[OpenBIOS] Video (particularly 440GX)

[Chong Chee Yong]

> Unfortunately it may not be that simple on motherboards with integrated
> graphics. There is only one NVRAM chip. This NVRAM is owned by the PIIX4E
> bridge. I am not sure how they would do the expansion ROM base address for
> the AGP stuff. 

If the system BIOS has to initialise the graphics chipset using codes from
itself (instead of the usual call to the graphics chipset BIOS), then the
system BIOS becomes less generic, probably customised for the
motherboard-graphics chip combination. I'm not sure if this would be the
usual case as it seems less cost-effective, because everytime the
graphics chip changes architecturally, the system BIOS has to change.

The expansion ROM base address register is part of the PCI configuration
space which is part of a usual PCI device (i.e. graphics chip).  When a
value (a physical address) is written to the expansion ROM base address
register, the graphics chip will respond to that address, within some
range.  However, if it is not implemented, this register is hardwired to
zero.  So, if it is implemented, you can read off the expansion ROM from
the address that you specify.  If the first 2 bytes are 0x55, 0xaa then it
is a valid expansion ROM, and the ???forgotten??? location is the
initialization routine entry point.  This PCI configuration space is
usually part of the graphics chipset (not on an NVRAM).  The expansion ROM
may however be on an NVRAM.

<I think NVRAM stands for non-volatile random access memory, correct me if
I'm wrong.>

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