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134 lines
4.1 KiB
Markdown
134 lines
4.1 KiB
Markdown
[← Home](../../README.md) · [Hardware](../README.md) · [AGA](README.md)
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# AGA Palette & Color System
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## Overview
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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.
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## Color Register Layout
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```
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ADDRESS: $DFF180 + (n × 2) for register n (0–255)
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```
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AGA extends the register space:
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```
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$DFF180–$DFF1BE COLOR00–COLOR31 (same addresses as OCS/ECS)
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$DFF180–$DFF3BE COLOR00–COLOR255 (full AGA range — needs BPLCON4 bank select)
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```
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The 256 color registers are accessed in 64-register **banks** selected by `BPLCON4`.
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## Writing 24-bit Colors
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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.
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### Manual 24-bit write sequence:
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```asm
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; Write color $FF8040 (R=$FF, G=$80, B=$40) to COLOR00
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; Step 1: Write high nibble ($F84 = top 4 bits of R,G,B)
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move.w #$0F84, COLOR00+custom ; $0RGB high nibble
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; Step 2: Set LOCT bit in BPLCON3 to enable low nibble write
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bset #9, BPLCON3+custom+1 ; bit 9
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; Step 3: Write low nibble ($F04 = low 4 bits of R,G,B → $0F, $08, $04 → $F84 again!)
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; Actually: low nibble of FF = F, low nibble of 80 = 0, low nibble of 40 = 0
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move.w #$0F00, COLOR00+custom ; low nibble
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; Step 4: Clear LOCT
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bclr #9, BPLCON3+custom+1
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```
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### Using LoadRGB32()
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The preferred OS call:
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```c
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#include <graphics/view.h>
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#include <proto/graphics.h>
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/* Table: count, index, then 0x00RRGGBB values, terminated by ~0 */
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ULONG table[] = {
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4, 0, /* 4 colors starting at index 0 */
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0x00FF0000, /* COLOR00: red */
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0x0000FF00, /* COLOR01: green */
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0x000000FF, /* COLOR02: blue */
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0x00FFFFFF, /* COLOR03: white */
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~0UL /* terminator */
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};
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LoadRGB32(viewport, table);
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```
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`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+.
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### Using LoadRGB4() — OCS/ECS compatible (12-bit)
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```c
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UWORD colors[32] = { 0x000, 0xF00, 0x0F0, ... };
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LoadRGB4(viewport, colors, 32); /* sets 32 colors from 12-bit table */
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```
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`LoadRGB4()` is safe on all chipsets but only provides 12-bit precision on AGA.
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## HAM8 Mode (Hold-And-Modify, 8-bit)
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HAM8 is the AGA extension of OCS's HAM6. It uses 8 bitplanes:
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- **Bits 7-6** of each pixel: mode select
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- `00` = index mode (look up COLOR00–COLOR63)
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- `01` = modify blue channel
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- `10` = modify red channel
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- `11` = modify green channel
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- **Bits 5-0**: 6-bit value for the selected channel (or 6-bit color index)
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Result: 2^18 = **262,144 simultaneous colors** from adjacent-pixel modification.
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Enabling HAM8:
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```asm
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; BPLCON0: 8 planes (BPU=8, BPU3=1, BPU2-0=000), HAM=1, ECSENA=1
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move.w #$9811, BPLCON0+custom
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```
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## Color Modes Summary
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| Mode | Planes | Colors | Method |
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|---|---|---|---|
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| Standard | 1–8 | 2–256 | Direct palette lookup |
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| EHB | 6 | 64 | Extra Half-Brite (OCS/ECS compat) |
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| HAM6 | 6 | 4096 | Hold-and-modify 4-bit channels |
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| HAM8 | 8 | 262,144 | Hold-and-modify 6-bit channels |
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| Dual Playfield | 3+3 | 8+8 | Two independent 8-color layers |
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## Color Bank Selection (BPLCON4)
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The 256 color registers are split into 4 banks of 64:
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| BPLAM | Bank | Registers |
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|---|---|---|
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| $00 | 0 | COLOR00–COLOR63 |
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| $40 | 1 | COLOR64–COLOR127 |
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| $80 | 2 | COLOR128–COLOR191 |
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| $C0 | 3 | COLOR192–COLOR255 |
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Dual playfield can use BPLCON4 to give each playfield a different 64-color bank.
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## OS Color Management
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`graphics.library` manages palette via the `ColorMap` structure attached to a `ViewPort`:
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```c
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struct ColorMap *cm = GetColorMap(256); /* allocate 256-entry AGA map */
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SetRGB32(vp, n, r, g, b); /* set one color (24-bit each) */
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LoadRGB32(vp, table); /* bulk load */
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FreeColorMap(cm);
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```
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## References
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- NDK39: `graphics/view.h` — ColorMap, LoadRGB32
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- ADCD 2.1 Autodocs: graphics — LoadRGB32, SetRGB32
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- http://amigadev.elowar.com/read/ADCD_2.1/Libraries_Manual_guide/node02B4.html
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- AmigaMail Vol. 2 — AGA color system articles
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