AGA Palette & Color System

Overview

AGA provides 256 color registers (COLOR00–COLOR255), each 24-bit RGB (8 bits per channel). This replaces OCS/ECS's 32 registers with 12-bit color.

Color Register Layout

ADDRESS:  $DFF180 + (n × 2)    for register n (0–255)

AGA extends the register space:

$DFF180–$DFF1BE  COLOR00–COLOR31   (same addresses as OCS/ECS)
$DFF180–$DFF3BE  COLOR00–COLOR255  (full AGA range — needs BPLCON4 bank select)

The 256 color registers are accessed in 64-register banks selected by BPLCON4.

Writing 24-bit Colors

Each color register holds 12 bits directly (OCS/ECS compatible). The upper 12 bits (the "low nibble") are written via a second access with LOCT set in BPLCON3.

Manual 24-bit write sequence:

; Write color $FF8040 (R=$FF, G=$80, B=$40) to COLOR00

; Step 1: Write high nibble ($F84 = top 4 bits of R,G,B)
move.w  #$0F84, COLOR00+custom   ; $0RGB high nibble

; Step 2: Set LOCT bit in BPLCON3 to enable low nibble write
bset    #9, BPLCON3+custom+1     ; bit 9

; Step 3: Write low nibble ($F04 = low 4 bits of R,G,B → $0F, $08, $04 → $F84 again!)
; Actually: low nibble of FF = F, low nibble of 80 = 0, low nibble of 40 = 0
move.w  #$0F00, COLOR00+custom   ; low nibble

; Step 4: Clear LOCT
bclr    #9, BPLCON3+custom+1

Using LoadRGB32()

The preferred OS call:

#include <graphics/view.h>
#include <proto/graphics.h>

/* Table: count, index, then 0x00RRGGBB values, terminated by ~0 */
ULONG table[] = {
    4, 0,             /* 4 colors starting at index 0 */
    0x00FF0000,       /* COLOR00: red   */
    0x0000FF00,       /* COLOR01: green */
    0x000000FF,       /* COLOR02: blue  */
    0x00FFFFFF,       /* COLOR03: white */
    ~0UL              /* terminator */
};
LoadRGB32(viewport, table);

LoadRGB32() (graphics.library LVO -$192) is the AGA-correct way to set colors. It handles the two-write LOCT protocol internally and is available from OS 3.0+.

Using LoadRGB4() — OCS/ECS compatible (12-bit)

UWORD colors[32] = { 0x000, 0xF00, 0x0F0, ... };
LoadRGB4(viewport, colors, 32);  /* sets 32 colors from 12-bit table */

LoadRGB4() is safe on all chipsets but only provides 12-bit precision on AGA.

HAM8 Mode (Hold-And-Modify, 8-bit)

HAM8 is the AGA extension of OCS's HAM6. It uses 8 bitplanes:

Bits 7-6 of each pixel: mode select
- 00 = index mode (look up COLOR00–COLOR63)
- 01 = modify blue channel
- 10 = modify red channel
- 11 = modify green channel
Bits 5-0: 6-bit value for the selected channel (or 6-bit color index)

Result: 2^18 = 262,144 simultaneous colors from adjacent-pixel modification.

Enabling HAM8:

; BPLCON0: 8 planes (BPU=8, BPU3=1, BPU2-0=000), HAM=1, ECSENA=1
move.w  #$9811, BPLCON0+custom

Color Modes Summary

Mode	Planes	Colors	Method
Standard	1–8	2–256	Direct palette lookup
EHB	6	64	Extra Half-Brite (OCS/ECS compat)
HAM6	6	4096	Hold-and-modify 4-bit channels
HAM8	8	262,144	Hold-and-modify 6-bit channels
Dual Playfield	3+3	8+8	Two independent 8-color layers

Color Bank Selection (BPLCON4)

The 256 color registers are split into 4 banks of 64:

BPLAM	Bank	Registers
$00	0	COLOR00–COLOR63
$40	1	COLOR64–COLOR127
$80	2	COLOR128–COLOR191
$C0	3	COLOR192–COLOR255

Dual playfield can use BPLCON4 to give each playfield a different 64-color bank.

OS Color Management

graphics.library manages palette via the ColorMap structure attached to a ViewPort:

struct ColorMap *cm = GetColorMap(256);     /* allocate 256-entry AGA map */
SetRGB32(vp, n, r, g, b);                  /* set one color (24-bit each) */
LoadRGB32(vp, table);                       /* bulk load */
FreeColorMap(cm);

References

NDK39: graphics/view.h — ColorMap, LoadRGB32
ADCD 2.1 Autodocs: graphics — LoadRGB32, SetRGB32
http://amigadev.elowar.com/read/ADCD_2.1/Libraries_Manual_guide/node02B4.html
AmigaMail Vol. 2 — AGA color system articles

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