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From: Michal Schulz <michal.schulz@gmx.de>
Subject: Re: [Qemu-devel] Re: [PATCH] Reboot CPU on triple fault
Date: Wed, 16 Apr 2008 12:25:49 +0200
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To: qemu-devel@nongnu.org

On Wednesday 16 April 2008, Michal Schulz wrote:

> > I have to admit that I'm all but an 286 PM expert. What exactly is a
> > triple fault doing there? If it leads to a reset as implemented here, I
> > can hardly imagine that it is of much use in regular context switching.
>
> It is required due to the "feature" of 286 CPU. Once 286 enters protected
> mode, it cannot be switched back to the real mode. The only way to perform
> it is resetting the CPU. A software which tries to go back from protected
> to real mode sets the warm-reset vector in BIOS memory area, sets the
> reboot reason variable in CMOS memory and issues a CPU reset.
>
> CPU may be resetted due to a triple fault :)

and some example code using it (with comments):

http://www.x86.org/ftp/source/3fault/reset.asm

-- 
Michal Schulz