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Amiga Hardware Models — Per-Model Architecture, Chip Inventory, and Expansion

Overview

The Amiga line spans nine years (1985–1994), three chipset generations, and four CPU families. While every model shares the same core custom-chip architecture — a DMA-driven display system with dedicated audio, blitter, and copper coprocessors — the implementation details vary dramatically. The A1000's 256 KB Chip RAM and side-car expansion bears little resemblance to the A4000's 32-bit Zorro III bus and on-board SCSI. Understanding these differences is essential for developers writing software that must run across the entire family, for reverse engineers analyzing per-model binaries, and for hardware restorers diagnosing chipset compatibility.

This article provides a per-model architectural breakdown: which custom chips are present, how memory is mapped, what expansion options exist, and which factory jumpers or revisions affect behavior. For the underlying memory-type concepts (Chip vs Fast vs Slow RAM), see memory_types.md. For address-space maps, see address_space.md.

Quick Reference Table

Model Year CPU MHz Chipset Chip RAM ROM Expansion
A1000 1985 68000 7.14 OCS 256 KB 256 KB Sidecar
A500 1987 68000 7.09 OCS 512 KB 256 KB Edge connector
A2000 1987 68000 7.14 OCS/ECS 512 KB–1 MB 256/512 KB Zorro II, ISA, CPU slot
A500+ 1991 68000 7.09 ECS 1 MB 512 KB Edge connector
A600 1992 68000 7.09 ECS 1 MB 512 KB PCMCIA, IDE, trapdoor
A3000 1990 68030 16/25 ECS 1–2 MB 512 KB Zorro III, ISA, SCSI
A1200 1992 68EC020 14.18 AGA 2 MB 512 KB PCMCIA, IDE, trapdoor
A4000 1992 68030/040 25 AGA 2 MB 512 KB Zorro III, IDE
A4000T 1994 68040/060 25 AGA 2 MB 512 KB Zorro III, SCSI
CDTV 1991 68000 7.09 OCS 1 MB 256/512 KB Internal A2000-compatible
CD32 1993 68EC020 14.18 AGA 2 MB 512 KB FMV slot (SX-1 expander)

Chipset Generations and Custom Chip Inventory

OCS (Original Chip Set) — 1985

Chip Part Number Function Notes
Agnus 8361 (NTSC) / 8367 (PAL) DMA scheduler, blitter, copper, disk 18-bit address (512 KB Chip RAM max)
Denise 8362 Display encoder, sprite DMA, mouse 320×200 to 640×400 interlace
Paula 8364 Audio, floppy, serial 4-channel DMA audio
Gary 5718 System glue, bus control Present on A500/A2000 only
Budgie — A1000-specific glue Replaced by Gary in later models

ECS (Enhanced Chip Set) — 1990

Chip Part Number Function Notes
Agnus 8372 (Fat) / 8372A (Super) DMA scheduler Fat: 1 MB address; Super: 2 MB address
Denise 8373 (ECS) Display encoder Supports 640×480 productivity modes
Paula 8364 (unchanged) Audio, floppy, serial Same as OCS
Gary 5718 System glue Unchanged from OCS

AGA (Advanced Graphics Architecture) — 1992

Chip Part Number Function Notes
Alice 8374 DMA scheduler, blitter (64-bit), copper 2 MB Chip RAM, 64-bit blitter fetches
Lisa 8375 Display encoder 24-bit color, HAM8, up to 1280×512
Paula 8364 (unchanged) Audio, floppy, serial Still 4-channel, 8-bit DMA audio
Akiko — CD32 only Chunky-to-planar converter, CD-ROM glue

Per-Model Deep Dives

A1000 (1985) — The Original

The A1000 is the progenitor. Its most distinctive feature is the Writable Control Store (WCS) — a 256 KB pseudo-ROM loaded from disk at boot. The Kickstart is not in a physical ROM chip; it is copied from a floppy into RAM that the CPU executes from at $FC0000.

graph TB
    CPU[68000 CPU<br/>7.09 MHz]
    Budgie[/Budgie<br/>Glue Logic/]
    Agnus{{Agnus 8361/8367<br/>DMA, Blitter, Copper}}
    Denise{{Denise 8362<br/>Display Encoder}}
    Paula{{Paula 8364<br/>Audio, Floppy, Serial}}
    WCS["WCS RAM<br/>256 KB Kickstart<br/>$FC0000"]
    CIAA[CIA-A 8520<br/>Keyboard, Parallel]
    CIAB[CIA-B 8520<br/>Mouse, Serial, RTC]
    ChipRAM["Chip RAM<br/>256/512 KB<br/>$000000"]
    SideSlot((Side Expansion<br/>86-pin))

    CPU -->|Address Bus| Budgie
    CPU -->|Data Bus D0-D15| Budgie
    Budgie -->|Chip Select| Agnus
    Budgie -->|Chip Select| Denise
    Budgie -->|Chip Select| Paula
    Budgie -->|Chip Select| WCS
    Budgie -->|Chip Select| CIAA
    Budgie -->|Chip Select| CIAB
    Budgie -->|Bus Control| SideSlot
    Agnus -->|DMA| ChipRAM
    Paula -->|Drive Ctrl| Floppy[Floppy Drive]
    CIAA -->|Serial| Keyboard[Keyboard<br/>Proprietary]
Spec Value
Chip RAM 256 KB stock, 512 KB max (internal expansion)
Kickstart Loaded from "Kickstart" disk to WCS; not socketed ROM
Expansion Side expansion (86-pin, similar to A500 but different)
Video 23-pin RGB analog + composite + monochrome
Keyboard External, proprietary interface

Developer note: The A1000's WCS means execbase->MaxLocMem reports less than 512 KB even with full expansion, because $FC0000–$FFFFFF is occupied by WCS. Software that probes for available memory rather than hardcoding addresses will run correctly.

A500 (1987) — The Icon

The best-selling Amiga model. Internally, it is an A1000 chipset repackaged into a keyboard case with a single floppy drive and a trapdoor expansion slot.

graph TB
    CPU[68000 CPU<br/>7.09 MHz]
    Gary[/Gary CSG 5719<br/>System Controller/]
    Agnus{{Agnus 8361/8367<br/>DMA, Blitter, Copper}}
    Denise{{Denise 8362<br/>Display Encoder}}
    Paula{{Paula 8364<br/>Audio, Floppy, Serial}}
    ROM["Kickstart ROM<br/>256 KB<br/>$F80000"]
    CIAA[CIA-A 8520<br/>Keyboard, Parallel]
    CIAB[CIA-B 8520<br/>Mouse, Serial]
    ChipRAM["Chip RAM<br/>512 KB / 1 MB<br/>$000000"]
    Trapdoor((Trapdoor<br/>86-pin))
    SideSlot((Side Expansion<br/>86-pin))

    CPU -->|Address Bus A1-A23| Gary
    CPU -->|Data Bus D0-D15| Gary
    Gary -->|Chip Select| Agnus
    Gary -->|Chip Select| Denise
    Gary -->|Chip Select| Paula
    Gary -->|/ROMCS| ROM
    Gary -->|Chip Select| CIAA
    Gary -->|Chip Select| CIAB
    Gary -->|Bus Ctrl| Trapdoor
    Gary -->|Bus Ctrl| SideSlot
    Agnus -->|DMA| ChipRAM
    Paula -->|Drive Ctrl| Floppy[Floppy Drive]
Spec Value
Chip RAM 512 KB stock, 1 MB max (trapdoor expansion)
Slow RAM 512 KB at $C00000 via trapdoor (Agnus-dependent)
Kickstart 256 KB ROM in socket; upgradeable to 512 KB ECS ROM
Trapdoor 86-pin: Chip RAM expansion, RTC option, 68000 relocator for accelerators
Side slot 86-pin: Zorro II-style expansion (A590 HD, etc.)

Key jumper/revision note: Early A500s (Rev 3–5) have the Agnus 8361 (18-bit address, 512 KB max Chip). Later revisions (Rev 6+) sometimes ship with Agnus 8372A (Super Fat Agnus), allowing 1 MB or 2 MB Chip RAM with modifications. The Denise chip is always 8362 (OCS) unless swapped by user.

A2000 (1987) — The Big Box

The A2000 is an A500 on a larger motherboard with five Zorro II slots, an ISA bridge, and a CPU slot. It was Commodore's attempt to capture the professional desktop market.

graph TB
    CPU[68000 CPU<br/>7.09 MHz]
    Gary[/Gary CSG 5719<br/>System Controller/]
    Buster[/Buster 5721<br/>Zorro II Arbiter/]
    Agnus{{Agnus 8361/8372<br/>DMA, Blitter, Copper}}
    Denise{{Denise 8362/8373<br/>Display Encoder}}
    Paula{{Paula 8364<br/>Audio, Floppy, Serial}}
    ROM["Kickstart ROM<br/>256/512 KB<br/>$F80000"]
    CIAA[CIA-A 8520]
    CIAB[CIA-B 8520]
    ChipRAM["Chip RAM<br/>512 KB-2 MB<br/>$000000"]
    CPUSlot((CPU Slot))
    ZorroII("5x Zorro II<br/>$200000-$9FFFFF")
    ISA((4x ISA Slots<br/>Passive Bridge))

    CPU -->|Address/Data| Gary
    Gary -->|Chip Select| Agnus
    Gary -->|Chip Select| Denise
    Gary -->|Chip Select| Paula
    Gary -->|/ROMCS| ROM
    Gary -->|Chip Select| CIAA
    Gary -->|Chip Select| CIAB
    Gary -->|Bus Grant| Buster
    Gary -->|Bus Ctrl| CPUSlot
    Buster -->|Arbitration| ZorroII
    ZorroII -->|Bridge| ISA
    Agnus -->|DMA| ChipRAM
    Paula -->|Drive Ctrl| Floppy[Floppy Drive]
Spec Value
Chip RAM 512 KB (early), 1 MB (Rev 6+ with Fat Agnus), up to 2 MB with Super Agnus mod
Fast RAM Zorro II cards up to 8 MB; CPU-slot accelerators up to 128 MB
Kickstart 256 KB (early), 512 KB (Rev 6+); socketed, easily swapped
Buster Zorro II bus arbiter chip; Rev 7–9 fix DMA bugs
ISA bridge Passive bridge; requires ISA card with Amiga driver

Revision history:

  • Rev 4.x: OCS, 512 KB Chip RAM, 256 KB ROM
  • Rev 6.x: ECS (Fat Agnus 8372), 1 MB Chip RAM, 512 KB ROM
  • Rev 6.5: Minor Buster fix
  • Rev 6.6: Final Commodore revision; best compatibility

Developer note: The A2000's Buster chip has known DMA arbitration bugs in early revisions. Zorro II cards that use DMA (e.g., GVP SCSI) may hang on Rev 4.x boards. Software cannot detect this; it is a hardware compatibility issue for users.

A500+ (1991) — ECS in a Budget Shell

Essentially an A500 with ECS chipset and 1 MB Chip RAM standard. It was a stopgap model between the A500 and A600.

graph TB
    CPU[68000 CPU<br/>7.09 MHz]
    Gary[/Gary CSG 5719<br/>System Controller/]
    Agnus{{Fat Agnus 8372<br/>DMA, Blitter, Copper}}
    Denise{{ECS Denise 8373<br/>Display Encoder}}
    Paula{{Paula 8364<br/>Audio, Floppy, Serial}}
    ROM["Kickstart ROM<br/>512 KB<br/>$F80000"]
    CIAA[CIA-A 8520]
    CIAB[CIA-B 8520]
    ChipRAM["Chip RAM<br/>1 MB / 2 MB<br/>$000000"]
    Trapdoor((Trapdoor<br/>86-pin))
    SideSlot((Side Expansion<br/>86-pin))

    CPU -->|Address/Data| Gary
    Gary -->|Chip Select| Agnus
    Gary -->|Chip Select| Denise
    Gary -->|Chip Select| Paula
    Gary -->|/ROMCS| ROM
    Gary -->|Chip Select| CIAA
    Gary -->|Chip Select| CIAB
    Gary -->|Bus Ctrl| Trapdoor
    Gary -->|Bus Ctrl| SideSlot
    Agnus -->|DMA| ChipRAM
    Paula -->|Drive Ctrl| Floppy[Floppy Drive]
Spec Value
Chip RAM 1 MB stock; 2 MB max with trapdoor mod (requires Agnus 8372A)
Kickstart 512 KB ROM (Kickstart 2.04)
Denise 8373 (ECS) — supports 640×480 productivity modes
Clock Battery-backed RTC standard (not optional as in A500)

The "2 MB Chip" mod: The A500+ can reach 2 MB Chip RAM by replacing the 8372 Fat Agnus with a 8372A Super Agnus and adding 1 MB of RAM chips on the motherboard. The Fat Agnus supports only 1 MB addressing; Super Agnus extends to 2 MB.

A600 (1992) — The Compact

Commodore's attempt at a portable-friendly Amiga. It removes the numeric keypad, uses a 2.5" hard drive, and adds PCMCIA and internal IDE.

graph TB
    CPU[68000 CPU<br/>7.09 MHz]
    Gayle["Gayle<br/>IDE + PCMCIA + Glue"]
    Agnus{{Super Agnus 8372A<br/>DMA, Blitter, Copper}}
    Denise{{ECS Denise 8373<br/>Display Encoder}}
    Paula{{Paula 8364<br/>Audio, Floppy, Serial}}
    ROM["Kickstart ROM<br/>512 KB<br/>$F80000"]
    CIAA[CIA-A 8520]
    CIAB[CIA-B 8520]
    ChipRAM["Chip RAM<br/>1 MB / 2 MB<br/>$000000"]
    IDE[(IDE Controller<br/>2.5-inch Internal)]
    PCMCIA("PCMCIA Type II<br/>$600000")
    Trapdoor((Trapdoor<br/>150-pin))

    CPU -->|Address/Data| Gayle
    Gayle -->|Chip Select| Agnus
    Gayle -->|Chip Select| Denise
    Gayle -->|Chip Select| Paula
    Gayle -->|/ROMCS| ROM
    Gayle -->|Chip Select| CIAA
    Gayle -->|Chip Select| CIAB
    Gayle -->|IDE Ctrl| IDE
    Gayle -->|Bus Ctrl| PCMCIA
    Gayle -->|Bus Ctrl| Trapdoor
    Agnus -->|DMA| ChipRAM
    Paula -->|Drive Ctrl| Floppy[Floppy Drive]
Spec Value
Chip RAM 1 MB stock; 2 MB max (trapdoor accelerator or Chip RAM expansion)
IDE Internal 2.5" IDE via Gayle chip; up to 2 devices
PCMCIA Type II slot; memory cards (up to 4 MB Fast RAM), Ethernet, modems
Gayle Replaces Gary; integrates IDE controller, PCMCIA interface, and memory decoding
Kickstart 512 KB ROM (Kickstart 2.05)

Developer note: The A600's Gayle chip has a known bug with IDE interrupt handling under heavy DMA load. Some CF card adapters require specific driver workarounds. PCMCIA memory cards map as Fast RAM at $600000 and can conflict with accelerator memory.

A3000 (1990) — The Professional

The first 32-bit Amiga with Zorro III, built-in SCSI, and a 68030 CPU. It was designed for the UNIX workstation market and is the most expandable ECS machine.

graph TB
    CPU[68030 CPU<br/>16 or 25 MHz<br/>+ 68882 FPU]
    FatGary[/Fat Gary CSG 5393<br/>32-bit System Controller/]
    Buster[/Super Buster<br/>Zorro III Arbiter/]
    Agnus{{Super Agnus 8372A<br/>DMA, Blitter, Copper}}
    Denise{{ECS Denise 8373<br/>Display Encoder}}
    Paula{{Paula 8364<br/>Audio, Floppy, Serial}}
    SDMAC[/SDMAC<br/>SCSI DMA/]
    SCSI[WD33C93A<br/>SCSI Controller]
    ROM["Kickstart ROM<br/>512 KB<br/>$F80000"]
    CIAA[CIA-A 8520]
    CIAB[CIA-B 8520]
    ChipRAM["Chip RAM<br/>1-2 MB<br/>$000000"]
    FastRAM["Fast RAM<br/>4-16 MB ZIP<br/>$08000000"]
    CPUSlot((CPU Slot))
    ZorroIII("4x Zorro III<br/>$100000-$7FFFFF")
    ISA((4x ISA Slots))

    CPU -->|Addr/Data<br/>32-bit| FatGary
    FatGary -->|Chip Select| Agnus
    FatGary -->|Chip Select| Denise
    FatGary -->|Chip Select| Paula
    FatGary -->|/ROMCS| ROM
    FatGary -->|Chip Select| CIAA
    FatGary -->|Chip Select| CIAB
    FatGary -->|/RAMSLOT| FastRAM
    FatGary -->|/SBIC| SCSI
    FatGary -->|"/FPUCS $E80000"| FPU[68882 FPU]
    FatGary -->|Bus Grant| Buster
    FatGary -->|Bus Ctrl| CPUSlot
    Buster -->|Arbitration| ZorroIII
    ZorroIII -->|Bridge| ISA
    SCSI -->|DMA Req| SDMAC
    SDMAC -->|DMA| FatGary
    Agnus -->|DMA| ChipRAM
    Paula -->|Drive Ctrl| Floppy[Floppy Drive]
Spec Value
Chip RAM 1 MB (early), 2 MB (later); fixed on-board
Fast RAM 4 MB standard, 16 MB max (ZIP chips on motherboard)
Zorro III 32-bit, up to 256 MB address space; backwards compatible with Zorro II
SCSI WD33C93A controller; DMA-capable; internal 50-pin + external DB25
Kickstart 512 KB ROM; unique "superkickstart" mode for ROM development
Video 23-pin RGB + monochrome; optional framebuffer card in Zorro III slot

The "Big Box" advantage: Unlike the A500/A600, the A3000 has no trapdoor. All expansion is via Zorro III slots or the CPU slot. The on-board Fast RAM uses ZIP (zig-zag inline package) DRAM, which is difficult to source today — many A3000 owners use Zorro III RAM cards instead.

A1200 (1992) — The Affordable AGA

The most popular AGA model. It packs Alice, Lisa, and 2 MB Chip RAM into a compact keyboard case similar to the A600.

graph TB
    CPU[68EC020 CPU<br/>14.18 MHz]
    Gayle["Gayle<br/>IDE + PCMCIA + Glue"]
    Alice{{Alice 8374<br/>DMA, Blitter 64-bit, Copper}}
    Lisa{{Lisa 8375<br/>Display Encoder<br/>24-bit Palette}}
    Paula{{Paula 8364<br/>Audio, Floppy, Serial}}
    ROM["Kickstart ROM<br/>512 KB<br/>$F80000"]
    CIAA[CIA-A 8520]
    CIAB[CIA-B 8520]
    ChipRAM["Chip RAM<br/>2 MB Fixed<br/>$000000"]
    IDE[(IDE Controller<br/>2.5-inch Internal)]
    PCMCIA("PCMCIA Type II<br/>$600000")
    Trapdoor((Trapdoor<br/>150-pin<br/>Full 68020 Bus))

    CPU -->|Address/Data<br/>32-bit| Gayle
    Gayle -->|Chip Select| Alice
    Gayle -->|Chip Select| Lisa
    Gayle -->|Chip Select| Paula
    Gayle -->|/ROMCS| ROM
    Gayle -->|Chip Select| CIAA
    Gayle -->|Chip Select| CIAB
    Gayle -->|IDE Ctrl| IDE
    Gayle -->|Bus Ctrl| PCMCIA
    Gayle -->|Bus Ctrl| Trapdoor
    Alice -->|DMA<br/>4x Fetch| ChipRAM
    Paula -->|Drive Ctrl| Floppy[Floppy Drive]
Spec Value
CPU 68EC020 @ 14.18 MHz (no MMU, no FPU)
Chip RAM 2 MB fixed; not expandable on motherboard
Fast RAM Trapdoor accelerators (up to 256 MB); PCMCIA memory (up to 4 MB)
IDE Gayle-based, same as A600; 2.5" internal + CF adapter popular
PCMCIA Type II; same caveats as A600 regarding memory mapping
Video 23-pin RGB (VGA-compatible scan rates) + composite; supports all AGA modes
Kickstart 512 KB ROM (Kickstart 3.0/3.1)

The 150-pin trapdoor: This connector carries the full 68020 bus, allowing accelerators to replace the CPU. Popular trapdoor cards include the Blizzard 1230/1240/1260 series, which provide 68030/040/060 CPUs, Fast RAM, and optional SCSI.

Developer note: The A1200's 68EC020 has no MMU. Software using virtual memory (e.g., Enforcer, GigaMem) requires an accelerator with a full 68030/040/060. The EC designation means the on-chip MMU is absent.

A4000 (1992) — The Desktop AGA

The A4000 is the big-box AGA counterpart to the A1200. It uses the same Alice/Lisa/Paula chipset but adds Zorro III slots, a desktop case, and either a 68030 or 68040 CPU.

graph TB
    CPU[68030/040 CPU<br/>25 MHz]
    Ramsey[/Ramsey<br/>DRAM Controller/]
    Budgie[/Budgie<br/>Bus Bridge/]
    Buster[/Super Buster<br/>Zorro III Arbiter/]
    Alice{{Alice 8374<br/>DMA, Blitter 64-bit, Copper}}
    Lisa{{Lisa 8375<br/>Display Encoder<br/>24-bit Palette}}
    Paula{{Paula 8364<br/>Audio, Floppy, Serial}}
    Gayle[Gayle<br/>IDE only]
    ROM["Kickstart ROM<br/>512 KB + Ext<br/>$F80000"]
    CIAA[CIA-A 8520]
    CIAB[CIA-B 8520]
    ChipRAM["Chip RAM<br/>2 MB Fixed<br/>$000000"]
    FastRAM["Fast RAM<br/>4-16 MB SIMM<br/>$07000000"]
    IDE[(IDE Controller<br/>3.5-inch Internal)]
    CPUSlot((CPU Slot))
    ZorroIII("5x Zorro III<br/>$100000-$7FFFFF")
    ISA((4x ISA Slots))

    CPU -->|Addr/Data<br/>32-bit| Ramsey
    Ramsey -->|DRAM Ctrl| FastRAM
    Ramsey -->|Bus Ctrl| Budgie
    Budgie -->|Chip Select| Alice
    Budgie -->|Chip Select| Lisa
    Budgie -->|Chip Select| Paula
    Budgie -->|/ROMCS| ROM
    Budgie -->|Chip Select| CIAA
    Budgie -->|Chip Select| CIAB
    Budgie -->|Chip Select| Gayle
    Budgie -->|Bus Grant| Buster
    Budgie -->|Bus Ctrl| CPUSlot
    Gayle -->|IDE Ctrl| IDE
    Buster -->|Arbitration| ZorroIII
    ZorroIII -->|Bridge| ISA
    Alice -->|DMA<br/>4x Fetch| ChipRAM
    Paula -->|Drive Ctrl| Floppy[Floppy Drive]
Spec Value
CPU 68030 @ 25 MHz (A4000/030) or 68040 @ 25 MHz (A4000/040)
Chip RAM 2 MB fixed
Fast RAM 4 MB (030 model) or 16 MB (040 model) on SIMM slots
IDE Gayle-based internal IDE; two 3.5" bays
Zorro III 5 slots; same 32-bit bus as A3000
Kickstart 512 KB ROM; some models include 512 KB extended ROM for SCSI boot
Video 23-pin RGB + monochrome; supports all AGA modes

A4000/040 note: The 68040 integrates CPU, FPU, MMU, and dual 4 KB caches onto a single 1.2-million-transistor die — a remarkable achievement for 1990. But Motorola had to cut somewhere. By contrast, the separate 68882 external FPU (available on the A3000 or A4000/030 via accelerator) implemented transcendental functions — FSIN, FCOS, FTAN, FASIN, FACOS, FATAN, FSINH, FCOSH, FTANH, FLOG2, FLOG10, FLOGN, FETOX, FTWOTOX, FTENTOX, plus FMOD/FREM — entirely in dedicated microcode ROM. The 68040 dropped the entire microcode ROM for these instructions to free silicon for the on-chip caches. Basic FPU operations (FADD, FSUB, FMUL, FDIV, FSQRT, FMOVE, FCMP) remain in hardware on both.

When the 68040 encounters a missing instruction, it raises a Line-F exception (vector $2C). The 68040.library (which AmigaOS loads automatically at boot when AttnFlags & AFF_68040) installs a trap handler that catches these exceptions and emulates the missing instructions in software using the hardware FADD/FMUL primitives. This works transparently — applications using FSIN() never know it's emulated — but it is 10–100× slower than the 68882's native microcode (~200 cycles hardware vs ~4,000 cycles emulated for FSIN). Code in performance-critical paths should use lookup tables or polynomial approximations rather than calling transcendental functions in tight loops.

Warning

68040.library is not optional. Without it, any FPU transcendental instruction causes an unhandled Line-F exception and an immediate Guru Meditation. The library must be present in LIBS: before any FPU-using software runs.

A4000T (1994) — The Tower

The final Commodore Amiga. A tower-case variant of the A4000 with SCSI replacing IDE as the primary storage interface.

graph TB
    CPU[68040/060 CPU<br/>25-50 MHz]
    FatGary[/Fat Gary CSG 5393<br/>32-bit System Controller/]
    Buster[/Super Buster<br/>Zorro III Arbiter/]
    Alice{{Alice 8374<br/>DMA, Blitter 64-bit, Copper}}
    Lisa{{Lisa 8375<br/>Display Encoder<br/>24-bit Palette}}
    Paula{{Paula 8364<br/>Audio, Floppy, Serial}}
    SCSI[NCR53C710<br/>SCSI-2 Controller]
    ROM["Kickstart ROM<br/>512 KB + Ext<br/>$F80000"]
    CIAA[CIA-A 8520]
    CIAB[CIA-B 8520]
    ChipRAM["Chip RAM<br/>2 MB Fixed<br/>$000000"]
    FastRAM["Fast RAM<br/>16 MB SIMM<br/>$08000000"]
    CPUSlot((CPU Slot))
    ZorroIII("5x Zorro III<br/>$100000-$7FFFFF")

    CPU -->|Addr/Data<br/>32-bit| FatGary
    FatGary -->|Chip Select| Alice
    FatGary -->|Chip Select| Lisa
    FatGary -->|Chip Select| Paula
    FatGary -->|/ROMCS| ROM
    FatGary -->|Chip Select| CIAA
    FatGary -->|Chip Select| CIAB
    FatGary -->|/RAMSLOT| FastRAM
    FatGary -->|/SCSICS| SCSI
    FatGary -->|"/FPUCS $E80000"| FPU[68882 FPU]
    FatGary -->|Bus Grant| Buster
    FatGary -->|Bus Ctrl| CPUSlot
    Buster -->|Arbitration| ZorroIII
    Alice -->|DMA<br/>4x Fetch| ChipRAM
    SCSI -->|DMA| Alice
    Paula -->|Drive Ctrl| Floppy[Floppy Drive]
Spec Value
CPU 68040 @ 25 MHz or 68060 @ 50 MHz (late production)
SCSI NCR53C710 SCSI-2 controller; DMA-capable; internal + external
IDE Not present as primary; some third-party tower conversions add IDE
Case Full tower; 6× 5.25" + 3× 3.5" bays
Kickstart 512 KB ROM + optional extended ROM for SCSI boot

Developer note: The 68060 revision of the A4000T is extremely rare. Most A4000T units are 68040-based. The 68060 requires 68060.library for FPU emulation and cache management, similar to the 68040.

CDTV (1991) — The Multimedia Pioneer

Commodore's CD-ROM-based multimedia console, based on A2000 hardware. It predates the CD32 and was aimed at the living room rather than the desktop.

graph TB
    CPU[68000 CPU<br/>7.09 MHz]
    Gary[/Gary CSG 5719<br/>System Controller/]
    Agnus{{Agnus 8361/8372<br/>DMA, Blitter, Copper}}
    Denise{{Denise 8362<br/>Display Encoder}}
    Paula{{Paula 8364<br/>Audio, Floppy, Serial}}
    DMAC[/DMAC<br/>CD-ROM DMA/]
    ROM[Kickstart ROM<br/>256/512 KB<br/>CDTV Extensions]
    CIAA[CIA-A 8520]
    CIAB[CIA-B 8520]
    ChipRAM["Chip RAM<br/>1 MB / 2 MB<br/>$000000"]
    CDROM[(CD-ROM Drive<br/>Caddy-Load)]
    NVRAM[NVRAM<br/>32 B Settings]
    IR[IR Receiver<br/>Remote Control]
    ExpSlot((Internal Expansion<br/>A2000-compatible))

    CPU -->|Address/Data| Gary
    Gary -->|Chip Select| Agnus
    Gary -->|Chip Select| Denise
    Gary -->|Chip Select| Paula
    Gary -->|/ROMCS| ROM
    Gary -->|Chip Select| CIAA
    Gary -->|Chip Select| CIAB
    Gary -->|Chip Select| DMAC
    Gary -->|Bus Ctrl| ExpSlot
    Agnus -->|DMA| ChipRAM
    DMAC -->|DMA| CDROM
    CIAA -.->|Serial| IR
    CIAB -.->|Serial| NVRAM
    Paula -->|Drive Ctrl| Floppy[Floppy Drive<br/>Optional External]
Spec Value
Chip RAM 1 MB stock; 2 MB with Super Agnus mod
CD-ROM Proprietary Commodore CD-ROM drive; caddy-loaded
NVRAM 32 bytes battery-backed RAM for settings
IR Infrared remote control; receiver on front panel
Kickstart Modified Kickstart with CDTV extensions
Expansion Internal A2000-compatible slots (memory, hard cards)

CD32 (1993) — The AGA Console

The world's first 32-bit game console. It is essentially an A1200 without a keyboard, with a CD-ROM drive, and the Akiko chip for chunky-to-planar conversion.

graph TB
    CPU[68EC020 CPU<br/>14.18 MHz]
    Alice{{Alice 8374<br/>DMA, Blitter 64-bit, Copper}}
    Lisa{{Lisa 8375<br/>Display Encoder<br/>24-bit Palette}}
    Paula{{Paula 8364<br/>Audio}}
    Akiko[/Akiko<br/>C2P + CD-ROM + NVRAM/]
    ROM["Kickstart ROM<br/>512 KB<br/>$F80000"]
    CIAA[CIA-A 8520]
    CIAB[CIA-B 8520]
    ChipRAM["Chip RAM<br/>2 MB Fixed<br/>$000000"]
    CDROM[(CD-ROM Drive<br/>2x Speed)]
    NVRAM[NVRAM<br/>1 KB Game Saves]
    FMVSlot((FMV Slot<br/>MPEG-1 Decoder))
    ExpPort((Expansion<br/>SX-1 / SX-32))

    CPU -->|Address/Data<br/>32-bit| Alice
    Alice -->|DMA| ChipRAM
    Alice -->|Video Ctrl| Lisa
    CPU -->|Data Bus| Akiko
    Akiko -->|C2P| ChipRAM
    Akiko -->|CD Ctrl| CDROM
    Akiko -.->|Serial| NVRAM
    CPU -->|Chip Select| Paula
    CPU -->|Chip Select| ROM
    CPU -->|Chip Select| CIAA
    CPU -->|Chip Select| CIAB
    CPU -->|Bus Ctrl| FMVSlot
    CPU -->|Bus Ctrl| ExpPort
Spec Value
Chip RAM 2 MB fixed
Akiko Chunky-to-planar converter (C2P); accelerates 3D rendering by ~4×
CD-ROM Double-speed; proprietary controller via Akiko
NVRAM 1 KB battery-backed for game saves
Expansion SX-1 or SX-32 add-on provides keyboard, floppy, IDE, and additional RAM
FMV Full Motion Video cartridge port for MPEG-1 decoding

Developer note: The CD32's Akiko C2P is a hardware chunky-to-planar converter that transforms 8-bit indexed pixels into Amiga bitplane format. Games using this (e.g., Doom, Gloom) achieve playable frame rates impossible on a stock A1200 without Akiko. See akiko_cd32.md for programming details.

CPU Feature Matrix

CPU Bus Address I-Cache D-Cache MMU FPU Models
68000 16-bit 24-bit — — — External 68881 A1000, A500, A500+, A600, A2000, CDTV
68EC020 32-bit 32-bit 256 B direct — — — A1200, CD32
68020 32-bit 32-bit 256 B direct — External 68851 External 68881/2 A1200 (accelerators)
68030 32-bit 32-bit 256 B 256 B On-chip External 68882 A3000, A4000/030
68040 32-bit 32-bit 4 KB 4-way 4 KB 4-way On-chip On-chip (partial) A4000/040, A4000T
68060 32-bit 32-bit 8 KB 4-way 8 KB 4-way On-chip On-chip (partial) A4000T (rare)

Note

68040 and 68060 have on-chip FPUs that omit transcendental instructions. AmigaOS provides 68040.library and 68060.library to trap the missing opcodes via Line-F emulation. See 68040_68060_libraries.md.

Kickstart ROM Evolution

OS Version ROM Size Part Models
1.0 / 1.1 256 KB WCS only A1000
1.2 / 1.3 256 KB Single A500, A2000, CDTV
2.04 512 KB Single A500+, A600, A3000
3.0 / 3.1 512 KB Single A1200, A4000, CD32
3.1 512 KB + 512 KB Ext Pair A4000 (with ext ROM for SCSI boot)

Chipset Detection at Runtime

Software can detect the chipset generation programmatically:

#include <graphics/gfxbase.h>
#include <proto/graphics.h>

struct GfxBase *GfxBase;

void DetectChipset(void)
{
    GfxBase = (struct GfxBase *)OpenLibrary("graphics.library", 0);
    if (!GfxBase) return;

    ULONG chipRev = GfxBase->ChipRevBits;

    if (chipRev & GFXF_AA_ALICE)       /* AGA detected */
    {
        /* Alice + Lisa present: AGA features available */
    }
    else if (chipRev & GFXF_HR_DENISE) /* ECS detected */
    {
        /* ECS Denise present */
    }
    else
    {
        /* OCS only */
    }

    /* Chip RAM size via exec */
    ULONG chipTotal = AvailMem(MEMF_CHIP | MEMF_TOTAL);
    /* 256 KB, 512 KB, 1 MB, or 2 MB */

    CloseLibrary((struct Library *)GfxBase);
}

Decision Guide: Which Model to Target?

Target Market Minimum Model Recommended Notes
Maximum compatibility A500 (OCS, 512 KB) A500 + 1 MB Chip Runs on every Amiga ever made
ECS features A500+ / A600 A600 1 MB Chip, productivity modes
AGA required A1200 A1200 + Fast RAM 2 MB Chip is baseline for AGA
Professional/desktop A3000 A4000 Zorro III, SCSI, big box
Console/CD-only CD32 CD32 Akiko C2P, no keyboard

Historical Context & Competitive Landscape

1985: The A1000 vs. Its Rivals

Platform CPU RAM Custom Chips Multitasking Price (1985)
Amiga 1000 68000 @ 7 MHz 256 KB OCS (Agnus/Denise/Paula) Preemptive (Exec) $1,295
Atari ST 68000 @ 8 MHz 512 KB None (Shifter = simple shift register) Cooperative (TOS) $799
Macintosh 68000 @ 7.8 MHz 128 KB None Cooperative $2,495
IBM PC/AT 80286 @ 6 MHz 256 KB None (ISA bus) None (MS-DOS) $3,995

The A1000's custom chipset was unprecedented. No competitor offered hardware sprites, DMA audio, blitter graphics, and a copper coprocessor in a single integrated design. The trade-off was complexity — developers had to learn the custom chips to extract performance.

1992: The AGA Generation

By 1992, the market had shifted:

Platform CPU RAM Graphics Audio Price
Amiga A1200 68EC020 @ 14 MHz 2 MB AGA (24-bit, HAM8) 4ch DMA $399
PC (VGA) 386SX @ 25 MHz 4 MB VGA 256-color Sound Blaster 8-bit $1,200+
Mac LC III 68030 @ 25 MHz 4 MB 512×384 16-bit 8-bit mono $1,349
Atari Falcon 68030 @ 16 MHz 1 MB VIDEL (16-bit) DSP 16-bit $799

The A1200 was aggressively priced but underpowered for its era. The 68EC020 at 14 MHz struggled against 386DX and 68030 systems. AGA added color depth but not raw fill-rate — the blitter remained fundamentally an OCS-era design. The CD32 console failed to find a market against the SNES and Mega Drive, which were cheaper and had larger game libraries.

Modern Analogies

Amiga Model Modern Equivalent Shared Concept
A1000 Original Macintosh 128K Groundbreaking but under-RAM'd; required expansion to be practical
A500 Commodore 64 / NES The "people's computer" — sold on price, expanded endlessly by users
A2000 IBM PC/AT with ISA slots Big-box expandability; professional positioning
A3000 SGI Indy / NeXTStation Professional workstation with UNIX aspirations
A1200 PlayStation 1 / 3DO Cutting-edge for its price class, but underpowered vs. PC
CD32 3DO / Philips CD-i Early CD-ROM console; failed due to software library, not hardware
A4000T Sun Ultra 1 Rare, expensive, professional tower; end of the line

Best Practices

  1. Target A500 (OCS, 512 KB) for maximum reach — every Amiga can run A500 software.
  2. Detect AGA at runtime — never assume; check GfxBase->ChipRevBits.
  3. Probe Chip RAM size — use AvailMem(MEMF_CHIP | MEMF_TOTAL) rather than hardcoding.
  4. Account for missing MMU/FPU — 68EC020 has neither; 68040 FPU is incomplete.
  5. Test on real hardware — emulation is accurate but timing-sensitive code (Copper, Blitter) behaves differently on real chips.
  6. Document minimum requirements — "Requires 1 MB Chip" or "AGA only" prevents user frustration.
  7. Respect expansion slot differences — A500 trapdoor ≠ A1200 trapdoor ≠ A600 trapdoor.

Pitfalls

1. Assuming All A1200s Have Fast RAM

The A1200 sold with 2 MB Chip RAM and no Fast RAM. Software that allocates large buffers with MEMF_ANY may exhaust Chip RAM on a stock A1200 even though the machine has 2 MB. Always check AvailMem() before large allocations.

2. The A600 PCMCIA Trap

PCMCIA memory on the A600/A1200 maps to $600000. Some accelerators also map Fast RAM to this region, causing a conflict that disables PCMCIA. If your software detects PCMCIA Fast RAM, warn the user about potential accelerator conflicts.

3. A4000/040 vs A4000/030 Differences

The A4000/040 has a 68040 with on-board FPU and MMU; the A4000/030 has a 68030 with external FPU optional. Software using FPU instructions will crash on a 030 unless it checks for FPU presence via execbase->AttnFlags & AFF_68040.

4. CD32 Without Akiko

Some third-party CD32 clones and emulators omit Akiko. Games that rely solely on Akiko C2P will fail. Always provide a software fallback path for C2P operations.

References