The "xor rax, rax" that I just saw at a quick glance makes me flinch. Still putting it on my reading list though. Sounds like a really interesting little toy project.
> In 64-bit mode, still use `xor r32, r32`, because writing a 32-bit reg zeros the upper 32. `xor r64, r64` is a waste of a byte, because it needs a REX prefix.