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

From: Sebastian Herbszt
Subject: Re: [Qemu-devel] Re: POST failure (loop) with isapc and seabios
Date: Fri, 27 Nov 2009 20:13:12 +0100

Jamie Lokier wrote:
Sebastian Herbszt wrote:
>We could have qemu do a soft reset (not reload rom) on a triple fault
>or keyboard controller reset, and then have SeaBIOS request a hard
>reset (have qemu reload rom) if it detects a soft reset that is not a
>"resume" request.
>I'm also not sure what qemu does today.

I don't think such an interface would add a long time benefit and
would prefer a proper solution which will not tie seabios even more
to qemu. One day it might be possible to run a commercial BIOS on
qemu, just like it is possible on Bochs.  Adding this interface
might prevent that.

What does a real BIOS on real hardware do?

Do real BIOSes make a decision as described above, and we're just
assuming they don't?

A real BIOS gets started, checks CMOS shutdown status byte and decides
whether to POST or resume execution somewhere else.

One way to answer may be: On a real PC with i440FX, what kind of reset
are the different reset methods (triple fault, keyboard etc.) normally
configured to do?

Should be those

power on -> hard reset
CPU shutdown bus cycle -> soft reset
TRC register (BISTE=0, SHRE=0, RCPU=1) -> soft reset
TRC register (BISTE=0, SHRE=1, RCPU=1) -> hard reset
keyboard controller -> soft reset
I/O port 92h -> soft reset
RC register (SRST=0, RCPU=1) -> soft reset
RC register (SRST=1, RCPU=1) -> hard reset

IIRC, HIMEM.SYS in DOS must use at least one of the
switch-to-real-mode methods to work, and that driver is needed by
Windows 3/3.11/95/98/ME, but I'm a bit vague on the details.

The highmem.sys version i got here seems to use CR0 to switch between real and
protected mode. I can't find where/if it loads a hosed IDT.

- Sebastian

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