I cut my teeth writing assembly on a ZX Spectrum (mostly hacking games for infinite lives, etc).

Over the past couple of years I've gone back to the Z80, working on CP/M emulation, testing, and similar things. It's fun to play the old infocom games, and mess around with BIOSes when all you have is 64k.

There's a lot of good CP/M reference material out there, and numerous emulators to run it on modern systems