ASCII art by Thomas Havemeister.

                  ->
+---------------------------------------+
|          +---------------+            |
|          |CPU/SALLY(6502)|        +-------+
|          +---------------+     <- |  I/O- |
|                  |     +----------|release|
|                 +-+    |          +-------+
| +---------+<-   |p|    |               |
| |   MMU   |-----| |    | <-+---------+-|----------+----------+
*-| memory- |     |r|    *---|   PIA   | | (trigger)|Controller|====\
| |managment|-----|-+--------| (6520)  | |+---------|   Ports  |====/
| +---------+<-   |o| -> |   +---------+-|-+  <-->  +----------+
|                 | |    |               |||           |    |
|   +-----+       |c|    | <-+---------+ |||           |(light pen/light gun)
|   | RAM |<-A/D  | |    *---|  ANTIC  | |||           |    |
*---|8-128|-------|e|----|---|(2nd CPU)|---------------+    |
|   |Kbyte|->D    | | -> |   +---------+ ||| +--------------+
|   +-----+       |s|    |       ||      ||| |
|                 | |    | <-+---------+-|||--------+(screen)
|  +-------+      |s|    *---|CTIA/GTIA|-|+| |      |
|  | Atari |<-A   | |----|---| /FGTIA  | | | | +----------+   +-----------+
|  | 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: 8K or 16K standard
        800: 8K, 16K, or 48K standard
        600XL: 16K
        1200XL/800XL/65XE/800XE/XEgs: 64K
        130XE: 128K
 * ROM: 400/800: 10K (OS)
        1200XL: 16K (OS) 
        XEgs: 32K (16K OS + 8K Atari BASIC + 8K Missile Command)
        all other XL/XE: 24K (16K OS + 8K Atari BASIC)
 * CPU: 400/800(most): 6502B
        400/800(late),XL/XE: Atari SALLY 6502
 * 800 includes two Cartridge Slots, all others include one
 * CTIA/GTIA/FGTIA: Most: GTIA.  Early 400/800: CTIA.  SECAM XL/XE: FGTIA
 * 400/800 have 4 Controller Ports, all others have 2
 * PBI is on 600XL/800XL only
 * ECI is on 130XE/800XE/later 65XE only
 * Some late XE's use a 68B21 for PIA; PIA is 6520/6520A on all others