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Re: [Qemu-devel] Re: POST failure (loop) with isapc and seabios

From: Jamie Lokier
Subject: Re: [Qemu-devel] Re: POST failure (loop) with isapc and seabios
Date: Wed, 25 Nov 2009 06:09:51 +0000
User-agent: Mutt/1.5.13 (2006-08-11)

Gleb Natapov wrote:
> > But QEMU is used to run old OSes too.
> > 
> That's OK. I don't expect BIOS to be reloaded if OS restart by jumping
> to BIOS reset code.

That's good then.

What about DOS and DOS-extender programs which do a soft reset by
triple-faulting the CPU (see Sebastian's notes on i440FX behaviour),
and asking the keyboard controller?

Both of those methods are used by DOS and DOS-extender programs to
switch from protected mode to real mode.  Keyboard controller was used
originally, but then someone figured out that triple fault can be used
(on most PCs) and is faster.

The switch to real mode is done by writing somewhere the BIOS checks,
so the BIOS just branches back to the application.

I think that may imply it has to be a "soft reset" as described by
Sebastian in the i440FX description, and I would think the BIOS must
not be reloaded.

But the BIOS must be reloaded from ROM, I'm guessing, if the keyboard
controller method is used and the word asking for a branch back to the
application has not been set.  Because that's how a modern OS (if not
using ACPI) asks for a system reset.

Do you think the above is (a) correct, and (b) what's implemented?

-- Jamie

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