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

[malc]

On Thu, 7 Apr 2011, Anthony Liguori wrote:

> 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.

It starts in big-real mode so only a20 can be a limiting factor..

> 
> 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.
> > 
> 
> 

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