On Mon, Oct 19, 2015 at 10:00 PM, Timothy Gu <timothygu99 at gmail.com> wrote: > + mov r6b, byte [src1q + iq] > + sub r6b, byte [src2q + iq] > + mov byte [dstq + iq], r6b Only r0b-r3b can be used for byte-sized instructions on x86-32.