On Tue, 1 Jan 2008, H. Peter Anvin wrote:

quoted text
> 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.

  Maciej
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