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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
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2 MB Chip RAM with Super Agnus
Overview
The original Agnus (8361/8367) could address only 512 KB of Chip RAM. The Fat Agnus (later OCS revision) addressed 1 MB. Super Agnus (8372A) extends the DMA address bus to 21 bits, allowing 2 MB of Chip RAM to be addressed by all DMA channels.
Requirements for 2 MB Chip RAM
All of the following must be present:
- Super Agnus 8372A — 2 MB variant (not all 8372A chips support 2 MB; check marking)
- 2 MB of Chip RAM physically installed — requires modified A3000 or a third-party board
- OS 2.0 or later — earlier OS does not manage the extended Chip RAM
On the A3000, 2 MB Chip RAM is the standard configuration. On A500/A2000 with Super Agnus, it requires a RAM expansion that adds 1 MB in the Chip RAM window.
Address Space Layout with 2 MB Chip RAM
$000000–$1FFFFF 2 MB Chip RAM (DMA accessible by all channels)
$200000+ Fast RAM (CPU only)
The Chip RAM extends from $000000 to $1FFFFF. Previously, $100000–$1FFFFF was "ranger" slow RAM, not DMA-accessible.
OS Detection and Use
AmigaOS automatically discovers Chip RAM size via the exec memory list:
/* Check available Chip RAM */
ULONG chip_free = AvailMem(MEMF_CHIP);
ULONG chip_total = AvailMem(MEMF_CHIP | MEMF_TOTAL);
The exec memory list is built at boot time from the chip RAM size detected by the ROM initialisation code, which queries Agnus's internal address counter.
AmigaOS ROM Initialisation (Exec init)
During cold boot, the Kickstart ROM probes Chip RAM size:
- Write a test pattern to $100000 (top of 1 MB range)
- Read back — if the value matches, 2 MB Chip RAM is present
- The exec
MemHeaderfor Chip RAM is extended to $1FFFFF
This is performed in the RomBoot() → InitCode() sequence before the exec memory system is fully initialised.
Implications for Programming
- Bitplane pointers can address any location in the 2 MB range
- Copper lists, sprite data, audio samples can all use the upper 1 MB
AllocMem(size, MEMF_CHIP)will draw from the full 2 MB pool- MEMF_24BITDMA is set on Chip RAM to indicate DMA accessibility within the 24-bit space
Common Pitfall: 1 MB vs 2 MB Super Agnus
Some Super Agnus chips (8372A rev 1) are hardware-limited to 1 MB despite the ECS part number. Identifying the 2 MB variant:
; Read the Agnus chip ID from VPOSR
move.w $DFF004, d0 ; VPOSR
and.w #$7F00, d0 ; mask to chip ID bits
cmp.w #$2300, d0 ; 8372A 2MB = ID $23?
beq .is_2mb_agnus
Software should not assume 2 MB Chip RAM — always use AvailMem() to determine the actual size.
References
- Commodore A3000 Technical Reference Manual — memory section
- AmigaMail Vol. 2 — Chip RAM expansion articles
- NDK39:
exec/memory.h— MEMF flags - ADCD 2.1 Hardware Manual — memory map section
- See also: memory_types.md — comprehensive Chip/Fast/Slow RAM comparison, per-model configurations