As much as I like the M68k, I don't think it would be easy to extend it to 64 bits. I'm looking at the MOVE instruction, encoded as: 00ssRRRmmmMMMrrr

As much as I like the M68k, I don't think it would be easy to extend it to 64 bits. I'm looking at the MOVE instruction, encoded as: 00ssRRRmmmMMMrrr ss = size 01 = 8 bits 10 = 32 bits 11 = 16 bits RRR = src register mmm = src mode MMM = dest mode rrr = dest register The obvious choice of setting the size bits to '00' for 64 bits is out of the question, because that overlaps many instructions (bit manipulations, bounds checking, several specialized move instructions). The whole instruction set is like this---where you would expect 64-bits to specified are a bunch of instructions instead.