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From: q4kx@vax5.cit.cornell.edu (Joel Sumner)
Newsgroups: comp.sys.apple2
Subject: Re: Apple IIGS's Memory Limit (17Megs)
Message-ID: <1991Jun22.110827.5652@vax5.cit.cornell.edu>
Date: 22 Jun 91 11:08:27 EDT
References: <864@generic.UUCP>
Distribution: comp
Organization: CIT, Cornell University
Lines: 22

In article <864@generic.UUCP>,
ericmcg@pnet91.cts.com (Eric Mcgillicuddy) writes: 
> The GS can address up to 16Megs of memory, however half of that is ROM and/or
> I/O spaces. The 8Megs can all be RAM, however there are only 2 bank selector
> lines for DMA, so only 4 banks in the memory expansion board can be accessed
> using DMA. How big these banks are depends on the address lines available,
> currently this is 1Meg per bank.

Be carful when you use the term 'bank' in the above context.  1 Memory Bank
as defined for the Apple IIgs memory map is 64K.  In the above example, you
mean 'rows'.  The memory expansion port only allows 4 rows of chips 
(256K/row with 256 Kbit chips or 1Meg/row with 1MBit chips) to be addressed
by DMA. Applied Engineering did try to make 6 addressable rows by reading
some bus line in addition to the 2 bits (4 rows) used in the standard DMA
hardware but I don't remember what degree of success they had.

-- 
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