[rep_lodsb]

I don't doubt that this specific processor special-cased XOR (regardless of how it was called in the assembly language)!

Merely pointing out that where both operations were available, there seems to have been a preference to use SUB instead, with some continuity from early business-oriented mainframes, to the 360, to the PC.

[fweimer]

You probably would prefer to use SUB with fault-checking to clear registers in general-purpose code, and only use XOR in early startup (and perhaps fault handlers), where error checking has to be suppressed. So both observations seem to align well?