Colour and the Copper

Every unit so far handed something to "the Copper" and moved on. Time to open the box, because the Copper is one of the things that makes an Amiga an Amiga.

The Copper is a tiny co-processor living inside the chip Agnus. It runs its own little program — the Copper list you've been writing — in step with the video beam as it sweeps down the screen. It's not the 68000; it runs alongside it, for free, while your CPU does other things. And it knows just two instructions:

• WAIT — pause until the beam reaches a given line. Written dc.w $VVHH,$fffe, where $VV is the line number.
• MOVE — write a value to a chip register. Written dc.w register,value.

That's the whole language. But "wait for a line, then change a register" is enough to do something the CPU would sweat over: change a colour part-way down the screen. Do it at several lines and you get a gradient — bands of colour — drawn by the Copper while the CPU sits idle.

What you'll see by the end

A gradient of colour bands, deep blue at the top fading down through cyan and green and yellow to red at the bottom. The CPU set none of it after start-up — the Copper painted every band, in step with the beam.

Wait, then move

;──────────────────────────────────────────────────────────────
; Meet the Machine (Amiga) - Unit 7: Colour and the Copper
;
; The Copper is a tiny co-processor inside the Amiga that runs its own program
; in step with the video beam. It knows two instructions: WAIT (pause until the
; beam reaches a line) and MOVE (write a value to a chip register). With them it
; changes the background colour part-way down the screen - a gradient the CPU
; never lifts a finger to draw.
;──────────────────────────────────────────────────────────────

CUSTOM      equ $dff000
DMACON      equ $096
INTENA      equ $09a
INTREQ      equ $09c
COP1LC      equ $080
COPJMP1     equ $088
BPLCON0     equ $100
COLOR00     equ $180

            section code,code_c

start:
            lea     CUSTOM,a5
            move.w  #$7fff,INTENA(a5)
            move.w  #$7fff,INTREQ(a5)
            move.w  #$7fff,DMACON(a5)
            lea     copperlist,a0
            move.l  a0,COP1LC(a5)
            move.w  d0,COPJMP1(a5)
            move.w  #$8280,DMACON(a5)   ; DMA on: master + Copper

forever:
            bra.s   forever

;──────────────────────────────────────────────────────────────
; The Copper list. Two instructions only:
;   WAIT:  dc.w $VVHH,$fffe   - pause until the beam reaches line $VV
;   MOVE:  dc.w register,value - write 'value' to the register
;──────────────────────────────────────────────────────────────
copperlist:
            dc.w    BPLCON0,$0200       ; 0 bitplanes - just the background
            dc.w    COLOR00,$0008       ; from the top: deep blue

            ; ----------------------------------------------- YOUR CODE START
            dc.w    $3001,$fffe
            dc.w    COLOR00,$000f       ; line $30: blue
            dc.w    $5001,$fffe
            dc.w    COLOR00,$00af       ; line $50: cyan
            dc.w    $7001,$fffe
            dc.w    COLOR00,$00f8       ; line $70: green
            dc.w    $9001,$fffe
            dc.w    COLOR00,$0ff0       ; line $90: yellow
            dc.w    $b001,$fffe
            dc.w    COLOR00,$0f60       ; line $B0: orange
            dc.w    $d001,$fffe
            dc.w    COLOR00,$0f00       ; line $D0: red
            ; ------------------------------------------------- YOUR CODE END

            dc.w    $ffff,$fffe         ; end of the Copper list

Read the Copper list as a script that runs top to bottom, once per frame:

dc.w    COLOR00,$0008       ; from the top of the screen: deep blue
dc.w    $3001,$fffe         ; WAIT until the beam reaches line $30...
dc.w    COLOR00,$000f       ; ...then MOVE a new colour into the background
dc.w    $5001,$fffe         ; wait for line $50...
dc.w    COLOR00,$00af       ; ...change it again

Each WAIT/MOVE pair says "when the beam gets here, make the background this." The background holds each colour until the next WAIT fires, so the screen shows solid bands. The CPU wrote the list once, at start-up; the Copper has re-run it every frame since, all by itself.

This is why the harness always pointed the hardware at a Copper list, even in Unit 1: the Copper is how you change the display in time with the beam — the one thing the CPU, running to its own clock, can't do in lockstep.

Assemble, master, and run

make

A seven-band gradient down the screen, painted by the Copper.

Try this: move the bands

Change the WAIT lines — $3001, $5001, and so on. The $VV digits are the line number (the screen runs from $00 at the top to about $FF at the bottom). Move a WAIT higher and its band starts higher; bunch them together for thin bands, spread them out for fat ones.

Try this: a smoother fade

Add more WAIT/MOVE pairs between the existing ones, stepping the colour by a little each time — $0008, $0009, $000a… The more bands you add, the smoother the fade, until it looks like a continuous gradient. (Real Amiga demos change the colour on every line this way — a "copper gradient" — and it costs the CPU nothing.)

If it doesn't work

• The whole screen is one colour. A WAIT is malformed — each is two words, dc.w $VVHH,$fffe, and the $fffe matters. Without it the Copper doesn't pause and the last colour wins.
• The bands are in the wrong place. Check the $VV line numbers are in increasing order down the list; the beam only moves down, so a WAIT for a line already passed never fires.
• Nothing shows / a black screen. The Copper DMA isn't on — the harness enables it with $8280 to DMACON. Leave that line as shown.

What you've learnt

The Copper is a co-processor that runs a list of WAIT and MOVE instructions in step with the video beam. With "wait for a line, then change a register" it paints colour bands down the screen — a gradient drawn entirely in hardware, while the CPU is free to do other work. It's been quietly running your display since Unit 1.

What's next

That's what the machine is — the 68000, its memory, the bitmap, the Copper. Now we turn to what it can do. Next — Test, Then Branch — the 68000 makes a decision: compare two values and jump one way or the other, the IF you build by hand.