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From: bash@cbnewsd.att.com (thomas.w.basham)
Newsgroups: comp.sys.ibm.pc.hardware
Subject: Mixing 1M and 256K simms- Why not?
Keywords: Reasons for disallowing?
Message-ID: <1991Feb23.045704.985@cbnewsd.att.com>
Date: 23 Feb 91 04:57:04 GMT
Organization: AT&T Bell Laboratories
Lines: 32


There seems to be a consensus (or myth) that a system using SIMM
memory can't mix 1M and 256K simms.  My machine uses two banks of
4 slots.  Bank 0 must be filled, bank 1 is optional.  Since the
system obviously has the ability to determine bank 1's status,
why is it beyond reason to simply see how high one can access bank 1,
add that to what's in bank 0, and store it in CMOS.  

Those who have 1M in 256K's and upgrade to 4M wouldn't have to sell
off their 256K's, but now have 5M.

My question is:  why wouldn't/isn't this done?  Are there valid
reasons (other than programming shortcuts) to keep memory to a
1/2/4/8 M system.

Or:

If this mixing can be done, why is so few people seem to know about this?

Seems to me it's nothing but poor BIOS programming that can't access
the separate banks with different ranges.  I've mixed ram types in
(homemade) micros before, albiet (considerably) smaller systems.  It's
really just a matter of knowing what the top address is, and not
exceeding it.

Very curious,

Tom
-- 
Tom Basham      AT&T Bell Laboratories   
bash@ihlpb.ATT.COM   I'm the NRA.  I also vote.  
I can't say what AT&T does.