About MMX /FPU as parameter-register

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hopcode

Joined: 04 Mar 2008
Posts: 563
Location: Germany

hopcode 30 Jan 2010, 01:22

I have used one time MMX/FPU registers to save and restore ESP/EBP in a proc.
I know they are volatile,like ECX/EDX etc., during context switching.

Is is dangerous to use them in the following way ? (i guess NO, because the whole remains in the same context, like respectively for ECX, or pending FPU instructions in other contexts do not affect too those in the thread)

Code:

 (pseudocode)
beginthread theproc
     ret

theproc:
     movd mm0,eax
        movd ecx,mm1
        call subproc
        ret

subproc:
     movd edx,mm0
        mov eax,ecx
 ret

Thanks,
hopcode
.
.
.

30 Jan 2010, 01:22

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20451
Location: In your JS exploiting you and your system

revolution 30 Jan 2010, 01:30

No registers are volatile due to context switching. The OS has to guarantee this else nothing will work.

But the FPU/SIMD registers may be corrupted by API or library calls. Other than that you can use them as much as you please without any concern.

30 Jan 2010, 01:30

hopcode

Joined: 04 Mar 2008
Posts: 563
Location: Germany

hopcode 30 Jan 2010, 01:48

OK,Alles Klar,thanks a lot for your fast answering.

btw: i was so sure about the fact that FPU/SIMD registers may be corrupted by API or library calls, (because i see them everyday running) that i have forgotten completely that an OS is behind all, and this is effectively the answer to my perplexity.
It is incredible how much clever Razz

i am

30 Jan 2010, 01:48

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