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Date:      Mon, 14 Apr 1997 17:09:05 -0700
From:      Arlie Davis <arlied@microsoft.com>
To:        "'Doug Russell'" <drussell@saturn-tech.com>, Kevin Van Maren <vanmaren@fast.cs.utah.edu>
Cc:        terry@lambert.org, freebsd-smp@freebsd.org
Subject:   RE: Tyan ATX 1668
Message-ID:  <7D06B4AA8B39D011A64900805F682CDA01001D72@RED-09-MSG.dns.microsoft.com>

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Or if the drivers correctly support INT / IRQ sharing, the way God
intended PCI drivers to work.

My copy of the PCI 2.x spec says that INTs should always always always
be shareable.  The whole idea of having four INTs is to _reduce_ but not
_eliminate_ INT channel sharing.  For example, a downstream PCI bus
(behind a PCI-to-PCI bridge) could have four PCI slots, each with four
INTs.  (The INTs would be shared in the usual round-robin fashion.)  The
INTs from that downstream bus are propagated upward to the main PCI bus
onto the four INTs of the hosting PCI slot.

This is why the Adaptec 3940 is so mind-blowing at first.  But the idea
is that you still _have to support_ INT sharing to fully support PCI.

So, having five PCI slots is completely "legitimate".  You should be
able to put a PCI card in both slot 4 and 5 which both use INT A (or any
common PCI INT) and it _should_ work just fine.  It doesn't, of course,
under most Intel-architecture OSes.

-- arlie

> -----Original Message-----
> From:	Doug Russell [SMTP:drussell@saturn-tech.com]
> Sent:	Monday, April 14, 1997 4:07 PM
> To:	Kevin Van Maren
> Cc:	terry@lambert.org; freebsd-smp@freebsd.org
> Subject:	Re: Tyan ATX 1668
> 
> 
> > Huh?
> > 
> > There *are* *5* slots.
> > Look at http://www.tyan.com/html/s166{2,8}.html  for a picture.
> 
> Physically there are 5 slots, but electrically slot 4 and 5 are the
> same.
> The PCI slots have 4 interrupts, each slot being rotated one further
> down
> the interrupt chain.  Slot 5 is just another slot 4, which, does work
> just
> fine under virtually any circumstance if you use one of them for a
> video
> card or something else that doesn't need an interrupt line.
> 
> Later......						<Doug>
> 
> 



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