It's been a few years since
my last look at calculating the date of
Easter Sunday using only Z80 machine code, but I thought I'd finally re-visit it. In particular, I wanted to look in detail at
Al Petrofsky's awesome optimizations of
my lame attempts.
If you want a master-class in optimizing integer calculations on archaic CPUs (and who doesn't?) look no further than Al's
solution. It's the product of deep understanding of:
- The Z80 instruction set,
- Integer multiplication-by-constants,
- Integer division-by-constants,
- Modulo arithmetic, and
- Easter computus.
At only two hundred or so opcodes, and no branches, it's a beautiful distillation.
