Path: utzoo!utgpu!water!watmath!clyde!burl!codas!killer!richardh
From: richardh@killer.UUCP (Richard Hargrove)
Newsgroups: comp.sys.ibm.pc
Subject: Re: What's the deal on the 80387 and real mode interrupts?
Summary: Intel reserved interrupts 00h thru 1fh
Message-ID: <2730@killer.UUCP>
Date: 9 Jan 88 03:07:48 GMT
References: <1269@looking.UUCP>
Organization: The Unix(R) Connection, Dallas, Texas
Lines: 39

In article <1269@looking.UUCP>, brad@looking.UUCP (Brad Templeton) writes:
> My documentation on the 387 states that an exception triggers trap 16 (decimal)
> in the 386.   If this maps into the 16th vector in real mode, as it seems to,
> this is the same as IBM's VIDEO Bios call!
> 

As the Intel documentation clearly states, (_iAPX 86/88, 186/188 User's Manual:
Programmer's Reference_, pg. 4-9 and Fig. 4-9 on pg. 4-10)

	"The dedicated and reserved portions of the interrupt pointer table
	 (locations 0H through 7FH), however, should not be used for any other
	 purpose to insure proper system operation and to preserve
	 compatibility with future Intel hardware and software products."

Guess what? Those are all the interrupt vectors associated with interrupts 00h
through 1fh! I think it took incredible stupidity or arrogance on the part of 
some designer at IBM to use the "unused" vectors in this range for the PC BIOS.
This actually caused some problems for the early AT BIOS implemenatations
because the 80286 claimed some of the reserved interrupts (the BOUND 
instruction traps to interrupt 5 if the tested value is out of the specified
bounds, and we all know what's at INT 5 - the print-screen driver!) and the
BIOS code wasn't written to handle them! The BOUND instruction could call
the INT 5 routine with random parameters!

> 
> Anyway, all this makes writing a good general exception handler for all three
> Intel 80x87 chips a tad complicated.  Could the screen clear be the result

Too say the least. Good luck. Just remember the above said designer (or design 
committee) when you run into problems.

You might try tracing through the code pointed to by the INT 13h handler to
see how it discriminates between the two (extremely) dissimilar interrupt 
sources. That is the critical problem. Then re-vector to a truly unused 
interrupt if the '387 exception needs to be handled.

richard hargrove
...!killer!richardh
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