Xref: utzoo comp.sys.mac.misc:13502 comp.sys.mac.hardware:12431
Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!sun-barr!olivea!apple!apple.com!rmh
From: rmh@apple.com (Rick Holzgrafe)
Newsgroups: comp.sys.mac.misc,comp.sys.mac.hardware
Subject: Re: 20megs - System used = 6megs left?
Message-ID: <14270@goofy.Apple.COM>
Date: 27 Jun 91 17:56:11 GMT
References: <5494@network.ucsd.edu>
Sender: usenet@Apple.COM
Organization: Apple Computer, Inc.
Lines: 46

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).

This has become a Frequently Asked Question. Here is the Frequently 
Provided Answer. :-)

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.

If you are running System 6.0.x, that's the end of the story. If you're 
running System 7, you have the following options, available from your 
Memory control panel.

If you turn on Virtual Memory, and move all your NuBus cards into the 
slots farthest from the power supply, your system will make available an 
extra megabyte for each empty slot. For example, if your IIfx has only a 
single video card installed, leaving 5 empty slots, you could get back 5 
Mb, making your total usable RAM 8 + 5 = 13 Mb.

You can get back *all* of your RAM by turning on 32-bit addressing (with 
or without Virtual Memory; with 20 Mb, you probably don't need VM!). If 
you do this, your System will still use about 2 Mb, and there will be 
about 18 Mb free for use by applications.

Warning: not all applications are "32-bit clean". Those that aren't will 
bomb in 32-bit mode. Use the Compatibility Checker stack that came with 
System 7 to check your disks for applications that may need upgrading.

Hope this helps.

==========================================================================
Rick Holzgrafe              |    {sun,voder,nsc,mtxinu,dual}!apple!rmh
Software Engineer           | AppleLink HOLZGRAFE1          rmh@apple.com
Apple Computer, Inc.        |  "All opinions expressed are mine, and do
20525 Mariani Ave. MS: 3-PK |    not necessarily represent those of my
Cupertino, CA 95014         |        employer, Apple Computer Inc."