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Path: utzoo!utgpu!water!watnot!watmath!clyde!cbatt!ukma!simon
From: simon@ukma.UUCP
Newsgroups: comp.sys.cbm
Subject: Re: Added RAM at $DE00 on C64
Message-ID: <5882@herkimer.ukma.ms.uky.csnet>
Date: Sun, 15-Feb-87 10:08:32 EST
Article-I.D.: herkimer.5882
Posted: Sun Feb 15 10:08:32 1987
Date-Received: Mon, 16-Feb-87 07:01:41 EST
References: <12450@watnot.UUCP> <2843@ihlpg.UUCP>
Sender: news@ukma.ms.uky.csnet
Reply-To: simon@herkimer.ms.uky.csnet (George Simon)
Distribution: na
Organization: U of Kentucky, Mathematical Sciences
Lines: 40



>> 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



	The reason you are changing multiple locations with one write has
something to do with the I/O block not being fully decoded.  I don't know
the exact specifics of the problem, but you should check out the address
lines (from the schematics) to see exactly how it is decoded.  

	The reason this area isn't decoded is that chips like the sid chip
only use a small range of addresses, so to save both design time and hardware
cost it is easier just to not decode some of the address lines.  This results
is the (sid) registers appearing in every block of 32 or so addresses.   

	Just from what you said about writing to location $DE09, maybe you
trace down address line 7 (bit value 64 ($40)) and make sure it is decoded when
the computer thinks it is looking at the I/O chips.

	However, as Mike said, there is already ram there, why add more?


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