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Date: Wed, 24 Nov 2004 09:47:13 -0500
From: Chuck Swiger <cswiger@mac.com>
Organization: The Courts of Chaos
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To: Scott Long <scottl@freebsd.org>
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cc: freebsd-current@freebsd.org
Subject: Re: Transparent bridges (a. k. a. HUB-to-PCI bridges)?
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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.

-- 
-Chuck