riscy
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Should we have SCSI?

To: riscy@pyramid.com
Subject: Should we have SCSI?
From: caret@pyramid.com (Neil Russell)
Date: Sat, 3 Jul 93 21:27:23 PDT
Reply-to: riscy@pyramid.com
Sender: riscy-request@pyramid.com
In my opinion, Adaptec have one of the best SCSI controllers that you
can get for a PC like machine.  Why is it good?  It provides master
DMA, scatter/gather modes, etc.;  In short, all the things that ISA makes
it difficult to get on an x86 machine.  But now we have the option
to do better.


SCSI on the motherboard:

Advantages:
        *  Higher bandwidth available to the SCSI bus gives higher
           transfer rates and lower latencies
        *  Access to main memory is a 32-bit data path, so consumes
           half of the main memory bandwidth (assuming the 3730 or
           3041 DMA)
        *  Much cheaper than a performance ISA SCSI card.
        *  Not limited to first 16 MB of memory
        *  Don't have to provide ISA bus master mode
Disadvantages:
        *  Cost of motherboard goes up (maybe)
        *  More software to write to drive it, especially if we
           do scatter/gather

If we intend to provide SCSI on ISA rather than the mother board,
we need to provide bus mastering capability to the ISA bus.  I think
that the chip count to do this would be around 10.  The chips would
probably be cheaper than a SCSI controller chip and connector, but they
would need extra board space (most of these chips are buffers
of some kind).  Also, there is a fairly large amount of time required
to design and test such a feature, and then the cost of the SCSI card.

Alternatively, a SCSI controller chip is designed to connect to a
system such as ours and would take relatively little effort to get
going.

Some cost guesses:

    ISA SCSI:
        *  Bus mastering logic:         $10
        *  Board space for logic:       $5
        *  SCSI controller card:        $300
    Motherboard SCSI:
        *  SCSI chip:                   $17
        *  Glue logic:                  $2
        *  SCSI connector:              $2
        *  Board space for this:        $2


For those wanting SCSI, the motherboard wins hands down.

In my last life, I used a MIPS magnum.  For those that don't know, this
is a MIPS workstation like machine which in our configuration had a
MIPS R3000/R3010 running at 33 MHz, 128 MB of memory, and several giga-
bytes of disk connected to a SCSI controller on the motherboard.
Its subjective performance is still the best I've ever seen, including
the 12 CPU Pyramid R3000 machine with its 35 GB of disk that I'm typing
on now.  Originally I put it down to a good operating system and a
fast CPU.  Now I'm sure that they have some good interface to that
SCSI bus.


Comments?
-- 
Neil Russell            (The wizard from OZ)
Pyramid Technology                      Email:  caret@pyramid.com
3860 N. First Street                    Voice:  (408) 428-7302
San Jose, CA 95134-1702                   FAX:  (408) 428-8845

 

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