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

From: Gleb Natapov
Subject: Re: [Qemu-devel] Re: POST failure (loop) with isapc and seabios
Date: Wed, 25 Nov 2009 14:20:39 +0200

On Wed, Nov 25, 2009 at 06:09:51AM +0000, Jamie Lokier wrote:
> 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.
If offset 0x0f in CMOS contains 0x0a then BIOS jumps to address stored
in memory address 0x467.

> 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.
Reading ich9 spec I see that on this chipset it is possible to configure
what kind of reset triple fault generates. Make it not very reliable. Was
this triple fault trick only needed on 286 anyway?

> 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?
Do different things during reset depending on CMOS values doesn't sound
right to me. I don't know what is implemented right now. I thought that
we reload BIOS on reset.


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