Re: MMX for kernel

Jason Flynn (flynnjs@mobile-systems.bt.co.uk)
Thu, 6 Feb 1997 13:40:02 +0000

Hi,
<stefan>
> Hmm. I've been thinking about this, and since MMX instructions use the
> FPU data paths, I'm not sure it's such a good idea to use MMX instructions.
> I read somewhere that some special instruction is used to kind of
> 'switch' between MMX and FPU mode, and that this takes some 50 cycles.
> Now, wouldn't that mean that we'd waste a lot of time here,
> for instance, if some programs make heavy use of FPU and at the same time
> makes a lot of system calls involving MMX instructions?
</stefan>

I not sure but :
isn't it the process of clearing the registers (cos MMX and FPU
use the sam ones) and clearing the pipeline that takes these cycles ?
And surely this would be done anyway during task switching so
the only extra overhead is if you mix types within one task.

Probably wrong,
J
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