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From: brian@galileo.uucp (Brian Donnell [PT4])
Newsgroups: comp.sys.mac.misc,comp.sys.mac.hardware
Subject: Re: 20megs - System used = 6megs left?
Message-ID: <1991Jun27.190533.13012@aio.jsc.nasa.gov>
Date: 27 Jun 91 19:05:33 GMT
References: <5494@network.ucsd.edu> <14270@goofy.Apple.COM>
Sender: brian@galileo.jsc.nasa.gov
Organization: NASA/Johnson Space Center
Lines: 19

>You are running in 24-bit mode. In this mode, the Mac's cpu only uses 24 
>bits for an address, which means it can address at most 16 Mb. But not all 
>of that 16 Mb is used to access RAM; large amounts are used to access ROM, 
>cards in the NuBus slots, and miscellaneous hardware. The upshot is that 
>only 8 Mb are left for addressing RAM. Your System is using about 2 Mb of 
>that, leaving 6 Mb available for apps. The remaining 12 Mb are shown as 
>belonging to the System only because they are not in use by or available 
>to apps; actually, they're unused.
>

I don't think this is completely correct.  As mentioned for System 7
virtual memory, System 6 and System 7 24-bit (w/o VM) can address more than
8 Mb (up to 13) depending on how many and which NuBus slots are empty.
However, System 6 (I don't know about System 7) will still only allow a single
application a maximum partition of 8 Mb - the system size.  You can get
around this by using just Finder in System 6.  Also, older versions of Macsbug
typically don't like address spaces larger than 8Mb.

Brian Donnell