I could make your words mine, just s/6502/Z80, it was really fun in the old days. :) But I'll add that x86 was pretty approachable, say up to the 386/486 times

I could make your words mine, just s/6502/Z80, it was really fun in the old days. :) But I'll add that x86 was pretty approachable, say up to the 386/486 times when I've written some minor stuff, mostly low-level graphics... even with quirks like segment registers, one could learn enough to hand-code expertly (beating the hell of any compiler of the time) with modest effort. Pentium onwards though, things got much more complicated esp. microarchitectural performance (have to consider out of order execution / ILP, use tricks like prefetching for cache perf etc.).