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From: ph@ssd.kodak.com (Pete Hoch)
Newsgroups: comp.sys.mac.misc,comp.sys.mac.hardware
Subject: Re: 20megs - System used = 6megs left?
Message-ID: <1991Jun27.195756.28067@ssd.kodak.com>
Date: 27 Jun 91 19:57:56 GMT
References: <5494@network.ucsd.edu> <14270@goofy.Apple.COM>
Sender: news@ssd.kodak.com
Organization: Eastman Kodak
Lines: 60

In article <14270@goofy.Apple.COM> rmh@apple.com (Rick Holzgrafe) 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).
>
>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.

WRONG!!!  WRONG!!!  WRONG!!!

The original poster has 20MEG of real RAM.  Since the largest memory
partition you can have in 24-bit mode it 14MEG, YOU CAN NOT TURN VM ON!!!

Beleve me I have tried.  I even set my machine to 32-bit mode, turned
VM on, and set my partition to 21MEG.  (I had to use 21 MEG because
VM disables itself if you try a partition <= real RAM size) Then I
switched back to 24-bit mode but poof VM was off again.

The only solution is to reduce your real RAM size to something less
then the VM partition you want to set up.  Then you can use VM again.

In my opinon this really sucks. 

So would everyone stop spreading this myth that you can have a 14 MEG
of memory in 24-bit mode on the Mac.  You can do it ONLY if you have
less than 14 MEG of real RAM!

By the way I would love to run in 32-bit mode but I have software
that I use every day that breaks in 32-bit mode.  And yes I am looking
for a replacement for this software.

Pete

-- 
Pete Hoch                         | ..somewhere..!kodak!ssd!bashow!ph   ..or..
Color Systems ISPD. 3/65/RL       | ph@ekcolorlink.ssd.kodak.com        ..or..
Eastman Kodak Co. 	          | ph@bashow.ssd.kodak.com
Rochester, NY 14650-1805          | 716-722-3285