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77 lines
2.9 KiB
Markdown
77 lines
2.9 KiB
Markdown
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[← Home](../../README.md) · [Hardware](../README.md) · [ECS](README.md)
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# 2 MB Chip RAM with Super Agnus
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## Overview
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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.
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## Requirements for 2 MB Chip RAM
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All of the following must be present:
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1. **Super Agnus 8372A** — 2 MB variant (not all 8372A chips support 2 MB; check marking)
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2. **2 MB of Chip RAM physically installed** — requires modified A3000 or a third-party board
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3. **OS 2.0 or later** — earlier OS does not manage the extended Chip RAM
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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.
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## Address Space Layout with 2 MB Chip RAM
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```
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$000000–$1FFFFF 2 MB Chip RAM (DMA accessible by all channels)
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$200000+ Fast RAM (CPU only)
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```
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The Chip RAM extends from $000000 to $1FFFFF. Previously, $100000–$1FFFFF was "ranger" slow RAM, not DMA-accessible.
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## OS Detection and Use
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AmigaOS automatically discovers Chip RAM size via the exec memory list:
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```c
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/* Check available Chip RAM */
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ULONG chip_free = AvailMem(MEMF_CHIP);
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ULONG chip_total = AvailMem(MEMF_CHIP | MEMF_TOTAL);
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```
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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.
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## AmigaOS ROM Initialisation (Exec init)
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During cold boot, the Kickstart ROM probes Chip RAM size:
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1. Write a test pattern to $100000 (top of 1 MB range)
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2. Read back — if the value matches, 2 MB Chip RAM is present
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3. The exec `MemHeader` for Chip RAM is extended to $1FFFFF
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This is performed in the `RomBoot()` → `InitCode()` sequence before the exec memory system is fully initialised.
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## Implications for Programming
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- **Bitplane pointers** can address any location in the 2 MB range
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- **Copper lists, sprite data, audio samples** can all use the upper 1 MB
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- `AllocMem(size, MEMF_CHIP)` will draw from the full 2 MB pool
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- **MEMF_24BITDMA** is set on Chip RAM to indicate DMA accessibility within the 24-bit space
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## Common Pitfall: 1 MB vs 2 MB Super Agnus
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Some Super Agnus chips (8372A rev 1) are hardware-limited to 1 MB despite the ECS part number. Identifying the 2 MB variant:
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```asm
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; Read the Agnus chip ID from VPOSR
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move.w $DFF004, d0 ; VPOSR
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and.w #$7F00, d0 ; mask to chip ID bits
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cmp.w #$2300, d0 ; 8372A 2MB = ID $23?
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beq .is_2mb_agnus
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```
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Software should not assume 2 MB Chip RAM — always use `AvailMem()` to determine the actual size.
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## References
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- Commodore A3000 Technical Reference Manual — memory section
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- AmigaMail Vol. 2 — Chip RAM expansion articles
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- NDK39: `exec/memory.h` — MEMF flags
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- ADCD 2.1 Hardware Manual — memory map section
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