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:

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):

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:

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):

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