Copper: The Amiga's Display Coprocessor

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: two bus cycles per instruction — except WAIT, which costs nothing while it waits.
Lists: two copper lists can be active (COP1/COP2).

Waiting is free

The Copper’s importance is usually put as “it changes registers mid-screen”. The sentence that explains why that is cheap is in the register description:

WAIT Wait until beam counter is equal to, or greater than. (keeps Copper off of bus until beam position has been reached)

A WAIT is not a loop. The Copper “examines the contents of the video beam position counter directly”, and while it waits it is off the bus entirely — using no memory cycles, competing with nothing. Only when the beam arrives does it resume fetching, and then “all instructions (except for WAIT) require two bus cycles”.

That is the whole economy of the chip. A Copper list is mostly waiting, and waiting is free. In a machine where the blitter, bitplanes, sprites, audio and the 680x0 are all contending for the same memory, Commodore added a coprocessor that spends most of its life not asking for any.

Copper danger mode

The manual’s other named bit is CDANG, in the Copper control register:

This is a 1-bit register that when set true, allows the Copper to access the blitter hardware. This bit is cleared by power-on reset, so that the Copper cannot access the blitter hardware.

CDANG stands for “Copper danger mode”. Out of reset the Copper is fenced off from the blitter registers, and a program that wants the two custom chips driving each other has to say so deliberately.

It is worth setting beside the blitter’s own BLTPRI, which Commodore documents as “blitter-nasty”. The Amiga hardware manual contains a bit called nasty and a bit called danger, both describing what one custom chip can do to the rest of the machine if allowed.

The connection runs the other way too. Bit 15 of a WAIT’s second word is BFD, “blitter-finished disable” — so a Copper wait can be made to depend not only on where the beam is but on whether the blitter has finished.

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
Writes only	The Copper writes to registers and tests beam position; it can’t read register values back. Conditional logic is limited to SKIP (skip the next instruction if past a beam position).
No RAM data access	The Copper fetches its instruction stream from chip RAM but cannot read or write data RAM directly — every action is a register write.
Timing bound	Executes during the display, not just vblank — but each instruction consumes Copper DMA cycles
Instruction timing	MOVE consumes 4 cycles; WAIT consumes 4 cycles when the wait fires (more if it has to wait), SKIP consumes 4 cycles
Protected registers	Some registers (DMACON, INTENA, INTREQ, ADKCON) require the COPCON “copper danger” bit before the Copper can write them

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.