Re: Transparent bridges (a. k. a. HUB-to-PCI bridges)?

Charles Swiger wrote:
> On Nov 24, 2004, at 10:19 AM, John Baldwin wrote:
> 
>>> I've been wrong before, but please double-check diagrams like:
>>>
>>> http://www.intel.com/design/chipsets/850/pix/850_800.gif
>>> http://www.viatech.com/en/products/chipsets/p4-series/pt880/
> 
> [ ... ]
> 
>> The northbridge is the host-pci bridge.  It contains a virtual PCI-PCI
>> bridge/bus that represents AGP.
> 
> 
> Agreed, although AGP is something of a special case.
> 
>> The chipset uses a propietary interconnect to the southbridge...
> 
> 
> Such as VIA's V-link.
> 
>> ...such that the devices the north and south bridges connect
>> to show up as one pci bus (bus 0).  You could build a system without a
>> southbridge (just PCI-X bridges or some such) and it would still have a
>> host-pci bridge.
> 
> 
> You and Scott are correct.  pciconf claims that something like a 440BX 
> northbridge (82443BX) contains both a HOST-PCI and a PCI-PCI bridge, 
> whereas the PIIX4 southbridge (82371AB) has a PCI-ISA bridge, as well as 
> ATA and the other things I'd mentioned.
> 
> I apologize if I confused the person I was trying to answer.
> 

Not a problem.  The V-Link and ICH buses are really just high-speed
tunnels for PCI.  Before their invention, the southbridge was directly
connected to the northbridge via PCI.

Scott