Comment by rwmj

The Z80 didn't even do 8 bit add. The ALU operates in two 4 bit cycles.

I am now wondering if it's possible to put a ZX81 emulator on one of these microcontrollers. It would need to emulate the Z80 but you've got plenty of spare cycles, and 3x the ROM and RAM of the original, so enough space for a small emulator!