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To: riscy@pyramid.com
Subject: Re: SIMMs
From: rohrer@fncrd8.fnal.gov (Keith Rohrer)
Date: Fri, 9 Jul 1993 13:48:22 -0600 (CDT)
In-reply-to: <9307091622.AA02441@titania.pac.sc.ti.com> from "Russell Kent" at Jul 9, 93 11:22:44 am
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 ($).
Then, if we run out of space, we can put the memory on a card, and only pay
for the edge connector in terms of 4+-layer MB space; the memory would
be sitting on a 2-layer or so card, which would be cheaper.  But bear in mind
that the motherboard has to be baby-AT size or (PERHAPS) full-size AT size,
to fit in most cases.

> Important questions:
>   - Will the 18 components in small surface mount packages use less
>     real estate than 8 SIMM sockets?
How can they?  You're talking about laying that number of SIMMS down flat on
the motherboard, instead of mounting them nearly perpendicular...
>   - How does mounting 18 discretes compare with 8 SIMM sockets cost-wise?
>     (PWB fabrication has fewer holes, but board stuffer has more stuffing).
How do you replace a bad chip?  How do you even swap the chips around to try
to see which ones are bad?  
>   - 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?
Can we also get the SIMMs in huge quantities?

> Russell Kent



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