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

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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 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, A500/A600/A3000)

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

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

$000000–$1FFFFF  2 MB Chip RAM
$200000–$07FFFFFF Fast RAM (on-board: 4–16 MB via Ramsey on A4000)
                   Trapdoor/PCMCIA on A1200
$A00000–$BEFFFF  Zorro II I/O
$BFD000          CIA-B
$BFE001          CIA-A
$C00000–$CFFFFF  Slow RAM / board I/O
$D80000–$D8FFFF  IDE / Gayle (A1200/A4000)
$DA0000–$DA3FFF  PCMCIA attribute memory (A1200)
$DC0000          RTC
$DFF000–$DFFFFF  Custom registers
$E00000–$E7FFFF  Kick mirror / WCS
$F00000–$F7FFFF  Extended ROM
$F80000–$FFFFFF  Kickstart ROM (512 KB)
$01000000+       Zorro III expansion (32-bit, A3000/A4000 only)

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

block-beta
    columns 1
    block:chip["Chip RAM\n$000000–$1FFFFF\n(DMA accessible)"]
    block:fast["Fast RAM\n$200000–$9FFFFF\n(CPU only, faster)"]
    block:zio["Zorro II I/O\n$A00000–$BEFFFF"]
    block:cia["CIA-A/B\n$BFD000/$BFE001"]
    block:slow["Slow/Ranger RAM\n$C00000–$C7FFFF"]
    block:rtc["RTC $DC0000"]
    block:custom["Custom Registers\n$DFF000–$DFFFFF"]
    block:rom["Kickstart ROM\n$F80000–$FFFFFF"]

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)