Portions of this section are based on the "System Overview" Section, written
by Atari's Cris Crawford, of Atari's De Re Atari (Atari#APX-90008).  The full
text of De Re Atari: http://www.atariarchives.org/dere/

The internal layout of the Atari 8-bit computer is very different from other
systems.  It of course has a microprocessor (a 6502), random-access memory
(RAM), read-only memory (ROM), and a peripheral interface adapter (PIA,
CO12298/CO14795, a standard 6520).  However, it also has three special-purpose
large-scale integration (LSI) chips known as ANTIC, one of CTIA/GTIA/FGTIA,
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.


6502/SALLY  Central Processing Unit (CPU)  --  6502B (400/800,most):CO14377
==========                     SALLY 6502 (400/800,late)(XL/XE,all):CO14806
The Microprocessor Unit (MPU), typically (and less-precisely) described as the
Central Processing Unit (CPU), in most Atari 400/800 computers is a standard
40-pin 6502 microprocessor.  More specifically, most Atari 400/800 computers
use a 6502B, which is a standard 6502 rated for a maximum operating frequency
of 3 MHz.  The 6502 was designed by Chuck Peddle and Bill Mensch for 
MOS Technology in 1975.  In addition to MOS Technology, the 6502B has also
been produced by Synertek and Rockwell.

Late production 400/800 computers and all of the Atari XL/XE computer models
(plus the Atari 5200 and 7800 game systems) contain Atari's customized
version of the 6502 chip, known (eventually) as SALLY.  The innovation of the
SALLY 6502 is the addition of the HALT' signal on pin 35.  The SALLY 6502 also
has a second R/W' signal on pin 36 (in addition to pin 34).  Pins 35 and 36
are not connected on a standard 6502.

The Atari's second microprocessor, ANTIC, must routinely interrupt the 6502 in
order to utilize the processor bus for itself for direct memory access (DMA).
HALT' on the SALLY 6502 facilitates this system design.  Atari's earlier
implementation of the same functionality in the 400/800 with the standard 6502
requires a series of 4 additional chips that are unnecessary in computers
designed for the SALLY 6502.  

Note that before finally adopting the name SALLY, Atari briefly referred to
their customized version of the 6502 by the name, "6502C".  The Atari SALLY
"6502C" is not to be confused with the standard 6502C, which is a standard
6502 rated for a maximum operating frequency of 4 MHz.

6502.org "the 6502 microprocessor resource": http://www.6502.org/


ANTIC --  400/800/1200XL,NTSC:CO12296        400/800,PAL:CO14887
=====     600XL/800XL/XE,NTSC:CO21697          XL/XE,PAL:CO21698
(The XL/XE PAL ANTIC is also used in SECAM XL/XE machines.)
ANTIC ("AlphaNumeric Television Interface Controller" --FD100001 Rev.02 p.1-8)
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 CTIA/GTIA/FGTIA.

ANTIC(NTSC) C012296 techical documentation by Atari: 
http://www.retromicro.com/files/atari/8bit/antic.pdf


CTIA/GTIA/FGTIA  --  CTIA(NTSC):CO12295       GTIA,PAL:CO14889
===============       GTIA,NTSC:CO14805   FGTIA(SECAM):CO20120
CTIA = "Color Television Interface Adaptor" --FD100001 Rev.02 p.1-10
GTIA = "Graphics Television Interface Adaptor" --FD100001 Rev.02 p.1-10
FGTIA = "French Graphics Television Interface Adaptor" (mc's guess)

The CTIA, GTIA, or FGTIA is the television interface chip.  ANTIC directly
controls most of the operations of the CTIA/GTIA/FGTIA, although the 6502 can
also be programmed to intercede and control some or all of the functions of
the CTIA/GTIA/FGTIA.  The CTIA/GTIA/FGTIA converts the digital commands
from ANTIC (or the 6502) into the video signal output.

In addition to its basic television/video interface function, the
CTIA/GTIA/FGTIA performs color-luminance control for the entire video signal,
player-missile control, and both priority control and collision detection
among player-missiles and the background.  The CTIA/GTIA/FGTIA also reads the
controller port trigger inputs, it reads console keys (Start/Select/Option),
and it controls the built-in speaker in the 400/800.

Early North American NTSC 400/800 models shipped with CTIA.  Later NTSC
400/800 models, all PAL 400/800's, and all NTSC XL/XE and PAL XL/XE systems
include GTIA.  SECAM 800XL, 130XE and XE game systems include FGTIA.

The GTIA is backwards-compatible with the CTIA, with the GTIA simply making
available three additional graphics modes (GTIA Modes 1-3).

Jerry Jessop adds:
  "The very first proto systems did have the GTIA, but it had some
  problems and was not released in the consumer version until 1981.  The
  GTIA was completed before the CTIA."

The FGTIA is software compatible with the GTIA.  However, in GTIA Mode 1 the
FGTIA can only display 8 distinct luminances, compared to the 16 distinct
luminances that can be displayed in GTIA Mode 1 by the GTIA.

The NTSC CTIA/GTIA were designed to interface with the NTSC ANTIC.
The PAL GTIA and the FGTIA were designed to interface with the PAL ANTIC.

Whether CTIA or GTIA/FGTIA is installed can be determined by observing what
happens as a result of trying to enter a GTIA graphics mode.  In Atari BASIC,
at the "READY" prompt, type POKE 623,64 [RETURN].  If the screen blackens, you
have the GTIA or FGTIA chip.  If it stays blue, you have the early CTIA chip.

Technical documentation by Atari:
GTIA(NTSC) C014805: http://www.retromicro.com/files/atari/8bit/gtia.pdf
FGTIA:
http://ftp.pigwa.net/stuff/collections/nir_dary_cds/Tech%2520Info/FGTIA.PDF


POKEY  --  CO12294
=====
POKEY (name derived from POtentiometer and KEYboard) 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.

POKEY Technical documentation by Atari:
http://www.retromicro.com/files/atari/8bit/pokey.pdf


FREDDIE  --  800XL(late),XE(all):CO61922/CO61991
=======
According to Atari's design specification, the "Freddie RAM" Memory Control
Unit (MPU) is a custom LSI chip providing dynamic RAM (DRAM) control
functions.  It replaces a number of small-scale integration (SSI) and medium-
scale integration (MSI) transistor-transistor logic (TTL) parts, including a
custom delay line.  FREDDIE multiplexes 16-bit RAM addresses from the
processor bus into 8-bit row and 8-bit column addresses for direct use in the
DRAM, and it generates row and column DRAM address timing strobes.

FREDDIE was initially designed by Atari Inc. in 1983 as chip that would cut
production costs for future XL computers.  FREDDIE was finally incorporated by
Atari Corp. into late-production 800XL computers and in all XE computers
systems.

"FREDDIE" or "FREDDY"?
Atari technical documentation consistently uses "FREDDIE" while Atari end-user
documentation (Owner's Manuals for all XE systems) consistently uses "FREDDY."
This FAQ List adopts the original convention from Atari's technical
documentation: "FREDDIE"

FREDDIE technical documentation by Atari:
www.atarimuseum.com/ahs_archives/archives/pdf/computers/8bits/freddie-mcu.pdf