amiga-bootcamp/01_hardware/common/address_space.md

← Home · Hardware

Amiga Address Space

Overview

The Amiga uses a 24-bit physical address bus on OCS/ECS machines (68000/68020 effective), giving 16 MB of addressable space.

AGA machines with 68030/040/060 and 32-bit-clean software can address the full 4 GB, but Chip RAM and custom registers remain in the lower 16 MB.

Memory Map — 24-bit (OCS/ECS: A1000, A500, A500+, A2000, A600, A3000, CDTV)

Address Range	Size	Region
$000000–$1FFFFF	2 MB max	Chip RAM (512 KB on OCS A500, 1–2 MB on ECS)
$200000–$9FFFFF	8 MB	Fast RAM (Zorro II autoconfig expansion)
$A00000–$BEFFFF	~2 MB	Zorro II I/O space
$BFD000–$BFDFFF	4 KB	CIA-B (8520 — keyboard, floppy motor, disk side select)
$BFE001–$BFE1FF	512 B	CIA-A (8520 — parallel port, serial flags, timers)
$C00000–$C7FFFF	512 KB	Slow RAM ("Ranger" — on Chip bus, not DMA-visible)
$C80000–$CFFFFF	512 KB	Zorro II expansion I/O (boards)
$D00000–$D7FFFF	512 KB	Zorro II expansion I/O
$D80000–$DBFFFF	256 KB	Reserved / board-specific
$DC0000–$DCFFFF	64 KB	Real-Time Clock (MSM6242B / RF5C01A)
$DD0000–$DEFFFF	128 KB	Reserved
$DF0000–$DFFFFF	64 KB	Custom chip registers ($DFF000–$DFF1FE)
$E00000–$E7FFFF	512 KB	Kick memory (WCS / Ranger slow RAM mirror)
$E80000–$EFFFFF	512 KB	Autoconfig space (Zorro II probe)
$F00000–$F7FFFF	512 KB	Extended Kickstart ROM (OS 3.1: second 256 KB)
$F80000–$FFFFFF	512 KB	Kickstart ROM (primary, mirrored at top of 16 MB)

Memory Map — 32-bit (AGA: A1200, A4000, CD32)

Address Range	Size	Region
$000000–$1FFFFF	2 MB	Chip RAM
$200000–$07FFFFFF	up to 126 MB	Fast RAM (on-board via Ramsey on A4000; trapdoor/PCMCIA on A1200)
$A00000–$BEFFFF	~2 MB	Zorro II I/O
$BFD000	—	CIA-B
$BFE001	—	CIA-A
$C00000–$CFFFFF	1 MB	Slow RAM / board I/O
$D80000–$D8FFFF	64 KB	IDE / Gayle (A1200/A4000)
$DA0000–$DA3FFF	16 KB	PCMCIA attribute memory (A1200)
$DC0000	—	Real-Time Clock
$DFF000–$DFFFFF	64 KB	Custom chip registers
$E00000–$E7FFFF	512 KB	Kick mirror / WCS
$F00000–$F7FFFF	512 KB	Extended Kickstart ROM
$F80000–$FFFFFF	512 KB	Kickstart ROM
$01000000+	up to 1.75 GB	Zorro III expansion (32-bit, A3000/A4000 only)

Per-Model Address Space Variations

The maps above show the common layout. Several models have unique regions:

A1000 — Writable Control Store (WCS)

Address Range	Size	Region
$F80000–$FFFFFF	512 KB	WCS RAM — Kickstart is loaded from floppy into this RAM at boot (A1000 has no ROM-based Kickstart)
$FC0000–$FFFFFF	256 KB	Bootstrap ROM (256-byte boot loader that loads Kickstart into WCS)

The A1000 is the only model where Kickstart lives in RAM, not ROM. If power is lost, Kickstart must be reloaded from the Kickstart floppy. The WCS is write-protected after loading via a hardware latch.

A2000 — Zorro II Bus and CPU Slot

Address Range	Size	Region
$200000–$9FFFFF	8 MB	Zorro II Fast RAM (5 expansion slots, autoconfig)
$E80000–$EFFFFF	512 KB	Autoconfig space — probed at boot for each Zorro II card
CPU slot	—	Directly wired 68000 socket — accepts accelerators (GVP G-Force, A2630)

The A2000 is the canonical "big-box" expandable Amiga. Its 5 Zorro II slots provide 8 MB of Fast RAM address space. Some later A2000 revisions (rev 6+) support Super Agnus for 2 MB Chip RAM.

CDTV — CD-ROM Controller and NVRAM

Address Range	Size	Region
$000000–$0FFFFF	1 MB	Chip RAM (stock, expandable to 2 MB with Super Agnus mod)
$E00000–$E3FFFF	256 KB	CDTV Extended ROM (CD filesystem, player software, DMAC)
$E40000–$E7FFFF	256 KB	CDTV Extended ROM (second bank)
$DC0000–$DC003F	64 B	Real-Time Clock (Oki MSM6242B)
$E90000–$E9FFFF	64 KB	DMAC (WD33C93 SCSI DMA controller for CD-ROM)
$F00000–$F3FFFF	256 KB	CDTV NVRAM (battery-backed, stores bookmarks and saves)

The CDTV is essentially an A500 with a CD-ROM drive, IR remote, and NVRAM in a consumer set-top box form factor. It uses the OCS chipset (original Agnus) with 1 MB Chip RAM. The DMAC at $E90000 handles DMA transfers between the CD-ROM's SCSI interface and memory.

CD32 — Akiko Chip and Flash ROM

Address Range	Size	Region
$000000–$1FFFFF	2 MB	Chip RAM (fixed)
$B80000–$B8FFFF	64 KB	Akiko custom chip (chunky-to-planar conversion, CD-ROM controller, NVRAM interface)
$DC0000–$DC003F	64 B	Real-Time Clock
$DFF000–$DFFFFF	64 KB	Custom chip registers (AGA — Alice/Lisa)
$E00000–$E7FFFF	512 KB	CD32 Extended ROM (CD filesystem, boot, CDDA player)
$F00000–$F7FFFF	512 KB	CD32 flash ROM (firmware, SysInfo)
$F80000–$FFFFFF	512 KB	Kickstart 3.1 ROM

The CD32's unique feature is the Akiko chip at $B80000, which provides:

• Chunky-to-Planar (C2P) conversion: a hardware DMA engine that converts linear 8-bit pixel arrays to planar bitplane format — the single most sought-after feature for game ports
• CD-ROM controller: handles the double-speed CD drive directly
• NVRAM interface: 1 KB battery-backed storage for game saves

Note

The CD32 has no Zorro slots, no CPU slot, no trapdoor connector, and no PCMCIA port. The only expansion path is the (rare) FMV module slot or the SX-1/SX-32 expansion unit that adds a keyboard port, IDE, and PCMCIA.

Memory Type Classification

AmigaOS classifies memory by access flags used in AllocMem():

MEMF Flag	Value	Description
MEMF_ANY	0	No constraint
MEMF_PUBLIC	1<<0	Accessible to all tasks and DMA
MEMF_CHIP	1<<1	Chip RAM — accessible to custom chips (DMA)
MEMF_FAST	1<<2	Fast RAM — CPU-only, no DMA, faster
MEMF_LOCAL	1<<8	Not mapped out (always present)
MEMF_24BITDMA	1<<9	Addressable within 24-bit space
MEMF_CLEAR	1<<16	Zero-fill before returning
MEMF_REVERSE	1<<17	Allocate from top of memory
MEMF_LARGEST	1<<18	Return size of largest free block
MEMF_TOTAL	1<<19	Return total memory of type

Chip RAM Requirement

Custom chip DMA can only access Chip RAM (MEMF_CHIP). This means:

• Graphics bitmaps rendered by Blitter/Copper must be in Chip RAM
• Audio sample data must be in Chip RAM
• Copper lists must be in Chip RAM
• Sprite data must be in Chip RAM

Fast RAM is CPU-only — generally used for code, non-DMA data structures, and stacks.

Diagram

flowchart TB
    chip["Chip RAM<br/>$000000–$1FFFFF<br/>(DMA accessible)"]
    fast["Fast RAM<br/>$200000–$9FFFFF<br/>(CPU only, faster)"]
    zio["Zorro II I/O<br/>$A00000–$BEFFFF"]
    cia["CIA-A/B<br/>$BFD000 / $BFE001"]
    slow["Slow / Ranger RAM<br/>$C00000–$C7FFFF"]
    custom["Custom Registers<br/>$DFF000–$DFFFFF"]
    rom["Kickstart ROM<br/>$F80000–$FFFFFF"]

    chip --- fast --- zio --- cia --- slow --- custom --- rom

    style chip fill:#2d6a4f,color:#fff
    style fast fill:#1b4332,color:#fff
    style slow fill:#774936,color:#fff
    style custom fill:#e63946,color:#fff
    style rom fill:#457b9d,color:#fff
    style zio fill:#264653,color:#fff
    style cia fill:#264653,color:#fff

Key Chip RAM Addresses

Address	Content
$000000–$000400	Exception vector table (copied from ROM)
$000004	SysBase pointer (exec library base)
$000100	Copper list scratch area (boot)
$000400–$001000	Reserved by OS
$001000+	Free Chip RAM (AvailMem result)

Warning

Writing to $000000–$000400 corrupts the exception table. Writing to $000004 corrupts SysBase. These addresses must never be allocated by user code; exec reserves them.

References

• NDK39: exec/memory.h — MEMF_ flag definitions
• ADCD 2.1 Hardware Manual: memory map chapter
• Commodore A1200/A4000 Technical Reference Manuals (local archive)
• See also: memory_types.md — Chip RAM vs Fast RAM vs Slow RAM, DMA accessibility, per-model configurations
• See also: chip_ram_expansion.md — 2 MB Chip RAM with Super Agnus
• See also: bus_architecture.md — Bus mechanics, register access patterns, cross-domain transfers