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From: af@cs.cmu.edu (Alessandro Forin)
Newsgroups: comp.arch
Subject: Re: Smart IO controllers - my thoughts
Summary: Similar adventures lead to similar conclusions
Keywords: smart I/O
Message-ID: <1991Mar22.155115.27087@cs.cmu.edu>
Date: 22 Mar 91 15:51:15 GMT
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Organization: School of Computer Science, Carnegie Mellon
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[My first attempt at posting this apparently failed, sorry if duplicated]

I share Larry's comments, and since I am not a hardware person I'd like
to perhaps hear the experts on the following, very concrete example.
Larry mentions SCSI, so that's my example. No blessing or flaming
of any particular manifacturer is intended in the following, pls get me
right.

When I wrote my scsi driver for Mach I stomped into the specs for the NCR
53C700 "Script" controller, and I liked it so much, even if it wasn't the
chip I had to fight with, that I modeled my chip-dependent code (for both
the NCR53c94 and the DEC SII 7061) after it.  Both of the latter chips have
the problem Larry mentions of generating too many interrupts, albeit the
53c94 behaves better.  Sounds like a general problem, but not one that
would surmise with the Script chip.

So the question is: how much would it cost to re-design the current
Decstation 5000 SCSI board (it uses the NCR53c94) to use the Script ?

In your thinking I do not believe you can assume much pin-compatibility
between the "old" scsi chip and the "new" scsi chip.
So I guess I am asking
	- how much does it take to design a scsi board from scratch ?
	- how 'bout a redesign ?
	- what is the cost of the parts in it, does "the chip" account
	  for most of it or not ? [This would answer the question of
	  using "expensive CPUs" as I/O processors also]
	- what's the price tag on the 53c700
	- where does a chip like this "loose" ?
	- how bad is the problem of coupling a DMA controller to an
	  off-the-shelf scsi chip ?
	- do hardware engineers like to spend their time working on
	  designing these boards or not ?
	- how could they make the ADAPTEC board affordable to PC owners.

I realize these might not be the right questions/all the questions,
but hey, it's a start.
sandro-