A.5.266 PXOR: MMX Bitwise XOR
PXOR mm1,mm2/m64 ; 0F EF /r [PENT,MMX]
PXOR xmm1,xmm2/m128 ; 66 0F EF /r [WILLAMETTE,SSE2]
PXOR performs a bitwise XOR operation between its two operands
(i.e. each bit of the result is 1 if and only if exactly one of the
corresponding bits of the two inputs was 1), and stores the result in
the destination (first) operand.