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

[Sebastian Herbszt]

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