mirror of https://github.com/alfishe/amiga-bootcamp.git synced 2026-06-13 00:26:28 +00:00

Ilia Sharin c4522ef2ed Add memory types article, expand blitter cookbook, update cross-references

New articles:
- 01_hardware/common/memory_types.md: comprehensive Chip/Fast/Slow RAM reference
  with DMA accessibility matrix, per-model configurations (A500-A4000),
  accelerator memory expansion (classic + modern), adaptive software behavior,
  pitfalls with impact analysis, FPGA/emulation notes
- AGENTS.md: documentation standards and methodology guidelines

Blitter programming (08_graphics/blitter_programming.md):
- Rewrote minterm truth table with narrative explanation and worked  example
- Added 7 advanced use cases with assembly/C code: shifted BOB, hardware scroll,
  area fill polygon, interleaved bitplane BOBs, double-buffered game loop,
  GUI window drag, tile map renderer
- Added Good/Bad Patterns section with 5 named antipatterns
- Added Practical Limitations table (10 constraints with workarounds)
- Expanded Common Minterms table with Description and Real-World Use Case columns

Cross-reference updates:
- Root README: added memory types to coverage, quick start, section index
- 01_hardware/README: updated common/ folder description
- address_space.md: linked to memory_types.md and chip_ram_expansion.md
- memory_management.md (exec): linked to hardware memory types reference
- bitmap.md, sprites.md, animation.md, audio.md: linked to memory_types.md
  explaining DMA Chip RAM requirement for each subsystem
- chip_ram_expansion.md: linked to comprehensive memory types article

2026-04-25 14:12:46 -04:00

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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)
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

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