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 Forums >>  IDA Pro  >>  IDA WAIT/FPU Problem

Topic created on: November 1, 2008 07:26 CDT by Piotr .


Just a short question, why IDA gives me one instruction instead of two - is it somekind of feature? For example here:

9B D9 3C 24 fstcw   [esp+4+var_4]

When it should be:
9B               wait
D93C24           fstcw word ptr [esp]


Is there any way to split this up to those two instructions?


Thanks.

  igorsk     November 2, 2008 12:24.15 CST
I think some compilers insert fwait before every instruction so IDA usually merges it with the following FPU instruction.

  Piotr     November 2, 2008 14:26.01 CST
Is there away to split it into two instructions? That may sound pretty odd but i've a reason to do so. Thanks.

  cseagle     November 2, 2008 17:58.44 CST
Intel defines the byte sequence 9b d9 ... as fstcw and the sequence d9 ... as fnstcw.  While the 9b is indeed an fwait, IDA seems to be adhering to the intel specification in this case.  I don't see any processor options that stand out which will allow you to easily split these apart.  You could consider writing a plugin that processes the custom_ana notification in order to modify the existing x86 processor module's behavior.

  Piotr     November 3, 2008 00:06.12 CST
Yo, seems you are right. Although i was pretty suprised that most of the disassemblers treat this instruction like two separate ones unlike IDA. However like you have mentioned it seems that both representations are correct. Thanks.

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