Commodore AmigaRenderingIntermediate

Copper Colour Bars

Change the background colour at chosen scanlines from the copper list — no CPU, no interrupt, no jitter — to paint horizontal bars or a smooth vertical gradient the beam draws for you.

coppercolour-barsgradientrastergraphics

Overview

The copper runs in lock-step with the video beam, so a WAIT for a scanline followed by a MOVE to a colour register changes that colour part-way down the screen. Stack a few and you get horizontal colour bars; change the colour a little on every line and you get a smooth gradient — a sky, a metallic sheen, a demo backdrop. The processor does nothing: the copper list is the effect, drawn fresh every frame, and because it’s beam-locked it never wobbles. This is the copper’s signature trick, and where the C64 needs a cycle-counted stable raster to hold a clean colour split, the Amiga gets it for free.

Code

; =============================================================================
; COPPER COLOUR BARS - AMIGA
; Change COLOR00 at chosen scanlines: hand-built bars, then a runtime gradient
; =============================================================================

COLOR00  equ $0180

; --- Hand-built: three colour bands down the screen ---
bars_list:
            dc.w    $2c07,$fffe         ; WAIT line $2C (top of the display)
            dc.w    COLOR00,$000f       ; deep blue
            dc.w    $6007,$fffe         ; WAIT line $60
            dc.w    COLOR00,$008f       ; lighter blue
            dc.w    $9407,$fffe         ; WAIT line $94
            dc.w    COLOR00,$00df       ; cyan
            dc.w    $ffff,$fffe         ; end of list (WAIT forever)

; --- Runtime: build a smooth vertical gradient into a copper list ---
; a0 -> buffer to build into, d0.b = first line, d1 = line count - 1,
; a1 -> table of d1+1 colour words ($0RGB)
build_gradient:
.line:
            move.b  d0,d3
            lsl.w   #8,d3
            or.w    #$0007,d3           ; (line << 8) | $07 | WAIT bit -> WAIT this line
            move.w  d3,(a0)+
            move.w  #$fffe,(a0)+        ; ...second WAIT word
            move.w  #COLOR00,(a0)+      ; MOVE COLOR00, ...
            move.w  (a1)+,(a0)+         ; ...the next colour from the table
            addq.b  #1,d0               ; next scanline
            dbf     d1,.line
            move.l  #$fffffffe,(a0)+    ; $FFFF,$FFFE — end of list
            rts

Trade-offs

Aspect Cost
CPU None — the copper writes each colour as the beam passes
Memory 8 bytes per line (one WAIT + one MOVE) — a full-screen gradient is ~2 KB
Limitation One register write per copper instruction; beam moves top-to-bottom only

When to use: Skies, backdrops, title screens, loading bars, any vertical colour ramp.

When to avoid: A single flat colour (one MOVE at the top is enough) — and horizontal gradients, which the copper can’t paint (it follows the beam down, not across).

Any register, not just COLOR00

Nothing about this is special to the background. Swap COLOR00 for a bitplane colour and the artwork recolours per band; write the sprite-colour registers and a sprite changes hue down its height; write two or three colours per line and the whole palette animates as the beam descends. The colour-bar loop is really “reprogram the display at this scanline”, and the split screen is the same idea aimed at the bitplane pointers instead of the palette.

Waiting past line 255

The WAIT vertical position is only 8 bits, so it can’t name a line above 255 directly — and PAL screens run to ~312. The idiom is to first wait out the top half with a throwaway WAIT for line 255, then wait for the low-numbered line in the second field:

            dc.w    $ffdf,$fffe         ; WAIT line 255 (VP=$FF, mask lets HP through)
            dc.w    $2c07,$fffe         ; now WAIT line $12C-256 = line $2C of the lower half

Forget it and a gradient meant for the bottom of a tall display simply stops at line 255. Most game displays sit inside the first 256 lines and never meet this, but full-overscan effects must handle the wrap.

Patterns: Copper List Basics, Copper Split Screen, Raster Splits (C64)

Vault: Copper | Commodore Amiga