Author Topic: FSTENV with 14-byte operand under BITS 32  (Read 7279 times)

nobody

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FSTENV with 14-byte operand under BITS 32
« on: January 28, 2009, 12:05:44 PM »
Is there any syntax which allows FSTENV instruction with 14-byte operand under BITS 32?

I have tried:

o16  fstenv [eax]

which assembles to:

66 9B D9 30

what is wrong, because the leading prefix 0x66 have no effect on fstenv operand.

It must be:

9B 66 D9 30

nobody

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Re: FSTENV with 14-byte operand under BITS 32
« Reply #1 on: January 28, 2009, 09:31:13 PM »
(bits 32)
call pm2rm
bits 16
fstenv [eax] ; *better* be less than 64k!
call rm2pm
bits 32

I'm pretty sure that's not what you have in mind. I see no indication that a size override prefix is going to have any effect, regardless where it's placed. Does 9B 66 D9 30 actually do what you want??? Seems to me that even if you *could* save ip instead of eip, and a truncated address... it would be totally useless.

I'm only certified for Beginner Questions, so I could be totally wrong, but what you want to do seems impossible to me...

Best,
Frank

nobody

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Re: FSTENV with 14-byte operand under BITS 32
« Reply #2 on: January 29, 2009, 10:16:03 AM »
> I see no indication that a size override prefix is going to have any effect, regardless where it's placed.

See Intel manual Volume 1: Basic Architecture, chapter 8.1.10 Saving the x87 FPU’s State with FSTENV/FNSTENV and FSAVE/FNSAVE:

"The manner in which this information is stored in memory depends on the operating
mode of the processor (protected mode or real-address mode) and on the operand size
attribute in effect (32-bit or 16-bit)."

Under BITS 32, operand size override provides 14-byte state information instead of default 28-byte one.

According to the documentation, "o16 fstenv [eax]" should work, but it places the prefix 66 at wrong place. It must be behind the WAIT prefix (9B) to take effect.

That's why I ask for another syntax which would provide the 14-byte operand.

Best regards,
MazeGen

Online Frank Kotler

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Re: FSTENV with 14-byte operand under BITS 32
« Reply #3 on: January 29, 2009, 08:41:27 PM »
fwait
o16 fnstenv [...]

???

Seems to produce the sequence you're looking for - 9B 66 D9 ...

Seems like kind of a PITA to have to write it that way - wait, oh wait, don't wait... Appears to store different information with/without the 66 - I haven't confirmed that it's "right"...

Best,
Frank