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SubjectRe: [PATCH] x86: provide a DMI based port 0x80 I/O delay override.
Maciej W. Rozycki wrote:
> On Tue, 1 Jan 2008, H. Peter Anvin wrote:
>
>> It's specifically a side effect *we don't care about*, except in the
>> by-now-somewhat-exotic case of 386+387 (where we indeed can't use it once user
>> code has touched the FPU -- but we can fall back to 0x80 on those, a very
>> small number of systems.) 486+ doesn't use this interface under Linux, since
>> Linux uses the proper exception path on those processors. If Compaq had wired
>> up the proper signals on the first 386 PC motherboards, we wouldn't have cared
>> about it on the 386 either.
>
> It was actually IBM who broke it with the 80286-based PC/AT because of
> the BIOS compatibility -- the vector #0x10 had already been claimed by the
> original PC for the video software interrupt call (apparently against
> Intel's recommendation not to use low 32 interrupt vectors for such
> purposes), so it could not have been reused as is for FP exception
> handling without breaking existing software. I suppose a more complicated
> piece of glue logic could have been used along the lines of what
> eventually went into the i486, but presumably the relatively low level of
> integration of the PC/AT made such additional circuits hard to justify
> even if it indeed was considered.
>

Supposedly the reason was that the DOS-less "cassette BASIC" delivered
by Microsoft used all the INT instructions except the reserved ones as a
weird bytecode interpreter. Bill Gates was fond of that kind of hacks.

-hpa


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