amiga-bootcamp/01_hardware/aga_a1200_a4000/aga_sprites.md

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AGA Sprite Enhancements

Overview

AGA Lisa extends the ECS sprite hardware with substantial changes to width, color depth, and DMA data format. While the fundamental 8-channel architecture and position/control register layout remain OCS-compatible, AGA sprites can be 4× wider and draw from any 16-color slice of the 256-color palette.

What Changed from ECS

Feature	ECS (Denise 8373)	AGA (Lisa)	Register
Maximum width	16 pixels	16, 32, or 64 pixels	FMODE bits 15-14
Color palette	COLOR16–COLOR31 fixed	Any 16-color bank of 256	BPLCON4 bits 7-0
Even/odd bank	Same bank for pair	Independent even/odd banks	BPLCON4 ESPRM/OSPRM
Scan doubling	Not available	SSCAN2 for 31kHz displays	BPLCON0
Border sprites	BRDSPRT (inherited)	Same, over 256-color backgrounds	BPLCON3 bit 3

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FMODE — Sprite Fetch Width ($DFF1FC)

The FMODE register controls DMA fetch width for sprites independently of bitplanes and blitter. Bits 15-14 (SPR_FMODE) set the sprite width:

SPR_FMODE (bits 15-14)	Sprite Width	Words per Line	DMA Slots per Channel	Alignment
00	16 pixels	2 (DATA + DATB)	2	Word (2-byte)
01	32 pixels	4 (2 longwords)	4	Long (4-byte)
10	64 pixels	8 (4 longwords)	8	Quad (8-byte)
11	Reserved	—	—	—

; Enable 64-pixel wide sprites
    move.w  #$C000, $DFF1FC     ; FMODE: SPR_FMODE = %10 (64px)
                                ; (preserves BPL_FMODE and BLT_FMODE at 1×)

Warning

SPR_FMODE is a global setting — all 8 sprite channels use the same width. You cannot mix 16px and 64px sprites in the same display. The Copper can change FMODE mid-frame but this requires careful DMA timing.

DMA Data Format Changes

At wider FMODE settings, the sprite data block in memory grows proportionally:

16px (SPR_FMODE = 00) — OCS-compatible:

POS word | CTL word
DATA_A   | DATB_A          ← 1 pair per line (2 words)
...
$0000    | $0000            ← terminator

32px (SPR_FMODE = 01):

POS word | CTL word
DATA_A0  | DATA_A1 | DATB_A0 | DATB_A1    ← 2 pairs per line (4 words)
...
$0000    | $0000                            ← terminator

64px (SPR_FMODE = 10):

POS word | CTL word
DATA_A0  | DATA_A1 | DATA_A2 | DATA_A3 |
DATB_A0  | DATB_A1 | DATB_A2 | DATB_A3    ← 4 pairs per line (8 words)
...
$0000    | $0000                            ← terminator

Caution

Sprite data pointers (SPRxPTH/L) must be aligned to the fetch width boundary. Misaligned pointers produce corrupted sprite data — the DMA engine reads incorrect word boundaries.

SPR_FMODE	Required Pointer Alignment
1× (16px)	2-byte (word)
2× (32px)	4-byte (longword)
4× (64px)	8-byte (quadword)

DMA Budget Impact

Wider sprites consume proportionally more DMA bandwidth:

SPR_FMODE	DMA Slots (all 8 channels)	Bus Budget Impact
1× (16px)	16 slots/line	~7% of scanline
2× (32px)	32 slots/line	~14% of scanline
4× (64px)	64 slots/line	~28% of scanline

At 4× mode, sprite DMA alone consumes nearly a third of available bus bandwidth, leaving significantly less for bitplanes, blitter, and CPU.

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BPLCON4 — Sprite Color Bank Selection ($DFF10C)

AGA separates sprite color bank selection for even and odd channels, allowing each half of a sprite pair to draw from a different 16-color slice of the 256-color palette:

bits 7-4:  ESPRM7:4    Even sprite color bank (upper nibble of color index)
bits 3-0:  OSPRM7:4    Odd sprite color bank (upper nibble of color index)

Color Index Calculation

The final 8-bit color register index for a sprite pixel is:

Even sprite: color_index = {ESPRM[7:4], sprite_pixel[3:0]}
Odd sprite:  color_index = {OSPRM[7:4], sprite_pixel[3:0]}

ESPRM/OSPRM Value	Color Registers Used	Palette Slice
$0	COLOR0–COLOR15	Shares with bitplane colors!
$1	COLOR16–COLOR31	OCS-compatible default
$2	COLOR32–COLOR47	—
...	...	...
$F	COLOR240–COLOR255	—

; Even sprites use colors 64-79, odd sprites use colors 128-143
    move.w  #$0048, $DFF10C     ; BPLCON4: ESPRM = $4, OSPRM = $8

Warning

Setting ESPRM or OSPRM to $0 causes sprite colors to overlap with bitplane palette entries (COLOR0–COLOR15). This is rarely intentional and can produce confusing visual artifacts.

Attached Sprite Color Banks

In attached mode (15-color pairs), the 4-bit pixel index uses the odd channel's OSPRM value as the color bank, since the combined pixel index spans 0-15 within that bank.

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SSCAN2 — Sprite Scan Doubling

On AGA machines using 31kHz (VGA-compatible) display modes, the SSCAN2 mechanism doubles each sprite scanline vertically to maintain correct aspect ratio. This is handled automatically by the display system when using graphics.library screen modes — direct hardware programming requires manual configuration.

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Programming Example

Complete example: 64px wide sprite using color bank 3 (COLOR48–COLOR63):

; Prerequisites: AGA machine, system taken over, custom registers accessible

; 1. Set sprite fetch mode to 64px
    move.w  #$C000, $DFF1FC         ; FMODE: SPR_FMODE = 10 (64px)

; 2. Set color bank for sprite 0 (even channel)
    move.w  #$0031, $DFF10C         ; BPLCON4: ESPRM = $3, OSPRM = $1

; 3. Point sprite 0 to quad-word aligned data in Chip RAM
    move.l  #SpriteData, d0
    move.w  d0, SPR0PTL+$DFF000
    swap    d0
    move.w  d0, SPR0PTH+$DFF000

; 4. Enable sprite DMA
    move.w  #$8020, $DFF096         ; DMACON: SET + SPREN

; ... SpriteData must be 8-byte aligned and use 64px data format

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See Also

• OCS Sprite Hardware — base register reference (SPRxPOS/CTL/DATA, color mapping, position formula)
• ECS Sprite Enhancements — border sprites, independent resolution (SPRES)
• Sprites — Graphics Programming Guide — OS API, techniques, decision guides, antipatterns
• AGA Chipset Internals — Alice/Lisa overview, FMODE architecture
• AGA Register Deltas — complete BPLCON4 definition
• DMA Architecture — FMODE bandwidth impact, sprite DMA slot budget, bus arbitration
• AGA Palette — 256-register 24-bit color system

References

• Amiga Hardware Reference Manual 3rd ed. — AGA sprite extensions
• ADCD 2.1 Hardware Manual — AGA sprite chapter
• NDK39: hardware/custom.h — FMODE, BPLCON4 definitions
• Commodore A1200 Technical Reference Manual — Lisa sprite section