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01_hardware/aga_a1200_a4000/aga_palette.md
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@ -1,12 +1,12 @@
[← Home](../../README.md) · [Hardware](../README.md) · [AGA](README.md)
# AGA Palette & Colour System
# AGA Palette & Color System
## Overview
AGA provides **256 colour registers** (COLOR00–COLOR255), each **24-bit RGB** (8 bits per channel). This replaces OCS/ECS's 32 registers with 12-bit colour.
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.
## Colour Register Layout
## Color Register Layout
```
ADDRESS: $DFF180 + (n × 2) for register n (0–255)
@ -18,11 +18,11 @@ $DFF180–$DFF1BE COLOR00–COLOR31 (same addresses as OCS/ECS)
$DFF180–$DFF3BE COLOR00–COLOR255 (full AGA range — needs BPLCON4 bank select)
```
The 256 colour registers are accessed in 64-register **banks** selected by `BPLCON4`.
The 256 color registers are accessed in 64-register **banks** selected by `BPLCON4`.
## Writing 24-bit Colours
## Writing 24-bit Colors
Each colour 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.
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:
@ -52,7 +52,7 @@ The preferred OS call:
/* Table: count, index, then 0x00RRGGBB values, terminated by ~0 */
ULONG table[] = {
4, 0, /* 4 colours starting at index 0 */
4, 0, /* 4 colors starting at index 0 */
0x00FF0000, /* COLOR00: red */
0x0000FF00, /* COLOR01: green */
0x000000FF, /* COLOR02: blue */
@ -62,13 +62,13 @@ ULONG table[] = {
LoadRGB32(viewport, table);
```
`LoadRGB32()` (graphics.library LVO -$192) is the AGA-correct way to set colours. It handles the two-write LOCT protocol internally and is available from OS 3.0+.
`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)
```c
UWORD colours[32] = { 0x000, 0xF00, 0x0F0, ... };
LoadRGB4(viewport, colours, 32); /* sets 32 colours from 12-bit table */
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.
@ -82,9 +82,9 @@ HAM8 is the AGA extension of OCS's HAM6. It uses 8 bitplanes:
- `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 colour index)
- **Bits 5-0**: 6-bit value for the selected channel (or 6-bit color index)
Result: 2^18 = **262,144 simultaneous colours** from adjacent-pixel modification.
Result: 2^18 = **262,144 simultaneous colors** from adjacent-pixel modification.
Enabling HAM8:
```asm
@ -92,19 +92,19 @@ Enabling HAM8:
move.w #$9811, BPLCON0+custom
```
## Colour Modes Summary
## Color Modes Summary
| Mode | Planes | Colours | Method |
| 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-colour layers |
| Dual Playfield | 3+3 | 8+8 | Two independent 8-color layers |
## Colour Bank Selection (BPLCON4)
## Color Bank Selection (BPLCON4)
The 256 colour registers are split into 4 banks of 64:
The 256 color registers are split into 4 banks of 64:
| BPLAM | Bank | Registers |
|---|---|---|
@ -113,15 +113,15 @@ The 256 colour registers are split into 4 banks of 64:
| $80 | 2 | COLOR128–COLOR191 |
| $C0 | 3 | COLOR192–COLOR255 |
Dual playfield can use BPLCON4 to give each playfield a different 64-colour bank.
Dual playfield can use BPLCON4 to give each playfield a different 64-color bank.
## OS Colour Management
## OS Color Management
`graphics.library` manages palette via the `ColorMap` structure attached to a `ViewPort`:
```c
struct ColorMap *cm = GetColorMap(256); /* allocate 256-entry AGA map */
SetRGB32(vp, n, r, g, b); /* set one colour (24-bit each) */
SetRGB32(vp, n, r, g, b); /* set one color (24-bit each) */
LoadRGB32(vp, table); /* bulk load */
FreeColorMap(cm);
```
@ -131,4 +131,4 @@ FreeColorMap(cm);
- 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 colour system articles
- AmigaMail Vol. 2 — AGA color system articles