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125 lines
3.6 KiB
Markdown
125 lines
3.6 KiB
Markdown
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[← Home](../README.md) · [Graphics](README.md)
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# Copper — Coprocessor Instructions and UCopList
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## Overview
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The **Copper** is a simple coprocessor in the Amiga custom chips that executes a list of instructions synchronized to the video beam. It can write to any custom chip register at any beam position, enabling per-scanline color changes, split screens, and hardware-level display effects without CPU intervention.
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---
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## Instruction Format
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The Copper has only three instructions, each 32 bits (one longword):
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### MOVE — Write a Register
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```
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[register_offset (9 bits)] [value (16 bits)]
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Bit layout: 0RRRRRRRR00000000 VVVVVVVVVVVVVVVV
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```
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- Register offset is relative to `$DFF000` (custom chip base)
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- Only even registers can be written (bit 0 = 0)
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- Example: `$0180, $0FFF` → write `$0FFF` to `COLOR00` (`$DFF180`)
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### WAIT — Wait for Beam Position
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```
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[vpos (8 bits)] [hpos (7 bits)] [1] [vmask (7 bits)] [hmask (7 bits)] [0]
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Bit layout: VVVVVVVVHHHHHH01 vvvvvvvvhhhhhhh0
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```
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- Pauses until the beam reaches at least the specified (vpos, hpos)
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- Masks allow waiting on partial positions (e.g. any horizontal, specific vertical)
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### SKIP — Conditional Skip
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```
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Same as WAIT but bit 0 of second word = 1
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```
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If the beam has already passed the specified position, skip the next instruction.
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---
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## Standard Copper Patterns
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### Per-Scanline Color Change (Rainbow)
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```
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WAIT $2C01,$FFFE ; wait for line $2C (44)
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MOVE $0180,$0F00 ; COLOR00 = red
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WAIT $2D01,$FFFE ; wait for line $2D (45)
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MOVE $0180,$00F0 ; COLOR00 = green
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WAIT $2E01,$FFFE ; wait for line $2E (46)
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MOVE $0180,$000F ; COLOR00 = blue
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...
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WAIT $FFDF,$FFFE ; wait past line 255 (enables access to lines 256+)
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WAIT $FFFF,$FFFE ; end-of-list (impossible position = halt)
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```
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### End of Copper List
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```
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$FFFF, $FFFE ; WAIT for beam position $FFFF — never reached
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```
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This is the standard "stop" marker. The Copper loops back to the start on the next vertical blank.
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---
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## System Copper Lists
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The OS manages copper lists through `GfxBase`:
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| Pointer | Description |
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|---|---|
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| `GfxBase->copinit` | System initialization copper list |
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| `GfxBase->LOFlist` | Long-frame copper list (even fields) |
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| `GfxBase->SHFlist` | Short-frame copper list (odd fields, interlace) |
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---
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## UCopList — User Copper Instructions
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Applications can inject copper instructions into the system list via `UCopList`:
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```c
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struct UCopList *ucl = AllocMem(sizeof(struct UCopList), MEMF_PUBLIC|MEMF_CLEAR);
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CINIT(ucl, 100); /* init, max 100 instructions */
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CWAIT(ucl, 44, 0); /* wait for line 44 */
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CMOVE(ucl, *((UWORD *)0xDFF180), 0x0F00); /* COLOR00 = red */
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CWAIT(ucl, 100, 0);
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CMOVE(ucl, *((UWORD *)0xDFF180), 0x000F); /* COLOR00 = blue */
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CEND(ucl); /* end of list */
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viewport->UCopIns = ucl;
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RethinkDisplay(); /* rebuild system copper list */
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```
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---
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## Custom Chip Register Addresses (Copper-Relevant)
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| Address | Name | Description |
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|---|---|---|
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| `$DFF180`–`$DFF1BE` | `COLOR00`–`COLOR31` | OCS/ECS palette (12-bit RGB) |
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| `$DFF100` | `BPLCON0` | Bitplane control (depth, resolution) |
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| `$DFF102` | `BPLCON1` | Scroll offsets |
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| `$DFF104` | `BPLCON2` | Priority control |
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| `$DFF08E` | `DIWSTRT` | Display window start |
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| `$DFF090` | `DIWSTOP` | Display window stop |
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| `$DFF092` | `DDFSTRT` | Data fetch start |
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| `$DFF094` | `DDFSTOP` | Data fetch stop |
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| `$DFF0E0`–`$DFF0FE` | `BPL1PT`–`BPL8PT` | Bitplane pointers |
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---
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## References
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- HRM: *Amiga Hardware Reference Manual* — Copper chapter
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- NDK39: `graphics/copper.h`, `graphics/gfxmacros.h`
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- ADCD 2.1: `CINIT`, `CMOVE`, `CWAIT`, `CEND`
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