Re: Cyrix M2

Richard B. Johnson (
Thu, 24 Apr 1997 14:26:55 -0400 (EDT)

On Thu, 24 Apr 1997, Mike Jagdis wrote:

> On Wed, 23 Apr 1997, Thayne Harbaugh wrote:
> > I remember a big discussion about MMX that happened not
> > too long ago. The conclusion was that the penalty to switch
> > between FP & MMX was too great for a true multitasking OS.
> (Cyrix had to use FP registers for MMX to be at all compatible
> with Intel. I don't think they liked having to do it :-). Intel
> did it to save the cost of engineering their silicon to hold a
> few more registers.)
It isn't a cost issue! Sources at Cyrix say they had to have a way for
an unknown operating system to be able to save and restore the new
registers without having to rewrite all the operating systems that use

It was presumed that, during a context switch FSAVE and FRSTOR will
be executed to save/restore the state of the FP Unit. If executed,
these instructions should automatically save and restore the MMX
register(s) also. I don't know if they have to extend the buffer
length for this operation. If so, the point is moot because the
operating systems will have to be rewritten to provide this additional

Dick Johnson
Richard B. Johnson
Project Engineer
Analogic Corporation
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