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From: wiltse@oceana.esd.sgi.com (Wiltse Carpenter)
Newsgroups: comp.sys.sgi
Subject: Re: A Question about SIMMS for IRIS 4D/25
Summary: 4MB simm story not correct on 4D/2x
Message-ID: <47911@sgi.sgi.com>
Date: 10 Jan 90 05:07:47 GMT
References: <9001051539.AA13873@aero4.larc.nasa.gov>
Sender: wiltse@oceana.esd.sgi.com
Organization: Silicon Graphics, Inc., Mountain View, CA
Lines: 20

In article <9001051539.AA13873@aero4.larc.nasa.gov>, blbates@AERO4.LARC.NASA.GOV ("Brent L. Bates AAD/TAB MS294 x42854") writes:
> 
>     I have been told that the 4D/20 and 4D/25 use the same memory.
> However, if you want 32Mb of memory on the main board you have to use
> 4Mb SIMMS, ALL of them must be the same.  So, if you currently have
> 1Mb SIMMS, you will have to remove them and replace them with the
> 4Mb SIMMS.

Whoever told you that 4D/20 and 4D/25 machines use the same memory was
right.  However, the 32MB maximum memory is achieved by using double-
high 1 MB SIMMs, not 4MB ones.  They don't all have to be the same, but
there are restrictions on mixing them.  Basically, it doesn't work to
have ``holes'' in the physical memory map.  Complicating the matter is
the way memory addressing works with dual height SIMMS: one layer at
a time.  So, all of the SIMM sockets must at least have single high
SIMMs in them before the first set of sockets can support double-high
ones.  4MB SIMMs haven't yet been qualified for the 4D/2x line and
may require a PROM fix to some machines; we're working on it.

	-Wiltse Carpenter