The Copper: Graphics Co-Processor

Synopsis

The Copper is a tiny, two-instruction co-processor that does one thing: it writes NextRegs at exact raster positions, perfectly synchronised to the video beam, with no CPU involvement. That sounds modest until you realise what it unlocks — per-scanline palette swaps, mid-frame layer reconfiguration, gradient skies, parallax via per-line scroll, and effects that are simply impossible to do reliably from the Z80 because the line interrupt’s jitter is too coarse.

Why this chapter sits where it does in the book: the Copper has nothing interesting to do until you already know which NextRegs are worth retiming. We placed it after every “single-layer” video chapter so each example can demonstrate something visually striking instead of an abstract toggle.

Topics:

• The Copper instruction set. Two instructions, both 16 bits.
  • MOVE: write value to NextReg nr immediately.
  • WAIT: pause the Copper until raster reaches (line, h-position).
• Copper memory. 2 KB (1024 instructions max), accessed via NextReg $60 (data) with auto-incrementing index in $61/$62.
• Run modes. $62 bits 7-6: stopped, run-once-then-stop, run-once-and-restart-at-V-blank, run-and-restart-at-(0,0).
• Synchronisation. How the Copper relates to the line interrupt (and why the Copper is more precise: the Copper waits for an exact H-position, the line interrupt fires at the start of the line ± Z80 latency).
• Limits. No conditionals, no jumps, no arithmetic. It’s a list of timed register writes. Anything more complex needs cooperation with a Z80 ISR.

What you should know first

• Interrupts — for the comparison.
• Palettes, Layer 2, Tilemap, Sprites — to know what’s worth retiming.

Planned exercises

• Sky gradient. A 192-instruction Copper program that writes a different palette colour to entry 0 on every Layer 2 scanline — instant gradient sky.
• Mid-screen mode swap. Top half of the screen uses ULA, bottom half uses LoRes, switched by a Copper MOVE at line 96.
• Parallax floor. Per-scanline X-scroll values for the tilemap, producing a perspective floor effect that no Z80 routine could keep up with.