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From: jb10320@uxa.cso.uiuc.edu (Desdinova)
Newsgroups: comp.sys.apple2
Subject: Re: non-DMA at 4 Megs (GS)
Message-ID: <1990Nov3.121558.7963@ux1.cso.uiuc.edu>
Date: 3 Nov 90 12:15:58 GMT
References: <6503@plains.NoDak.edu> <3143@skivs.UUCP> <4866@sage.cc.purdue.edu>
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In article <4866@sage.cc.purdue.edu> ericm@sage.cc.purdue.edu (Eric Mulholland) writes:
>
>    This has been brought up in the past but I don't remember a definate
>answer to it.  I know that memory above 4 megs is not DMA accessable.
>My question is it 4 megs TOTAL memory or 4 megs of EXPANDED memory?
>I have a 4 meg rancard (RamPRO IIgs) and I'm curious to wether the top
>256K of ram on it is DMA or not.  As you guessed, I have a rom 1 IIgs.

   The limit of memory expansion is based on one and only one thing-
only four rows of RAM are directly supported by the IIgs and it's
DMA scheme.  Four rows = 1Meg with 256K chips, or 4Meg with 1Meg chips
(SIMMs or what not).

   Designers wishing to go past the four row limit can decode the address
lines themselves to get more than 4 rows, and by keeping track of the DMA
bank can support DMA with this non-standard configuration.

> Y_,_|[]|   Eric Mulholland
>{|_|_|__|   ericm@sage.cc.purdue.edu
>//oo--OO    ...!pur-ee!sage.cc!ericm
Cute!

--
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