> If you decode the instruction, it makes sense to use XOR:

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

Except, apparently, on the pentium Pro, according to this comment: https://randomascii.wordpress.com/2012/12/29/the-surprising-..., which says:

“But there was at least one out-of-order design that did not recognize xor reg, reg as a special case: the Pentium Pro. The Intel Optimization manuals for the Pentium Pro recommended “mov” to zero a register.”