Re: [PATCH 7/9] x86/asm/entry/32: tidy up some instructions

From: H. Peter Anvin
Date: Wed Apr 01 2015 - 16:57:54 EST

On 04/01/2015 01:52 PM, Denys Vlasenko wrote:
> On 04/01/2015 05:50 PM, Linus Torvalds wrote:
>> On Wed, Apr 1, 2015 at 4:10 AM, Denys Vlasenko <dvlasenk@xxxxxxxxxx> 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.

For Intel processors it is true for Pentium Pro and later processors, as
far as I know.


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