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Comment by becurious | Hacker News Reader

Cycle counting was key on the Spectrum - for obvious things like the tape load routines but also for advanced techniques like the ‘Rainbow processor’ - updating the attribute bytes (those responsible for the infamous color clash) as each scan line progressed you could get different colors on each scan line.

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Once made a tape-loading like pattern, and tried to get it as stable (not moving up or down on screen) as possible.

Managed to produce a program where with key presses, you could change delay in the loop in +/- 1 clockcycle increments. Mind you: fastest Z80 opcodes take 4 cycles.

How then? Well, there's also opcodes that take 5 cycles. Or 6. Or 7. And 8=2*4, 9=4+5, etc. Program just automated the insertion/removal of those in the inner loop. Of course I had to pick instructions that didn't mess with some Z80 registers.

Great fun (& educational) figuring out stuff like that. Fun times...

There was some game (and I think a program in a book) where the border color would be changed at a specific scan line to get a horizon that would span the entire screen.

I pretty much knew all the clock cycle counts for the instructions as a teenager, and you would code assembler with them always in mind.

> ...updating the attribute bytes (those responsible for the infamous color clash) as each scan line progressed...

Ah clever! Didn't think of that. Probably the closest thing to "racing the beam" since the Atari 2600 :)

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