On 11/29/2010 03:42 PM, Alan Cox wrote:
quoted text
>> The usual layout is that the PCI bus is a direct child of
>> the root node, and the ISA bus is a child of the PCI bus.
>> That reflects the "Northbridge + Southbridge" wiring that
> 
> That isn't strictly true either. On many PC devices the ISA bus (or LPC
> bus nowdays) has no heirarchy as such because ISA cycles get issued if
> the PCI cycles don't generate a response. In addition some cycles go to
> both busses on some chipsets and there are various bits of magic so the
> I/O spaces and particularly the memory spaces are intertwined.
> 
> So it's not a subordinate bus really, its a bit weirder. PCMCIA is
> probably a sub-bus when you've got a PCI/PCMCIA adapter but ISA in
> general is a bit fuzzy.
> 

Actually, it can go both ways -- there are ISA/LPC busses which are true
childs of PCI busses -- in particular, are subject to the decoding
restrictions of the host bridge -- and there are those that aren't
logically even if they are physically.  The reason for this is that
subtractive decoding can be done either at the back end (as in a classic
PCI/ISA system with a single PCI bus) or at the front end (as in
HyperTransport for example.)

	-hpa
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