Re: fpu IO port reservation (arch/i386)

From: Bill Davidsen
Date: Fri Sep 28 2007 - 15:29:23 EST

Andi Kleen wrote:
"Maciej W. Rozycki" <macro@xxxxxxxxxxxxxx> writes:

Hi Peter,

Does anybody know why we reserve this range of IO ports for 'fpu'?
AFAIK from all the IO maps I can find on the internet for various x86
chipsets only 0x00f0 is actaully ever used.
There are two ports used: 0xf0 is the busy latch reset and 0xf1 is the coprocessor reset. They are legacy ports resulting from the interesting way the FPU has been wired by IBM in their PC design.

Was it really needed on 386s? I didn't think there was a IBM 386 PC.

There were 386 PC clones for sure, and they almost certainly needed it and still do. IBM was doing the MCA thing at that time and it was a wonderful time for the clone makers.

None of them is used by Linux for i486 and newer systems, which can support the FPU in its native configuration.

I can remove it from x86-64 at least.

AFAIK you are just right, I'm pretty sure there will be systems needing it for 386 and 486, and maybe the old Pentium systems as well. A lot of system vendors wanted it so software for the old systems would still work.

Bill Davidsen <davidsen@xxxxxxx>
"We have more to fear from the bungling of the incompetent than from
the machinations of the wicked." - from Slashdot
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