Path: utzoo!utgpu!news-server.csri.toronto.edu!rpi!batcomputer!cornell!vax5.cit.cornell.edu!q4kx
From: q4kx@vax5.cit.cornell.edu (Joel Sumner)
Newsgroups: comp.sys.apple2
Subject: Re: Apple IIGS's Memory Limit
Message-ID: <1991Jun17.100712.5532@vax5.cit.cornell.edu>
Date: 17 Jun 91 10:07:12 EDT
References: <1991Jun17.065317.13965@clark.edu>
Distribution: comp
Organization: CIT, Cornell University
Lines: 37

In article <1991Jun17.065317.13965@clark.edu>,
andy@pro-palmtree.cts.com (Andy Stein) writes: 
>     While it may be nonstandard and not fully addressable, isn't the Apple 
> IIGS's memory limit 17 Meagabytes? If one has a ROM 03 GS, and Applied
> Engineering's RamKeeper and two GS RAM Pluses with 8 Megabyes each, isn't
> the total memory capacity of the computer 17.125 Megabytes, with the ROM
> 03's 1.125 Megabyte on board memory? Maybe I'm totally offbase, but that's 
> what AE advertised.

Sorry, but this is incorrect.  The 65816 processor has an address space
of 2^24 or 16 million, 771 thousand, + a few.  Therefore, the most memory
the processor can directly address is 2^24 bytes.  Some of this memory,
though, is reserved by Apple and some of it is already in use.  The main
memory that came with the original Apple IIgs was in addresses
$00/0000-$01/FFFF and $E0/0000-E1/FFFF (Adds up to 256K).  The 128K of ROM
is mapped to $FE/0000-$FF/FFFF.  When you add memory via slinky cards
in the Apple RAM Expansion slot, it gets mapped from $02/0000 on up.
You would think that you still have 15 megs or so to expand though. This is
not the case.  When Apple built the IIgs, it reserved the memory above
$08/0000 for Apple purposes (ROM expansion, etc...) This you can only
have actual RAM from $00/0000-$7F/FFFF or 8 megabytes.
Therefore, the maximum RAM you can have (without some sort of trick that
they may invent in the next 5 years) is 8 Megs.  None of this is including
the 64K dedicated to the DOC.  I am not sure if that is included in the
memory map or not.

For more information, look in the Apple IIgs Hardware Reference.  It is
probably in the firmware reference too.  And I am sure that Matt and Dave
will both jump on this post and clear up any inaccuracies I have
written.

-- 
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