Path: utzoo!attcan!uunet!cs.utexas.edu!samsung!munnari.oz.au!metro!ipso!fawlty!johnmac
From: johnmac@fawlty.towers.oz (John MacLean)
Newsgroups: comp.sys.apple2
Subject: DMA memory size clarification.
Message-ID: <526@fawlty.towers.oz>
Date: 12 Nov 90 21:30:59 GMT
Organization: Tower Technology, Lane Cove, NSW
Lines: 36

In article <4867@sage.cc.purdue.edu> you write:
>My question is about DMA not amount of ram.  Everyone says the GS can
>only handle 4 megs of DMA ram (8 megs total).  I have a rom 1 GS (256K)
>and a 4 meg ramcard (supports DMA) thus giving me 4.25 megs of memory.
>Of this 4.25 megs, can DMA access all of it or is the last 256K on the
>ramcard non-DMA?
>-- 
> Y_,_|[]|   Eric Mulholland
>{|_|_|__|   ericm@sage.cc.purdue.edu
>//oo--OO    ...!pur-ee!sage.cc!ericm

The responses up until now have been somewhat confusing, so I'll try
and clarify things:

Any memory on the motherboard is accessable via DMA.
The DMA limitation is 4 banks of chips on your *memory expansion card*.
Anything beyond this is not accessable via DMA.
With 256K chips in your memory expansion card:
- With a ROM1 you can DMA up to 1.25 Meg.
- With a ROM3 you can DMA up to 2.125 Meg.
Thus with 1 Mbit chips in your memory expansion card:
- With a ROM1 you can DMA up to 4.25 Meg.
- With a ROM3 you can DMA up to 5.125 Meg.
I presume if you had 4 Mbit chips (Simms I guess, I do not think anyone
makes a GS card that takes 4 Mbit chips), you could DMA the whole eight
megabytes. The boundary condition as to what happens over the 8 meg line
if you had 8.25 or 9.125 Meg is beyond me - I'll let you know when I get
another 7 Meg.
Hope that is somewhat clearer.

John MacLean.
-- 
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