Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP
Path: utzoo!linus!philabs!cmcl2!harvard!seismo!hao!hplabs!hpda!hpisoa2!hpitg!uthub!koko@uthub
From: koko@uthub
Newsgroups: net.micro.apple
Subject: Re: bank switching for large memories
Message-ID: <296@uthub>
Date: Fri, 25-Apr-86 22:09:00 EDT
Article-I.D.: uthub.296
Posted: Fri Apr 25 22:09:00 1986
Date-Received: Wed, 14-May-86 05:44:30 EDT
References: <1840@ihlpg>
Lines: 35

> 
> > After looking at several memory expansion boards for my Apple ][+, I think I
> > understand the concept, but I would like more details of how the bank
> > switching really works.
> > ...
> > For the 128Kb expansion, what mechanism is used?  How big are the "chunks"
> > that are switched?  Are all chunks switched into the same region in the
> > address spectrum, or can a 16Kb expansion coexist peacefully with a 128Kb
> > expansion? Can two 128Kb expansions coexist in harmony?  Do the companies
> > that make the 1 MEG (!) expansions use a different technique?  Has any sort
> > of standard evolved?  I noticed that some expansion boards need a ribbon
> > cable connected to the motherboard, others do not.  Why???
> > ...
> >
> > tim born

	All of the memory expansion boards which are available use dynamic
RAM.  This means that the RAM chips need dynamic refresh logic.  On the cards
without the ribbon cable, there is a binary counter which runs continuously
and generates a refresh address for the memory chips every memory cycle.
On the cards with the cable, however, the refresh address is siphoned off
from the motherboard through the cable and then is gated into memory chips
on the expansion board.  This saves some chips, and hence some power and
heat.  (Note that the displaced RAM chip is connected, pin-for-pin, to the
cable when it is plugged into the expansion board.)  A disadvantage of
this approach is that the board only adds six RAM chips instead of eight,
only 12K of expansion RAM instead of 16K is added.  (However, for running
CP/M with the Z80 card, this does not matter.)  Also, since each chip is
only one bit wide -- 4116's are used -- there is a need for multiplexing
logic for mapping the bits into a rectangular 12-K by 8 block.


			Mike Kokodyniak, University of Toronto
> > ...ihnp4!ihlpg!timborn
>