riscy
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Re: SIMMs

To: riscy@pyramid.com
Subject: Re: SIMMs
From: kent@tifsim.pac.sc.ti.com (Russell Kent)
Date: Fri, 9 Jul 93 11:22:44 CDT
In-reply-to: <9307091020.AA16867@resi.waldorf-gmbh.de>; from "Andreas Busse" at Jul 9, 93 12:20 pm
Reply-to: riscy@pyramid.com
Sender: riscy-request@pyramid.com
I've been thinking about memory.  I'm a software weenie, so if I've
seriously mis-understood, please *gently* correct me.

Some folks have pointed out that since this is a RISC CPU, then the
executables will be significantly larger (40% was the number I read).
This implies that 8Mb of RAM really is not sufficient.  The next logical
memory size is 16Mb (you could go to 12Mb, but that makes 2-way interleaving
impossible I believe).  To take full advantage of 2-way interleaving though,
you need two banks, which therefore implies 16 1Mb SIMMs.

This has the nice feature of being expandable to 32Mb, 40Mb, and 64Mb.
(Since any bank of 8 could be upgraded to 4Mb SIMMs.  Upgrading a single
bank of 4 would eliminate the 2-way interleave capability, right?)

BUT, it requires all that expensive 4-, 6-, or 8- layer M/B real estate.
Plus drill holes through all those layers is not a pleasant thought (routing
headaches, reliability, cost).  And 16 SIMM sockets ($).

Since Waldorf has (seems to have) agreed to produce boards that have surface
mount components, would it be reasonable to mount discrete memory chips
on the M/B?  Here was what I was considering:

  18 1Mbx4 70ns (is 70ns fast enough?) surface mount DRAMs on the M/B.
      (18 because I assume the 3730 part wants parity bits)
   8 standard SIMMs on M/B

This way every board has 8Mb to start with.  This is enough for a bare-bones
no X11 system (or one that swaps alot :-).  One can easily add an additional
8Mb to make a 16Mb system.  Or add 32Mb to make a 40Mb system.  Or give up
2-way interleaving (if that's possible) and make a 12Mb, 24Mb, or 28Mb system.

Important questions:

  - Will the 18 components in small surface mount packages use less
    real estate than 8 SIMM sockets?

  - How does mounting 18 discretes compare with 8 SIMM sockets cost-wise?
    (PWB fabrication has fewer holes, but board stuffer has more stuffing).

  - If one makes as few as 56 boards you break the 1000's in the DRAM count
    (which gets you to the first cost break).  But cost-wise how does this
    compare to SIMMs?

Russell Kent
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