mirror of
https://github.com/alfishe/amiga-bootcamp.git
synced 2026-06-12 16:16:28 +00:00
127 lines
3.8 KiB
Markdown
127 lines
3.8 KiB
Markdown
[← Home](../../README.md) · [Hardware](../README.md) · [AGA](README.md)
|
||
|
||
# AGA Chipset Internals — Alice & Lisa
|
||
|
||
## Alice (MOS 8374) — AGA Agnus
|
||
|
||
Alice is the successor to Super Agnus and is the DMA controller and Copper/Blitter engine for AGA machines.
|
||
|
||
### Key Enhancements over Super Agnus
|
||
|
||
**64-bit DMA fetch bus (FMODE):**
|
||
Alice can fetch 2 or 4 words per DMA cycle via the `FMODE` register ($DFF1FC). This dramatically increases the bandwidth available to the blitter and bitplane DMA.
|
||
|
||
**Extended bitplane depth:**
|
||
Alice supports up to **8 bitplanes** (256 colors), compared to OCS/ECS's 6-plane limit.
|
||
|
||
**BPLCON4:**
|
||
Alice adds `BPLCON4` to control bitplane bank selection — which 64-entry block of the 256-entry color table is used by the bitplanes.
|
||
|
||
### ALICE_ID
|
||
|
||
Alice can be identified via `VPOSR`:
|
||
```asm
|
||
move.w $DFF004, d0 ; VPOSR
|
||
lsr.w #8, d0
|
||
```
|
||
|
||
| VPOSR[15:8] | Chip |
|
||
|---|---|
|
||
| $22 | Alice AGA (standard) |
|
||
| $23 | Alice AGA (some A4000 revisions) |
|
||
|
||
---
|
||
|
||
## Lisa (AGA Denise)
|
||
|
||
Lisa is the display chip successor to ECS Denise, providing 8-bit color output (256 color registers) and extended sprite capabilities.
|
||
|
||
### Key Enhancements over ECS Denise
|
||
|
||
**256 color registers:**
|
||
Lisa provides COLOR00–COLOR255, each 24-bit (32-bit register with low byte unused).
|
||
|
||
**4 color banks for bitplanes:**
|
||
`BPLCON4` selects which 64-register bank (0–3) the bitplanes use for lookup. This allows dual-playfield each using a different 64-color palette.
|
||
|
||
**Sprite bank selection:**
|
||
`BPLCON3` bits select which color bank sprite pairs use.
|
||
|
||
**Extended sprite width:**
|
||
Sprites can be 16 or 64 pixels wide in AGA mode.
|
||
|
||
**Lisa ID:**
|
||
Readable from `$DFF07C` (DENISEID):
|
||
```asm
|
||
move.w $DFF07C, d0 ; DENISEID = $00F8 for AGA Lisa
|
||
```
|
||
|
||
---
|
||
|
||
## FMODE — DMA Fetch Width Register ($DFF1FC)
|
||
|
||
The most critical AGA-specific register. Controls the data bus width for blitter and bitplane DMA:
|
||
|
||
```
|
||
bits 15-14: SPR_FMODE — sprite fetch mode
|
||
bits 13-12: BPL_FMODE — bitplane fetch mode
|
||
bits 9-8: BLT_FMODE — blitter fetch mode
|
||
|
||
00 = 1× (16-bit, OCS/ECS compatible)
|
||
01 = 2× (32-bit)
|
||
10 = 4× (64-bit)
|
||
11 = reserved
|
||
```
|
||
|
||
**Setting full 64-bit blitter mode:**
|
||
```asm
|
||
move.w #$00C0, $DFF1FC ; FMODE: BLT_FMODE=11 (64-bit)
|
||
; Also set BPL and SPR modes as needed
|
||
```
|
||
|
||
> [!CAUTION]
|
||
> Writing FMODE on OCS/ECS machines writes to the `LISAID` location (read-only) — no hardware damage, but the read-back value is incorrect. Always verify AGA presence before writing FMODE.
|
||
|
||
---
|
||
|
||
## DMA Bandwidth with FMODE
|
||
|
||
| FMODE | Bus width | Blitter speed | Bitplane DMA |
|
||
|---|---|---|---|
|
||
| 1× (OCS compat) | 16-bit | 1× | 1× |
|
||
| 2× | 32-bit | 2× | 2× |
|
||
| 4× | 64-bit | 4× | 4× |
|
||
|
||
At 4× mode, the AGA blitter can fill/copy at ~70 MB/s theoretical on a 7 MHz bus.
|
||
|
||
---
|
||
|
||
## BPLCON0 Extended Bits (AGA)
|
||
|
||
In AGA mode, `BPLCON0` bit 4 (`ECSENA`) must be **1** to enable AGA features. Additional BPU bit (bit 4 of the count) allows 7 and 8 planes:
|
||
|
||
```
|
||
bits 14-12: BPU2-0 — lower 3 bits of bitplane count
|
||
bit 4: BPU3 — MSB of bitplane count (AGA: allows 7, 8 planes)
|
||
```
|
||
|
||
To use 8 bitplanes (256 colors):
|
||
```asm
|
||
move.w #$9411, BPLCON0+custom ; HIRES=1 (if needed), BPU=8 (BPU3=1, BPU2-0=000), ECSENA=1
|
||
```
|
||
|
||
---
|
||
|
||
## References
|
||
|
||
- ADCD 2.1 Hardware Manual — AGA chapter
|
||
- NDK39: `hardware/custom.h` — struct Custom (with AGA extensions)
|
||
- Commodore A1200 Technical Reference Manual — Alice/Lisa section
|
||
- AmigaMail Vol. 2 — AGA programming articles
|
||
|
||
## See Also
|
||
|
||
- [Akiko — CD32 Custom Chip](akiko_cd32.md) — CD32-exclusive ASIC (C2P, CD-ROM, NVRAM) that sits alongside Alice/Lisa/Paula
|
||
- [AGA Blitter](aga_blitter.md) — 64-bit FMODE blitter details
|
||
- [AGA Palette](aga_palette.md) — 256-register 24-bit color system
|
||
- [DMA Architecture](../common/dma_architecture.md) — scanline slot allocation, FMODE bandwidth impact, bus arbitration
|