5 KiB
OCS Chipset — A500 / A1000 / A2000
Overview
The Original Chip Set (OCS) ships in the Amiga 1000 (1985), A500 (1987), and early A2000 boards. It consists of three custom chips: Agnus, Denise, and Paula, supported by the MOS 8520 CIA pair.
Chip Summary
| Chip | MOS Part | Primary Responsibilities |
|---|---|---|
| Agnus | 8361 (PAL), 8367 (NTSC) | DMA controller, Copper coprocessor, Blitter, address generation |
| Denise | 8362 | Display: bitplane fetch decode, sprite decode, color output |
| Paula | 8364 | Audio DMA (4 channels), floppy disk I/O, serial port, interrupts |
Contents
| File | Topic |
|---|---|
| chipset_ocs.md | Chip internals, DMA priorities, bus arbitration |
| custom_registers.md | Full OCS register map $DFF000–$DFF1FE |
| copper.md | Copper coprocessor: WAIT/SKIP/MOVE, copperlist format |
| blitter.md | Blitter: channels A/B/C/D, minterms, line mode, fill |
| paula_audio.md | Audio DMA: AUDxLCH/LCL/LEN/PER/VOL, interrupt |
| paula_serial.md | Serial port: SERPER/SERDATR, baud rate |
| sprites.md | Hardware sprites: SPRxPTH, control words, attach mode |
| cdtv_hardware.md | CDTV platform: DMAC, CD-ROM, IR remote, NVRAM |
OCS Machines — Per-Model Details
A1000 (1985) — Writable Control Store
The A1000 is the original Amiga. Its most distinctive feature is the Writable Control Store (WCS) — Kickstart is loaded from floppy into RAM at every cold boot, rather than residing in ROM.
| Feature | Details |
|---|---|
| CPU | 68000 @ 7.09 MHz |
| Chip RAM | 256 KB base (daughterboard adds 256 KB = 512 KB total) |
| Kickstart | Loaded from floppy into 256 KB WCS RAM at $F80000 |
| Bootstrap ROM | 256 bytes at $FC0000 — just enough to load Kickstart from floppy |
| Expansion | 86-pin sidecar bus (predecessor to Zorro) |
WCS boot process:
- Power on → bootstrap ROM displays "Insert Kickstart disk"
- User inserts Kickstart 1.x floppy
- Bootstrap loads 256 KB Kickstart image into WCS RAM
- Hardware write-protect latch activates → WCS becomes read-only
- System resets and boots from the now-protected WCS as if it were ROM
Note
The WCS is the reason the A1000 can run different Kickstart versions without swapping ROM chips. Third-party "Kickstart Eliminators" add actual ROM chips, bypassing the floppy-loading step entirely.
The A1000 daughterboard (256 KB Chip RAM expansion) sits inside the case on top of the motherboard. The 86-pin sidecar connector on the right side accepts external expansion chassis for memory, hard drives, and other peripherals.
A2000 (1987) — The Expandable Workhorse
The A2000 is the first "big-box" Amiga, designed for professional expansion. It shipped in two major variants:
| Variant | Board Revisions | Chipset | Chip RAM | Notes |
|---|---|---|---|---|
| A2000-A (German) | Rev 3.x | OCS (Agnus 8361) | 512 KB | Original design by Commodore Germany |
| A2000-B (US) | Rev 4.x | OCS (Fat Agnus 8370/8372) | 512 KB + 512 KB slow | Redesigned for US market |
| A2000-C | Rev 6.x | ECS (Super Agnus 8372A) | 1 MB Chip | Late production with ECS chips |
Expansion Architecture
| Slot Type | Count | Bus Width | Address Space | Description |
|---|---|---|---|---|
| Zorro II | 5 | 16-bit | $200000–$9FFFFF (8 MB) |
Autoconfig expansion cards |
| CPU slot | 1 | Direct 68000 | — | Directly wired to CPU socket — accepts accelerators (A2630, GVP G-Force) |
| Video slot | 1 | 36-pin | — | Internal video signals for genlocks and framebuffers |
| ISA (PC bridgeboard) | 2 | 8/16-bit | — | For A2088/A2286 PC compatibility cards |
The CPU slot is the A2000's most important expansion feature. Accelerator cards plug directly into the 68000 socket, replacing the CPU with 68020/030/040/060 processors. This makes the A2000 the most upgradeable classic Amiga.
Note
Late A2000 boards (rev 6+) shipped with ECS chips and are sometimes listed under ECS. Architecturally they remain A2000 boards with the same Zorro II bus and expansion layout.
CDTV (1991) — CD-ROM Set-Top Box
The CDTV is an A500-class OCS computer in a consumer set-top box form factor. See the dedicated article: CDTV Platform Hardware — covers the DMAC/WD33C93 SCSI CD-ROM controller, 512 KB Extended ROM, 64 KB NVRAM, infrared remote, and real-time clock.
OCS Limitations vs ECS/AGA
- Max 512 KB Chip RAM on A500 rev 5 and earlier (Agnus 8361/8367 addresses 512 KB only)
- A500 rev 6+ allows 1 MB with Fat Agnus (part of later OCS run)
- No productivity display modes (ECS adds BEAMCON0)
- 32 colors max (or 64 EHB, or HAM 12-bit) in standard bitplane modes
- Blitter is 16-bit; no 64-bit fetch (AGA adds FMODE)
- No ECS Denise border features
References
- ADCD 2.1 Hardware Manual: http://amigadev.elowar.com/read/ADCD_2.1/Hardware_Manual_guide/node0000.html
- Amiga Hardware Reference Manual 3rd ed., Chapter 5–8