Path: utzoo!utgpu!watmath!isishq!Geoffrey.Welsh
From: Geoffrey.Welsh@isishq.FIDONET.ORG (Geoffrey Welsh)
Newsgroups: comp.sys.cbm
Subject: Re: C-128 MMU techniques and memory maps
Message-ID: <948.23A6D6EA@isishq.FIDONET.ORG>
Date: 14 Dec 88 14:28:01 GMT
Organization: International Student Information Service -- Headquarters
Lines: 55


 > From: aliu@nunki.usc.edu (Alejandro Liu)
 > Message-ID: <2140@nunki.usc.edu>
 
 >       I have been reading some of the specs about the C-128,
 > because a
 > program  I am writing, and I need some information.  First
 > I heard
 > that the 128's MMU can handle 1 Meg of memory, has that
 > thing has been
 > realeased?  Second, How do you use the MMU to do Bank Switching?
 
   To answer your first question: No. The MMU was designed to handle 

up to four 64K banks for a total of 256K. The problem is that there 
aren't enough pins on the MMU chip to handle the extra lines, so Commodore 
never connected them. As a result, you'd have to take the MMU out of 
its casing and re-wire the MMU to a larger carrier in order to take 
advantage of that ability.
 
   This is not a job even for the most skilled hacker!
 
   To answer your second question: get the C128 Programmer's Reference 

Guide from Bantam and/or 128 Internals from Abacus. Explaining the 
MMU to someone who has no idea what's going on is damned difficult, 
if not impossible.
 
   The C128 PRG also provides memory maps for C64 and C128 modes with 

Kernal jump tables and info on how to use those routines plus specs 
and info on most of the peripheral chips in the 128.
 
 > And last but not least, How do you
 > make the MMU
 > do virtual memory for pages 0 and 1.  I know that this
 > is possible.
 
   You've got to be kidding! Virtual memory on a C128? The closest 

thing you'll get is the ability to relocate pages zero and one (i.e. 
z-page and the stack).
 
   True virtual memory would require that hardware exceptions occurr 

when certain address ranges are used. I suppose the REU would provide 
a good place to swap memory blocks to & from (certainly disk drives 
are out of the question!), but the question would then be "what use 
is that to existing applications?"... none.


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