[Fwd: Re: [OpenBIOS] Why protected mode?]

[Andrew Kohlsmith]

for some reason this message didn't make it ot the list...

Andrew

-------- Original Message --------
Subject: Re: [OpenBIOS] Why protected mode?
Date: Thu, 15 Jul 1999 08:20:22 -0400
From: Andrew Kohlsmith <akohlsmith at benshaw.com>
Organization: Benshaw, Inc.
To: openbios at elvis.informatik.uni-freiburg.de
References: <3.0.32.19990707182234.01c84ec0 at pop.netschools.net>

> >simulator, does it not? My recollection is that the only way out of
> >protected mode (back in 286 days) was through a keyboard reset sequence.
> 
> That was only on the '286, which required a processor reset to get back
> into real mode and was assisted by the keyboard controller, one of the
> Great PC Hacks.  It is not as traumatic with '386 and beyond.

Actually that was the kludge to fix it.  The *really* savvy way to do it was
a triple fault.  Something along the lines (going from memory here) like
trashing the stack and executing an illegal instruction.  The processor
would take exception to the illegal instruction (fault 1), go to push
CS,IP,F to the stack, which wasn't there (fault 2), cause exception to that
and go to push them again (fault 3) so it would reset.  Thousands of times
faster than playing with the keyboard controller to hit reset.

That's all from memory so it may be a little incorrect, but the theory is
sound.  It may have been only a double fault, but either way that method was
MUCH faster than resetting through the keyboard controller.

Andrew
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