Re: Interesting pentium-memcpy results

Robert L Krawitz (
Tue, 29 Jul 1997 10:22:51 -0400 (EDT)

Date: Tue, 29 Jul 1997 16:19:31 +0200 (MET DST)
From: Ingo Molnar <>

the 'soft' routine is not complete, eg. it does not save FPU flags .. but
it saves the 'main bulk', the 80 bit FPU registers. Maybe there are other
things too to save? Everything that might be affected by those fildl and
fistl instructions should be saved.

There's also the issue of making certain that the FPU is in a sane
state. If the FPU's in a weird state, it's possible that fildq/fistpq
might not do the right thing, although I don't know that for certain.

also, to reduce the size of the saved context, it's not necessary to use
all FPU registers. I think 4 of them would be enough. This halves context
saving cost. Probably there are enough delay slots to do the looping 'for

The performance advantage from using all FPU registers is significant,
although I don't remember offhand what it was on my system.

Robert Krawitz <> 

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