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Re: What goes on a Motherboard?

To: Steven.D.Ligett@Dartmouth.EDU
Subject: Re: What goes on a Motherboard?
From: Pat Mackinlay <SMACKINLA@cc.curtin.edu.au>
Date: 29 Jun 1993 13:46:48 +0800
Cc: riscy@pyramid.com
>Let's assume that we're designing a motherboard that will have some sort of
>I/O bus.  Then, I think that we want to put as little on the motherboard as
>we can, so it'll be cheaper and easier to design.  What HAS to go on the
>motherboard?  High-bandwidth things.  Low latency things.  Things to make it
>easier to get the motherboard going.  Just enough to make the board useful. 
>Little else.  The more things we leave off, the better.  My list has

I personally agree with Steve's list here.

>Some would argue that the last two aren't needed on the motherboard.  Some
>would argue that ethernet is needed.  Ethernet doesn't need much bandwidth.

Again, I agree with this. To me, Ethernet isn't really a big deal. I think 
we would be better off attaching any Ethernet devices to whatever I/O bus 
we use - as Steve says, Ethernet isn't a high bandwidth device, and doesn't 
really "deserve" to be on the motherboard.

>I thought that a 3041 (a little 3051 w multiple bus width support) should run
>the I/O bus.  The 3041 could also be the graphics accelerator.

This is my only real gripe with Steve's suggestion - I'd prefer to use a 
"dedicated" I/O chip like the 3730 (or possibly the C&T thing?) to drive 
the slower I/O devices. Seeing as we have to have a DRAM controller anyhow, 
I think it'd be better to use a single device to handle both DRAM and I/O. 

A second processor will bring a couple of its own problems into the deal, 
and I don't really think the payoff is worth it. I think the main CPU will 
be perfectly capable of dealing with the interrupt latency and bandwidth 
requirements of the I/O chips - it _is_ a 35 MIPS chip...

I would much rather see a more "conventional" design, using one of the I/O 
chips that have been suggested, the 3730 or the C&T thing (the one Neil's 
looking at). Because they're specifically targetted at this type of system, 
they will be pretty easy to use and integrate, I'd imagine, and think they 
would be the most cost-effective way of doing I/O. The other important 
point is that these chips have DRAM control as well, which has to be 
implemented somewhere. One other advantage is that they're going to take up 
very little board space and reduce the time needed to design the board.

Steve: think "toaster"... <grin>

Pat -- "There's only one thing left to do Mama, I got to ding a ding dang
        my dang a long ling long" (Jesus Built My Hotrod -- Ministry)
GCS d* -p+ c++ l++ m--- s+/- !g w- t- r


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