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From: rmh@apple.com (Rick Holzgrafe)
Newsgroups: comp.sys.mac.system
Subject: Re: Why does 24-bits => 8 MB RAM use; also a 24-bit story
Message-ID: <14117@goofy.Apple.COM>
Date: 17 Jun 91 21:21:29 GMT
References: <4112@nosc.NOSC.MIL>
Sender: usenet@Apple.COM
Organization: Apple Computer, Inc.
Lines: 30

In article <4112@nosc.NOSC.MIL> samuels@halibut.nosc.mil (Lawrence J. 
Samuels) writes:
> I'm sorry to repeat this 'frequently-asked question' (hey, maybe 
> we need out _own_ FAQ list! -) ), but why, oh why, is it
> that 24-bit machines, such as my IIcx, can 'use' only 8MB of
> physical RAM?  As we've all heard too many times, 2^24 = 16 MB, etc., 
> but in 24-bit mode, RAM above 8 MB can't be used, as I understand.

It's because addresses are used to access more than just RAM. Of the 
"missing" 8 Mb, 1 Mb is used to access ROM, 1 Mb is used to access 
hardware (sound chips, serial chips, etc.), and 1 Mb is assigned to each 
of the NuBus slots.

That 6 Mb to the NuBus slots applies whether or not the slots are 
occupied, and whether or not your particular Mac actually has 6 slots. 
(Your IIcx has only three.) If you turn on VM, and cluster all your NuBus 
cards in the slots farthest from the power supply, the system will let you 
use those extra 1 Mb's for RAM, giving you more than 8 Mb of usable RAM.

I'm not sure about IIcx's in particular, but a Mac II with a single video 
card in the far slot can access 13 Mb of RAM, *if* VM is turned on.

Hope this helps.

==========================================================================
Rick Holzgrafe              |    {sun,voder,nsc,mtxinu,dual}!apple!rmh
Software Engineer           | AppleLink HOLZGRAFE1          rmh@apple.com
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