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:
1.**Super Agnus 8372A** — 2 MB variant (not all 8372A chips support 2 MB; check marking)
2.**2 MB of Chip RAM physically installed** — requires modified A3000 or a third-party board
3.**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:
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:
1. Write a test pattern to $100000 (top of 1 MB range)
2. Read back — if the value matches, 2 MB Chip RAM is present
3. 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 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:
```asm
; 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.
