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110 lines
4.1 KiB
Markdown
110 lines
4.1 KiB
Markdown
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[← Home](../../README.md) · [Hardware](../README.md)
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# Amiga Address Space
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## Overview
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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.
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## Memory Map — 24-bit (OCS/ECS, A500/A600/A3000)
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```
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$000000–$1FFFFF Chip RAM (max 2 MB on ECS, 512 KB on OCS A500)
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$200000–$9FFFFF Fast RAM (expansion via Zorro II autoconfig)
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$A00000–$BEFFFF Zorro II I/O space
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$BFD000–$BFDFFF CIA-B (8520, keyboard, floppy motor, disk side)
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$BFE001–$BFE1FF CIA-A (8520, parallel port, serial flags, timer)
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$C00000–$C7FFFF Slow RAM ("Ranger", DMA-visible but not fast)
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$C80000–$CFFFFF Zorro II expansion I/O (boards)
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$D00000–$D7FFFF Zorro II expansion I/O
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$D80000–$DBFFFF Reserved / board-specific
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$DC0000–$DCFFFF Real-Time Clock (MSM6242B / RF5C01A)
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$DD0000–$DEFFFF Reserved
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$DF0000–$DFFFFF Custom chip registers ($DFF000–$DFF1FE)
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$E00000–$E7FFFF Kick memory (WCS / Ranger slow RAM mirror)
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$E80000–$EFFFFF Autoconfig space (Zorro II probe)
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$F00000–$F7FFFF Extended Kickstart ROM (OS 3.1: second 256 KB)
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$F80000–$FFFFFF Kickstart ROM (512 KB mirror at top of 16 MB)
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```
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## Memory Map — 32-bit (AGA, A1200/A4000)
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```
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$000000–$1FFFFF 2 MB Chip RAM
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$200000–$07FFFFFF Fast RAM (on-board: 4–16 MB via Ramsey on A4000)
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Trapdoor/PCMCIA on A1200
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$A00000–$BEFFFF Zorro II I/O
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$BFD000 CIA-B
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$BFE001 CIA-A
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$C00000–$CFFFFF Slow RAM / board I/O
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$D80000–$D8FFFF IDE / Gayle (A1200/A4000)
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$DA0000–$DA3FFF PCMCIA attribute memory (A1200)
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$DC0000 RTC
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$DFF000–$DFFFFF Custom registers
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$E00000–$E7FFFF Kick mirror / WCS
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$F00000–$F7FFFF Extended ROM
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$F80000–$FFFFFF Kickstart ROM (512 KB)
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$01000000+ Zorro III expansion (32-bit, A3000/A4000 only)
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```
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## Memory Type Classification
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AmigaOS classifies memory by access flags used in `AllocMem()`:
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| MEMF Flag | Value | Description |
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|---|---|---|
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| `MEMF_ANY` | 0 | No constraint |
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| `MEMF_PUBLIC` | 1<<0 | Accessible to all tasks and DMA |
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| `MEMF_CHIP` | 1<<1 | Chip RAM — accessible to custom chips (DMA) |
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| `MEMF_FAST` | 1<<2 | Fast RAM — CPU-only, no DMA, faster |
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| `MEMF_LOCAL` | 1<<8 | Not mapped out (always present) |
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| `MEMF_24BITDMA` | 1<<9 | Addressable within 24-bit space |
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| `MEMF_CLEAR` | 1<<16 | Zero-fill before returning |
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| `MEMF_REVERSE` | 1<<17 | Allocate from top of memory |
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| `MEMF_LARGEST` | 1<<18 | Return size of largest free block |
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| `MEMF_TOTAL` | 1<<19 | Return total memory of type |
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### Chip RAM Requirement
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Custom chip DMA can only access **Chip RAM** (`MEMF_CHIP`). This means:
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- Graphics bitmaps rendered by Blitter/Copper must be in Chip RAM
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- Audio sample data must be in Chip RAM
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- Copper lists must be in Chip RAM
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- Sprite data must be in Chip RAM
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Fast RAM is **CPU-only** — generally used for code, non-DMA data structures, and stacks.
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## Diagram
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```mermaid
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block-beta
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columns 1
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block:chip["Chip RAM\n$000000–$1FFFFF\n(DMA accessible)"]
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block:fast["Fast RAM\n$200000–$9FFFFF\n(CPU only, faster)"]
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block:zio["Zorro II I/O\n$A00000–$BEFFFF"]
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block:cia["CIA-A/B\n$BFD000/$BFE001"]
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block:slow["Slow/Ranger RAM\n$C00000–$C7FFFF"]
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block:rtc["RTC $DC0000"]
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block:custom["Custom Registers\n$DFF000–$DFFFFF"]
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block:rom["Kickstart ROM\n$F80000–$FFFFFF"]
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```
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## Key Chip RAM Addresses
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| Address | Content |
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|---|---|
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| $000000–$000400 | Exception vector table (copied from ROM) |
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| $000004 | `SysBase` pointer (exec library base) |
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| $000100 | Copper list scratch area (boot) |
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| $000400–$001000 | Reserved by OS |
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| $001000+ | Free Chip RAM (AvailMem result) |
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> [!WARNING]
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> 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.
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## References
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- NDK39: `exec/memory.h` — MEMF_ flag definitions
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- ADCD 2.1 Hardware Manual: memory map chapter
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- Commodore A1200/A4000 Technical Reference Manuals (local archive)
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