Skip to content

← Home · Hardware · OCS

Copper Coprocessor

Overview

The Copper (co-processor) is built into Agnus. It executes a simple instruction list (the copperlist) in sync with the video beam, allowing precise per-scanline changes to any writable custom register — without CPU intervention.

The Copper can only write to custom registers (it cannot access Chip RAM directly), but it can change bitplane pointers, colors, BPLCON0, sprite pointers, and any other $DFF0xx register on a cycle-accurate basis.

Copper Instruction Set

The Copper has exactly three instructions, each exactly 32 bits (two 16-bit words):

1. MOVE — Write a Register

Word 1:  [RRR RRRR RRR0]   Destination register address (must be even, bit 0 = 0)
Word 2:  [DDDD DDDD DDDD]  Data to write

Example — set COLOR00 to red at scanline 100:

DC.W  $0180, $0F00    ; MOVE COLOR00, $0F00 (red)

2. WAIT — Wait for Beam Position

Word 1:  [VVVV VVVH HHHH HH1]   Vertical pos[8:1], Horizontal pos[8:3], bit0=1
Word 2:  [EVVV VVVH HHHH HHx0]  Enable, VP mask, HP mask, bit0=0

The Copper stalls until the beam reaches (V,H) AND (Vmask,Hmask).

Standard full-precision WAIT:

DC.W  $6401, $FF00    ; WAIT for line $64 (100), any H — full V mask

WAIT for end of frame (copper stop):

DC.W  $FFFF, $FFFE    ; WAIT $FF,$FF — impossible position → stop copper

3. SKIP — Conditional Skip

Word 1:  [VVVV VVVH HHHH HH1]   Same format as WAIT
Word 2:  [EVVV VVVH HHHH HHx1]  Same as WAIT but bit 0 = 1 (SKIP flag)

If beam has passed the position, skip the next instruction. Used for double-buffered copper switching.

Copperlist Format

A copperlist is an array of 32-bit instruction pairs in Chip RAM, terminated by:

DC.W  $FFFF, $FFFE

Example — color cycle on vertical blank:

Copperlist:
    DC.W  $0180, $0000   ; COLOR00 = black
    DC.W  $4401, $FF00   ; WAIT line 68 (display area start)
    DC.W  $0180, $0F00   ; COLOR00 = red
    DC.W  $6401, $FF00   ; WAIT line 100
    DC.W  $0180, $00F0   ; COLOR00 = green
    DC.W  $FFFF, $FFFE   ; END

Copper Pointers and Control

Register Offset Description
COP1LCH $080 Copper list 1 pointer high word
COP1LCL $082 Copper list 1 pointer low word
COP2LCH $084 Copper list 2 pointer high word
COP2LCL $086 Copper list 2 pointer low word
COPJMP1 $088 Strobe: restart copper from list 1 (any write)
COPJMP2 $08A Strobe: restart copper from list 2
COPCON $02E Copper danger bit (CDANG)

COPCON bit 1 (CDANG): When set, Copper is allowed to write to registers $40–$7F (blitter registers). Should be 0 in normal use to prevent runaway copperlists from corrupting the blitter.

Activating the Copper

; Load copperlist address into copper list 1
move.l  #Copperlist, d0
move.w  d0, COP1LCL+custom    ; low word
swap    d0
move.w  d0, COP1LCH+custom    ; high word

; Restart copper (triggers on next VBlank)
move.w  d0, COPJMP1+custom    ; value irrelevant, strobe only

; Enable copper DMA
move.w  #$8280, DMACON+custom ; SET + MASTER + COPEN

Dual-Playfield and HAM via Copper

Common copper techniques:

Split-screen different palettes: Change COLOR00–COLOR31 registers mid-screen via a WAIT at the split scanline.

Mid-screen bitplane pointer change: Redirect BPL1PTH/BPL1PTL to a different bitmap half-way through the display — used for large vertical scrolling without double-buffering the full screen.

BPLCON0 mid-screen: Switch between HIRES and LORES, or between 6-plane and 4-plane modes, on different lines.

Raster bars: Write a different color to COLOR00 on every scanline using sequential WAIT+MOVE pairs.

Graphics Library vs Direct Copper

AmigaOS's graphics.library manages the Copper list internally: - MrgCop() merges system and user copper lists - LoadView() installs a View structure's copper list - WaitTOF() waits for the top-of-frame (VBlank) before the copper restarts

Direct copper access should be done via GetColorMap(), SetRGB4() and official graphics calls, or through a custom View/ViewPort/ColorMap structure passed to LoadView().

References

  • ADCD 2.1 Hardware Manual — Copper chapter
  • NDK39: hardware/custom.h, graphics/copper.h
  • Amiga Hardware Reference Manual 3rd ed., Chapter 6