From owner-freebsd-alpha  Mon Jun 24 15:10:11 2002
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Date: Mon, 24 Jun 2002 15:09:33 -0700 (PDT)
From: Matthew Jacob <mjacob@feral.com>
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Reply-To: mjacob@feral.com
To: Peter Wemm <peter@wemm.org>
Cc: Andrew Gallatin <gallatin@cs.duke.edu>,
	"M. Warner Losh" <imp@village.org>, ticso@cicely.de,
	alpha@FreeBSD.ORG
Subject: Re: Wither the nexus driver? 
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On Mon, 24 Jun 2002, Peter Wemm wrote:

> Matthew Jacob wrote:
> > 
> > note that on 8X00 and 4100 systems that the PCI chipset is *not* the root
> > nexus (or shouldn't be)
> [..]
> > This is has always been a peeve of mine about *BSD usage of root 'nexus'- the
> > assumption that the PCI chipset is the one and only nexus.
> 
> Logically, shouldn't the "nexus" be the cpu's backplane?
> 
> ie:
> root -> cpu -> nexus -+-> hostpci -> ....
>                       +-> otherbus etc
>                      

A CPU is just another peripheral. The system nexus is, at least to my
understanding, the main system interconnect that all major components tie
into.


If you have to have a tree representation, root == nexus. So,

root/nexus -> cpu
	   -> memory
	   -> iobus

So, whatever the "MAIN SYSTEM INTERCONNECT" is would be the system nexus. For
8X00 systems, it's the TLSB. For PCs it really probably really *is* the
motherboard PCI/memory controller chipset (i.e., PIIIX). On SparcStation 1 it
was the memory/SBus ASIC.





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