On Mon, Nov 21, 2016 at 7:58 AM, H. Peter Anvin <h...@zytor.com> wrote:
> On 11/20/16 20:54, h...@zytor.com wrote:
>>
>> I believe i686+ writes zero, older CPUs leave unchanged.
>
> I should point out that, at least from my memory, the same applies to
> instructions like "movl <seg>". I can't even remember for sure how the
> behavior differs between "movl <seg>,<reg32>" and "movl <seg>,<mem>";
> I'd have to do some digging.
I have this distinct feeling that there issues with *both* the
register and memory versions.
Because I have this dim memory that on early microarchitectures, even
"mov segment to register" would always only do a 16-bit move, even if
it was encoded as a 32-bit "movl". Although that may be partly because
I know "gas" had some confusion about operand sizes and segment
register instructions, so there might have been toolchain issues too.
I just dug out my old 486 manual on _paper_ (Christ, I still had it):
"Intel486(tm) Microprocessor Family Programmer's Reference Manual".
The "mov" instruction is only documented for r/m16, and it has a
footnote saying "In protected mode, use 16-bit operand size prefix".
I definitely know that the "only write 16 bits" was the case for
memory accesses, but I think it might have been the case even for
register moves. After all, "mov segment register" is actually a
completely different instruction from the normal "mov" instructions,
even if it often shows up together with them in the instruction
descriptions.
The i686 cleaned up a lot of things, but I think this might be an area
where there were differences between i486 and Pentium and all the
clone chips too.
Linus
