From: tholen@galileo.ifa.hawaii.edu (Dave Tholen)
Subject: Re: How to upgrade PS/2 model 90 from 8 to 16Mb?
Organization: University of Hawaii
Date: Wed, 12 Aug 1992 22:57:00 GMT

Richard Stanton writes:

> Not having done this before, do I just buy and plug in some extra memory
> chips (in which case, what speed and format for a 25MHz model 90), or
> do I have to rip out and throw away the old memory and replace it with
> denser chips? 
> 
> Also, what upgrades are possible (i.e. can I just increase to 9Mb, 10Mb or
> whatever I like, or are there restrictions on the total amount of memory?

I just upgraded a 25 MHz Model 90 from 8 to 16 Mbytes.  You'll want to use
70 nsec SIMMS in this machine.  The only real restriction is that SIMMS must
be added in matched pairs, due to the interleaving scheme the Model 90 uses.
So, to add 8 Mbytes, I bought two 4 Mbyte SIMMS.  The machine already had
8 Mbytes, configured as four 2 Mbyte SIMMS, all in card 1 (closest to the
processor card.  I added the two 4 Mbyte SIMMS to slots J1 and J3 on card 2,
following the instructions in the Quick Reference Guide.  The machine
promptly told me the memory was bad.  A call to IBM verified correct
installation.  Over the weekend, however, I swapped SIMMS around, and
discovered that if I put the 4 Mbyte SIMMS in J2 and J4 of card 1, and the
displaced 2 Mbyte SIMMS in J1 and J3 of card 2, suddenly the whole 16 Mbytes
was working.  A followup call to IBM (obviously a different tech support
person this time) verified that there is a known situation in which certain
4 Mbyte SIMMS won't work in the configuration I tried first.  The solution
is to either get a different set of SIMMS (and there's no guarantee they
will work either -- sounds like it's trial and error) or to try different
orderings of the SIMMS.  Apparently the problem only affects the 4 Mbyte
SIMMS, or possibly how they interact with the 2 Mbyte SIMMS.  Something
about the way the system determines the amount of memory -- allocating
4 Mbytes and then taking away 2 if the SIMM is really a 2 Mbyte SIMM.  At
this point the technical stuff is going over my head.

Anyway, yes, you can just buy and plug in extra memory, as long as you
have enough slots free, remembering to add SIMMS in matched pairs.  You
only need to rip out old SIMMS if all your slots are full and you still
want to add more memory.  With 8 slots total, if you used 8 Mbyte SIMMS
you can bring the Model 90 up to 64 Mbytes total.  If you have the shipped
configuration of four 2 Mbyte SIMMS populating card 1, you can bring it
up to 40 Mbytes by using four 8 Mbyte SIMMS in card 2 before having to rip
out old memory.  I believe that the smallest SIMMS they offer are 1 Mbyte,
and because they must be added in matched pairs, you can increment from
8 Mbytes to 10 Mbytes, but not 9.  You can work out the possible
configurations for yourself, knowing that there are 8 slots available,
and that SIMMS come in 1, 2, 4, and 8 Mbyte flavors, and that they must
be installed in matched pairs (2, 4, 6, 8, 10, 12, 14, 16, 20, 24, 28, 32,
40, 48, 56, and 64 Mbytes are possible).

And if the installed configuration doesn't work the first time, try swapping
the locations.  It may solve the problem.
Newsgroups: comp.sys.ibm.ps2.hardware