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Path: utzoo!utgpu!water!watnot!watmath!clyde!cuae2!ihnp4!ihlpg!cuda
From: cuda@ihlpg.UUCP
Newsgroups: comp.sys.cbm
Subject: Re: Added RAM at $DE00 on C64
Message-ID: <2843@ihlpg.UUCP>
Date: Fri, 13-Feb-87 09:23:11 EST
Article-I.D.: ihlpg.2843
Posted: Fri Feb 13 09:23:11 1987
Date-Received: Sat, 14-Feb-87 06:45:05 EST
References: <12450@watnot.UUCP>
Distribution: na
Organization: AT&T Bell Labs, Naperville, IL
Lines: 28

> 
> I am having a problem with the expansion port of the C64. The interface I have
> made puts 256 bytes of RAM in the I/0 1 block ($DE00 to $DEFF). I am using 
> HM-6116 chips with 150ns access time. When I write to location $DE09, the
> information at location $DE49 sometimes changes with it. I have tried several 
> different chips, but the problem persists. Writing to other addresses does not
> seem to create the situation.
>  
> I would really appreciate any suggestions on how to correct this problem, or 
> what is causing it. 
> 
> Terry.
> 
> {watmath,alberta,ubc-vision}!watnot!thfisher

There is already RAM at those locations so I am a little confused as
to why you would need to put ram there externally.  In fact (if I 
remember correctly there are 3! things addressed at those locations.
I/O registers, RAM, and character ROM.  The ram under all those locations
is usable but you will need to be carefull about how you do it.  I 
believe that one of the I/O bits on the 6510 controls the area between
RAM and I/O and some bit in one of the 6526 control the character memory
at that point.  The safest and most non-interfeering way to use it
is to switch from I/O to RAM in the uP and read/write what you need
then switch the I/O back in.  The Programmers Reference Guide can tell
you which bit needs to be changed.

Mike Nelson