices can be configured in one of the following three ways:
     - 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 frequencies/pitches and 2 voices with one of
       256 frequencies/pitches each

    Each of the main voices may produce 8 types of tones: pure tones (square
    wave type), or tones produced with one of 7 types of "noise" which is
    known as "distortion" on the Atari.

    Each of the main voices may be produced at one of 16 volumes.

    Direct control of the position of the speaker cone, known today as pulse-
    code modulation or PCM, is available at a bit depth of 4 bits, for a
    volume resolution of 16 possible values (4-bit PCM).  This is known as
    "Volume Control Only" or "Volume Only" sound on the Atari.

    The Console Speaker was intended only for system keyclick and buzzer, but
    it may also be programmed as 1-bit PCM.

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.

Atari 8-bit computers were supplied with the proprietary Atari Operating
System contained in Read Only Memory (ROM) as an integral feature of the
computer.  The Atari OS is described elsewhere in this FAQ List.
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