[LinuxBIOS] Question about protect mode?

Yu-ning Feng fengyuning1984 at gmail.com
Sat Jun 2 10:25:23 CEST 2007

2007/5/31, Juergen Beisert <juergen127 at kreuzholzen.de>:
> > Maybe a silly question: Isn't it that PCI needs initialization before
> > we could access it? How could we get there when the processor is
> > fetching the 1st instruction?
> In this case the ROM device is connected to the PCI bus, but not connected
> _as_ a PCI device!. It shares the PCI bus address/data lines only to save an
> additional bus.
> If you reuse (or misuse?) the AD[31...0] lines you can connect up to a 16MiB
> ROM device with 8 bit data width (AD[31...8] as addressbus, AD[7..0] as data
> bus). Add also three separate additional lines (read, write, chip select) and
> you are done. Whenever the chipset generates a ROM device access cycle, it
> does not generate a valid PCI cycle as it only uses the AD[31...0] lines! So
> this does not hurt any other *real* PCI device on the same bus.
> But this may work only at system start. Later on some PCI master devices could
> inhibit this mode. But it doesn't matter: At this point of time the ROM
> content runs from system RAM, so there is no more need to access the real ROM
> device (maybe only to reprogramm it).
> Hope it helps.
> Juergen

Please check whether I have understood correctly.

The processor lauches a cycle with address = 0xFFFF_FFF0. The north
bridge chipset explains this address. In this case, it signals the
read line and chip select line which connect the ROM device, and
select an address of the ROM device through PCI bus AD[31..8]. To the
north bridge, the ROM device is like a common BIOS ROM except that it
uses PCI bus to exchange information. Then the ROM device uses
AD[7..0] to send 1 byte of data back to the north bridge. That 1 byte
of data is a portion of the 1st instruction.



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