Some x86 instructions can have prefixes that modify their behavior in a meaningful way. Such a prefix can be applied generally to any instruction, but it's expe

Some x86 instructions can have prefixes that modify their behavior in a meaningful way. Such a prefix can be applied generally to any instruction, but it's expected to have no effect when applied to an instruction it doesn't make sense with. But it turns out the CPU actually misbehaves in some cases when this is done. Intel released a CPU firmware update to fix it.