Re: [Qemu-devel] Re: [PATCH] Reboot CPU on triple fault

[Michal Schulz]

On Wednesday 16 April 2008, Kevin Wolf wrote:

> > For old MS-DOS apps / OSes, I have the impression this can happen
> > hundreds of times per second.  It's part of task context switching and
> > BIOS calls.
>
> 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 :)

-- 
Michal Schulz