REX_R
#define REX_RW (REX_PREFIX | REX_R | REX_W)
#define REX_BRW (REX_PREFIX | REX_B | REX_R | REX_W)
if (reg != NULL && (REX_R & rex_prefix) != 0)
x->d86_rex_prefix |= REX_R;
x->d86_rex_prefix |= REX_R;
x->d86_rex_prefix |= REX_R;