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

[John Baldwin]

On Wednesday 24 November 2004 09:47 am, Chuck Swiger wrote:
> Scott Long wrote:
> > Chuck Swiger wrote:
>
> [ ... ]
>
> >> A host-PCI bridge is typically part of the "southbridge" chip of
> >> modern motherboards; on Intel motherboards this is also called the ICH
> >> chip, such as the 82801AA/BA/CA/etc.  VIA Southbridges include the
> >> VT8233/8235/8237/etc.
> >
> > Nope.  The southbridge typically holds a PCI-ISA bridge.  The host-pci
> > bridge is usually found in the northbridge part of the chipset.  The
> > whole point it to bridge the CPU to one or more PCI buses.
>
> 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", or MCH, connects to the CPU, AGP, RAM, and the
> southbridge.
>
> The "southbridge", or ICH, connects to PCI, ATA, USB, BIOS chip, and the
> northbridge.  Newer southbridge chips may add integrated LAN,
> 1394/Firewire, integrated AC'97 audio, and such via external codec chips
> like the VT6103 PHY.

The northbridge is the host-pci bridge.  It contains a virtual PCI-PCI 
bridge/bus that represents AGP.  The chipset uses a propietary interconnect 
to the southbridge 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.

-- 
John Baldwin <jhb at FreeBSD.org>  <><  http://www.FreeBSD.org/~jhb/
"Power Users Use the Power to Serve"  =  http://www.FreeBSD.org