[chasd00]

> - mov ax, 0 - needs 4 bytes (66 b8 00 00) - xor ax,ax - needs 3 bytes (66 31 c0)

iirc doesn't word alignment matter? I have no idea if this is how the IBM PC XT was aligned but if you had 4 byte words then it doesn't matter if you save a byte with xor because you wouldn't be able to use it for anything else anyway. again, iirc.

[Narishma]

No, the 8088 used in the PC has a 2 byte word size. More importantly, it only has an 8-bit data bus, so alignment didn't really matter because it fetched instructions one byte at a time.