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 initialization code, which queries Agnus's internal address counter.

AmigaOS ROM Initialization (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 MemHeader for Chip RAM is extended to $1FFFFF

This is performed in the RomBoot() → InitCode() sequence before the exec memory system is fully initialized.

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

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