Copper — Coprocessor Instructions and UCopList

Overview

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.

Instruction Format

The Copper has only three instructions, each 32 bits (one longword):

MOVE — Write a Register

[register_offset (9 bits)] [value (16 bits)]
Bit layout: 0RRRRRRRR00000000 VVVVVVVVVVVVVVVV

Register offset is relative to $DFF000 (custom chip base)
Only even registers can be written (bit 0 = 0)
Example: $0180, $0FFF → write $0FFF to COLOR00 ($DFF180)

WAIT — Wait for Beam Position

[vpos (8 bits)] [hpos (7 bits)] [1] [vmask (7 bits)] [hmask (7 bits)] [0]
Bit layout: VVVVVVVVHHHHHH01 vvvvvvvvhhhhhhh0

Pauses until the beam reaches at least the specified (vpos, hpos)
Masks allow waiting on partial positions (e.g. any horizontal, specific vertical)

SKIP — Conditional Skip

Same as WAIT but bit 0 of second word = 1

If the beam has already passed the specified position, skip the next instruction.

Standard Copper Patterns

Per-Scanline Color Change (Rainbow)

WAIT   $2C01,$FFFE    ; wait for line $2C (44)
MOVE   $0180,$0F00    ; COLOR00 = red
WAIT   $2D01,$FFFE    ; wait for line $2D (45)
MOVE   $0180,$00F0    ; COLOR00 = green
WAIT   $2E01,$FFFE    ; wait for line $2E (46)
MOVE   $0180,$000F    ; COLOR00 = blue
...
WAIT   $FFDF,$FFFE    ; wait past line 255 (enables access to lines 256+)
WAIT   $FFFF,$FFFE    ; end-of-list (impossible position = halt)

End of Copper List

$FFFF, $FFFE          ; WAIT for beam position $FFFF — never reached

This is the standard "stop" marker. The Copper loops back to the start on the next vertical blank.

System Copper Lists

The OS manages copper lists through GfxBase:

Pointer	Description
`GfxBase->copinit`	System initialization copper list
`GfxBase->LOFlist`	Long-frame copper list (even fields)
`GfxBase->SHFlist`	Short-frame copper list (odd fields, interlace)

UCopList — User Copper Instructions

Applications can inject copper instructions into the system list via UCopList:

struct UCopList *ucl = AllocMem(sizeof(struct UCopList), MEMF_PUBLIC|MEMF_CLEAR);

CINIT(ucl, 100);                    /* init, max 100 instructions */
CWAIT(ucl, 44, 0);                  /* wait for line 44 */
CMOVE(ucl, *((UWORD *)0xDFF180), 0x0F00); /* COLOR00 = red */
CWAIT(ucl, 100, 0);
CMOVE(ucl, *((UWORD *)0xDFF180), 0x000F); /* COLOR00 = blue */
CEND(ucl);                          /* end of list */

viewport->UCopIns = ucl;
RethinkDisplay();                   /* rebuild system copper list */

Custom Chip Register Addresses (Copper-Relevant)

Address	Name	Description
`$DFF180`–`$DFF1BE`	`COLOR00`–`COLOR31`	OCS/ECS palette (12-bit RGB)
`$DFF100`	`BPLCON0`	Bitplane control (depth, resolution)
`$DFF102`	`BPLCON1`	Scroll offsets
`$DFF104`	`BPLCON2`	Priority control
`$DFF08E`	`DIWSTRT`	Display window start
`$DFF090`	`DIWSTOP`	Display window stop
`$DFF092`	`DDFSTRT`	Data fetch start
`$DFF094`	`DDFSTOP`	Data fetch stop
`$DFF0E0`–`$DFF0FE`	`BPL1PT`–`BPL8PT`	Bitplane pointers

References

HRM: Amiga Hardware Reference Manual — Copper chapter
NDK39: graphics/copper.h, graphics/gfxmacros.h
ADCD 2.1: CINIT, CMOVE, CWAIT, CEND

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