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1.4) What is the internal layout of the 8-bit Atari?

The following text was written by Chris Crawford and appears in De Re
Atari (Atari#APX-90008), a book published and copyright by Atari, Inc.,
1981-1982.  It has been very slightly modified here for generality.

     "The internal layout of the Atari 8-bit computer is very different
from other systems.  It of course has a microprocessor (a 6502), RAM,
ROM, and a (PIA).  However, it also has three special-purpose (LSI) chips
known as ANTIC, GTIA, and POKEY.  These chips were designed by Atari
engineers primarily to take much of the burden of housekeeping off of the
6502, thereby freeing the 6502 to concentrate on computations.  While
they were at it, they designed a great deal of power into these chips.
Each of these chips is almost as big (in terms of silicon area) as a
6502, so the three of them together provide a tremendous amount of
power.  Mastering the Atari 8-bit computers is primarily a matter of
mastering these three chips.

     ANTIC ("Alpha-Numeric Television Interface Circuit") is a microprocessor
dedicated to the television display.  It is a true microprocessor; it has an
instruction set, a program (called the display list), and data.  The display
list and the display data are written into RAM by the 6502.  ANTIC retrieves
this information from RAM using direct memory access (DMA).  It processes the
higher level instructions in the display list and translates these instructions
into a real-time stream of simple instructions to GTIA.

      CTIA ("Color Television Interface Adapter") / GTIA ("George's Television
Interface Adapter") is a television interface chip.  ANTIC directly controls
most of GTIA's operations, but the 6502 can be programmed to intercede and
control some or all of GTIA's functions.  GTIA converts the digital
commands from ANTIC (or the 6502) into the signal that goes to the
television.  GTIA also adds some factors of its own, such as color
values, player-missle graphics, and collision detection.

     POKEY is a digital input/output (I/O) chip.  It handles such
disparate tasks as the serial I/O bus, audio generation, keyboard scan,
and random number generation.  It also digitizes the resistive paddle
inputs and controls maskable interrupt (IRQ) requests from peripherals.

     All four of these LSI chips function simultaneously.  Careful
separation of their functions in the design phase has minimized conflicts
between the chips.  The only hardware level conflict between any two
chips in the system occurs when ANTIC needs to use the address and data
buses to fetch its display information.  To do this, it halts the 6502
and takes control of the buses."

The 130XE and XEGS contain a small additional LSI called FREDDY, a RAM
address multiplexer.  According to mailto:sup8pdct@closer.brisnet.org.au
(James Bradford), "FREDDY is a type of memory controller.  It takes the
address and clock from the CPU and multiplexes it with the appropriate timings
and signals to use DYNAMIC memory.  FREDDY also buffers the system clock
crystal and divides it down then feeds that to GTIA.  The XEGS has a FREDDY
but it doesn't have the extended RAM.  Even if it did, you would still need
the chip that does the REAL bank switching.  It is a small 16-pin chip
(Atari/Best Electronics catalog number CO25953: rev9/page 42).  It gets RAS
from FREDDY, the bank select bits from PIA, A14, A15 and the 6502 halt signal
to control which bank of 8 chips RAS goes to.  A14 and A15 then go to FREDDY
for the address range of the extra memory bank (or normal address range with
no bank switching).  The ANTIC/6502 select bits in combination with the 6502
halt line, control the switching of the PIA bank number bits to A14/A15 and
which bank of memory RAS goes to.  Why people say FREDDY does the bank
switching is beyond me.  An 800XL can look like a 130XE with that 16-pin chip
installed (That's right NO FREDDY) and an extra 8 RAM chips."


Hardware Arrangement (With thanks to mailto:peter@soemtron.sb.sub.de (Peter))

                  ->
+---------------------------------------+
|            +------------+             |
|            | CPU(6502C) |         +-------+
|            +------------+      <- |  I/O- |
|                  |     +----------|release|
|                 +-+    |          +-------+
| +---------+<-   |p|    |               |
| |   MMU   |-----| |    | <-+---------+-|----------+----------+
*-| memory- |     |r|    *---|   PIA   | | (trigger)|Controller|====\
| |managment|-----|-+--------| (6520)  | |+---------|   Ports  |====/
| +---------+<-   |o| -> |   +---------+-|-+  <-->  +----------+
|                 | |    |               |||           |    |
|   +-----+       |c|    | <-+---------+ |||           |(lightpen)
|   | RAM |<-A/D  | |    *---|  ANTIC  | |||           |    |
*---|8-128|-------|e|----|---|(2nd CPU)|---------------+    |
|   |Kbyte|->D    | | -> |   +---------+ ||| +---------------
|   +-----+       |s|    |       ||      ||| |
|                 | |    | <-+---------+-|||--------+(screen)
|  +-------+      |s|    *---|  GTIA   |-|+| |      |
|  | Atari |<-A   | |----|---|  /CTIA  | | | | +----------+   +-----------+
|  | BASIC |------|o| -> |   +---------+ | | | | summary  |===| modulator |
*--|8 Kbyte|->D   | |    |               | | | |connection|===| ^^^^^^^^^ |
|  |  ROM  |      |r|    | <-+---------+ | | | +----------+   +-----------+
|  +-------+      | |    +---|  POKEY  |-|-|-+      |(sound)        |
|                 | |--------|         |-|-|--------+               |
|  +-------+      |b| ->     +---------+ | +----------+             |
|  |AtariOS|<-A   | |                 |  |            |             |
*--|10/16Kb|------|u|                 +--|----------+ |         tv/monitor
|  |  ROM  |->D   | +-----------------   |          | |         **********
|  +-------+      |s|              | |   |          | |
|                 | |              | |   |          | |
|                 +-+              +-+   |          | |
|                  |                |    |          | |
+--------------*---|------------*---|    |          | |
               |   |            |   |    |          | |
             +-----------+    +-----------+    +------------+
             |ParallelBus|    | Cartridge |    |   Serial   |
             |Interface/ |    |   Slot    |    |Input/Output|
             | Enhanced  |    |    ROM    |    |    (SIO)   |
             | Cartridge |    +-----------+    +------------+
             | Interface |          |                |
             +-----------+          |                |
                   |                |                |
        - memory expansion    -cartridge with   - disk drive
        - Z80 card             programs         - printer
        - 80 char card         (games , dos )   - modem

NOTES
 * RAM: 400/800:8/16/48K,1200XL/800XL/65XE/800XE/XEGS:64K,600XL:16K,130XE:128K
 * ROM: 400/800:10K OS, 1200XL:16K OS, all others:16K OS + 8K Atari BASIC
 * CPU: 400/800:6502, all others:6502C
 * 800 includes two Cartridge Slots, all others include one
 * early release 400/800 have CTIA instead of GTIA
 * 400/800 have 4 Controller Ports, all others have 2
 * PBI is on 600XL/800XL only
 * ECI is on 130XE/800XE only