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From: brian@galileo.uucp (Brian Donnell [PT4])
Newsgroups: comp.sys.mac.misc
Subject: Re: 20megs - System used = 6megs left?
Summary: System 6.0.x and 24-bit 7 CAN Address more than 8 Megs
Keywords: RAM 6.0.x 7.0 24-bit
Message-ID: <1991Jun26.192234.869@aio.jsc.nasa.gov>
Date: 26 Jun 91 19:22:34 GMT
References: <5494@network.ucsd.edu> <1991Jun26.170014.14999@midway.uchicago.edu>
Sender: brian@galileo.jsc.nasa.gov
Organization: NASA/Johnson Space Center
Lines: 42

In article <1991Jun26.170014.14999@midway.uchicago.edu> xdab@ellis.uchicago.edu (David Baird) writes:
>In article <5494@network.ucsd.edu> simon@ivem1.ucsd.edu (Simon) writes:
>>Hello. I have a Mac IIfx and 20 megs installed inside, 4x4meg SIMMS,
>>and 4x1meg SIMMs.  The problem is, the system only show 6 or so megs for use
>>while the System seems to be taking the rest of the available ram (13megs).
>>
>>Does anybody know how to get the memory back into unused ram?  Or is the
>>SIMMs bad?  Or are they in wrong?  I don't think it's in wrong; the 4
>>meg SIMMs are in Bank B while the 1 meg SIMMs are in Bank A.
>>
>>Any help would be appreciated.
>>-Simon
>
>You did not say if you were running System 6.0.x or System 7.0. If 
>you are running System 7.0 turn on 32-bit memory with the
>Memory cdev in the Control Panels.
>
>If you are running System 6.0.x or System 7.0 without 32-bit memory
>turned on, your machine can only access 8 megs of Ram. The System
>will keep all the memory because it doesn't know what to do with
>it otherwise. (OK, I know thats not a techie explanation, but
>it does "explain" the situation.    :-)     ).
>

No - that is not quite right.  System 6 and 24-bit System 7 can address
16 Megs of RAM.  However, 2 Megs is reserved for ROM, and 1 Meg is reserved
for each filled NuBus slot.  It is important to put all your cards in the
highest numbered slots possible to avoid fragmenting the address space.

For example, if you have just one card (the video board) in the slot 6
of the Mac IIfx, then System 6 or 24-bit System 7 will be able to address
up to 13 Megs for your applications (16 - 2 - 1).

System 7 32-bit has similar limitations except that they start at the 1 Gig
boundary instead of 16 Meg (I think - somebody correct me if I'm wrong on
this one).  In other words, the same machine above in 32-bit mode should be
able to address 997 Megs (1Gig - 2 - 1).  Why the System didn't put things
at the top of the 32-bit address space (thus allowing 3997 Megs for
applications) I don't now.

Brian Donnell
NASA/JSC