Copper: The Amiga's Display Coprocessor

Overview

The Copper (short for coprocessor) lives inside Agnus and executes simple programs synchronised to the video beam position. With just three instructions—WAIT, MOVE, and SKIP—programmers created effects that seemed impossible: smooth colour gradients, split-screen displays, and sprite multiplexing.

Fast facts

Location: integrated into Agnus chip.
Instructions: 3 (WAIT, MOVE, SKIP).
Execution: one instruction per memory cycle during display.
Lists: two copper lists can be active (COP1/COP2).

Instruction format

All copper instructions are 32 bits (two words):

MOVE instruction

First word:  Register offset (bit 0 = 0)
Second word: Data to write

Example: $0180, $0F00 — write $0F00 to COLOR00 (red).

WAIT instruction

First word:  VP7-VP0 HP8-HP1 (position) | $0001
Second word: VPM7-VPM0 HPM8-HPM1 | $FFFE

VP = vertical position (line number).
HP = horizontal position (divided by 2).
Mask bits allow partial matching.

Example: $4007, $FFFE — wait for line $40, any horizontal position.

SKIP instruction

First word:  VP7-VP0 HP8-HP1 (position) | $0001
Second word: VPM7-VPM0 HPM8-HPM1 | $FFFF (note: bit 0 = 1)

Skip next instruction if beam is past specified position.

Common techniques

Copper rainbow

Change background colour each scanline:

    dc.w    $2c07,$fffe      ; Wait line $2c
    dc.w    $0180,$0100      ; Dark red
    dc.w    $2d07,$fffe      ; Wait line $2d
    dc.w    $0180,$0200      ; Slightly brighter
    dc.w    $2e07,$fffe      ; Wait line $2e
    dc.w    $0180,$0300      ; Brighter still
    ; ... continue for smooth gradient

Split screen

Different display modes in upper/lower screen:

    ; Upper screen: hires
    dc.w    $0100,$8200      ; BPLCON0 = hires
    dc.w    $8007,$fffe      ; Wait for line $80
    ; Lower screen: lowres
    dc.w    $0100,$1200      ; BPLCON0 = lowres

Sprite multiplexing

Reposition sprite after it passes:

    dc.w    $5007,$fffe      ; Wait for sprite's last line
    dc.w    $0140,$8050      ; SPR0POS = new position
    dc.w    $0142,$8060      ; SPR0CTL = new control
    dc.w    $0144,sprite2    ; SPR0DATA = new graphics

Copper limitations

Limitation	Details
Can’t read	Only writes, no conditional logic beyond SKIP
Can’t access RAM	Only hardware registers
Timing bound	Executes during display, not vblank
Protected registers	Some registers (like DMACON) require “copper danger” bit

Copper danger

Certain registers are protected by default. To allow copper writes to these:

    move.w  #$8002,$dff096   ; Set COPCON bit in DMACON

Protected registers include DMACON and INTENA.

Copper list management

Register	Address	Purpose
COP1LCH	$DFF080	Copper list 1 high word
COP1LCL	$DFF082	Copper list 1 low word
COP2LCH	$DFF084	Copper list 2 high word
COP2LCL	$DFF086	Copper list 2 low word
COPJMP1	$DFF088	Restart copper 1 (strobe)
COPJMP2	$DFF08A	Restart copper 2 (strobe)

Example copper list structure

copperlist:
    dc.w    $008e,$2c81      ; DIWSTRT - display window start
    dc.w    $0090,$f4c1      ; DIWSTOP - display window stop
    dc.w    $0092,$0038      ; DDFSTRT - data fetch start
    dc.w    $0094,$00d0      ; DDFSTOP - data fetch stop
    dc.w    $0100,$1200      ; BPLCON0 - 1 bitplane
    dc.w    $0180,$0000      ; COLOR00 - black background
    dc.w    $0182,$0fff      ; COLOR01 - white foreground
    dc.w    $ffff,$fffe      ; End - wait forever

Why the Copper mattered

The Copper democratised advanced display effects. What required expensive hardware on other systems—or wasn’t possible at all—became a matter of writing a simple list. Demo coders and game programmers alike exploited this to create visuals that defined the Amiga’s reputation.