Re: mmx support with smp

Jeremy A. Gilbert (
Wed, 9 Jul 1997 10:36:01 -0400 (EDT)

On Wed, 9 Jul 1997, Ricky Beam wrote:

> But isn't there some other things going on insode the CPU re: FPU/MMX use?
> If you context switch a task off a processor the is using MMX, then load
> any other process expecting FPU setup...
> As I remember, there is a price (a heavy one) for switching back and forth
> between MMX and FPU usage. The kernel may not be able to tell what mode
> the application has put the CPU in. We shall see, I'm sure.

I'm really not sure. I'm not a x86 hacker by any means. But it
makes sense to me that if you are going to add new registers to a chip and
you don't want to break anything, you better do your best to make existing
context switching routines continue to work with the new registers. In
this case, it would appear that Intel was recognizing this rubric when
they overylayed the registers for the FPU/MMX. If there was a status flag
that switchs between MMX and FPU mode (and I am by no means sure that
there is), it probably is embedded in some other CPU state identifier is
preservered. (An extra bit in a status register somewhere, maybe.)

Jeremy Gilbert +
Webmaster & UNIX Systems Support +
Department of Computer Science +
Brandeis University +