Re: [Qemu-devel] How does the QEMU load the binary files bios.bin and vgabios-cirrus.bin?

[Anthony Liguori]

On 04/07/2011 10:31 AM, Gleb Natapov wrote:
> On Thu, Apr 07, 2011 at 08:28:37AM -0500, Anthony Liguori wrote:
> > On 04/07/2011 03:22 AM, Bei Guan wrote:
> > > Hi,
> > >
> > > I have some questions about the qemu's bios. How does the QEMU
> > > load the binary files bios.bin and vgabios-cirrus.bin? Which
> > > function or code file need I to pay more attention to?
> > >
> > > For the loading of vgabios-cirrus.bin and bios.bin, I just trace
> > > them into the same funciton rom_add_file() in hw/loader.c. Is it
> > > the right function, which loads the bioses?
> > >
> > > And then another question, how qemu give the control to bios when
> > > the bios file is loaded? Maybe this question is not in the scope
> > > of qemu, however, can you give me some cue point.
> > I had some stuff written up locally so I posted it to the wiki at
> > http://wiki.qemu.org/Documentation/Platforms/PC
> >
> > The x86 architecture defines the initial state of the chip to have
> > the CS register have a base of 0xF000 and an IP of 0xFFF0.  The
> > result is that the actual memory address of the first instruction
> > falls at the end of the BIOS ROM segment.  This is the entry point
> > to the BIOS.
> Actually after reset on x86 IP=0x0000fff0, CS=0xf000, CS.BASE= 0xffff0000,
> CS.LIMIT=0xffff. So the execution begins at 0xfffffff0 where ROM is
> mapped initially.

That impossible because 1) the processor starts in 16 bit mode so such 
an address cannot be generated 2) the processor has a20 held to zero 
which makes that the processor cannot generate a load to an address with 
the 20th bit set to anything but zero.

The CS base starts out at 0xf0000 and IP is 0xfff0.  That gives a real 
address of 0xffff0.  This is usually a trampoline to somewhere else in 
the space.

The mapping of BIOS to the top of 4GB is just a convention that modern 
BIOSes use because the legacy space isn't big enough for most modern BIOSes.

Regards,

Anthony Liguori

> > The VGABIOS is treated like any other option ROM and is initialized
> > during option ROM scanning.
> >
> > Regards,
> >
> > Anthony Liguori
> >
> > > Any reply are appreciated. Thanks.
> > >
> > > Gavin
> --
>                         Gleb.