Maciej W. Rozycki wrote:
quoted text
> 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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