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SubjectRe: [PATCH 7/9] x86/asm/entry/32: tidy up some instructions
On 04/01/2015 05:50 PM, Linus Torvalds wrote:
> On Wed, Apr 1, 2015 at 4:10 AM, Denys Vlasenko <dvlasenk@redhat.com> wrote:
>>
>> I did not know that. I was sure they are always zero extended.
>
> On all half-way modern cpu's they are. But on some older cpu's
> (possibly just the original 386) the segment move instructions
> basically are always 16-bit, and the operand size is ignored (so the
> 32-bit version is just smaller and faster to decode, because it
> doesn't have a 16-bit operand size prefix)
>
> Iirc, the same is true for the values pushed to memory on exceptions,
> so the 'cs/ss' values on the exception stack may not be reliable in
> the upper 16 bits.
>
> I don't remember if the same might be true of "pushl %Sseg". The intel
> architecture manual says segment registers are zero-extended on push.

BTW, AMD64 docs do explicitly say that MOVs from segment registers
to gpregs are zero-extending.



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