Re: ROMs replacement.. solved!

From: Gerrit Heitsch <>
Date: Fri, 23 Nov 2012 18:30:37 +0100
Message-ID: <>
On 11/23/2012 06:19 PM, wrote:
> On 2012-10-18, at 18:52, Gerrit Heitsch wrote:
>>> Why stop there? Replace the TTL with micro-sized versions, mount them inbetween the pin header on the bottom, and make the board the same size as the eprom!
>>> Just kidding!  ;-)
>> No, the next step would be to design one such circuit for a 250469 board, the one where the BASIC and KERNAL are in one ROM and only the char-ROM is extra.
>> Should be a bit easier since _KERNAL and _BASIC are already added together by an AND gate on the board. That way one doesn't need a 74LS11, a 74LS08 should do.
> I just had a look at the schematics of the narrow board and I wonder what the A14 is doing at the U4 (23128)..

Remember that BASIC ROM and KERNAL ROM in the C64 are not 'next' to each 
other. There is a 8 KB gap between them. So in order to select the 
correct 8 KB block in the 23128 ROM, you can't use A13. It would be HIGH 
at $A000-$BFFF and $E000-$FFFF. But you can use A14 since that is LOW at 
$A000-$BFFF and HIGH at $E000-$FFFF.

Also, _CS for that ROM is already a combination of _BASIC and _KERNAL 
done by using an AND gate from a 74LS08.


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Received on 2012-11-23 18:00:47

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