[Open-graphics] From PCI to VGA registers

[Paul Brook]

> > I just can't find what that way is. My
> > naive guess is that it might be that the system BIOS only calls the VGA
> > BIOS code for the selected card, and it sets up legacy decode in a
> > card-specific way, but I can just find no way to confirm that, and I've
> > been googling for quite some time now.

That matches my suspicion too.

> Time to disassemble someone's BIOS code and find out?  :)
>
> Note that anyone who does that needs to be careful about how they
> contribute to developing a new BIOS, because we don't want to be
> accused of stealing code.

Bochs and qemu use an open source BIOS and vgabios implementation (the 
actually have two - one for a "real" cirrus card, and one for a fake emulated 
VGA device).
I've never tried multiple heads inside qemu, so I can't guarantee it'll do the 
right thing there. It may be a helpful way of poking at a normal startup 
sequence though.

Paul