Re: 6502/6510 ROM selection daughterboard

From: Marko Mäkelä <>
Date: Sun, 3 Jun 2012 13:32:24 +0300
Message-ID: <20120603103224.GA3147@x220>
Hi Patryk,

>>Basically, you would need a CPLD on the daughterboard that would mimic 
>>the PLA and hijack a few addresses for itself. On power-up, the CPLD 
>>would initialize its bank selection lines so that the 6510 sees a 
>>custom on-board ROM in place of the KERNAL. This custom ROM would 
>>display the menu. Once you have selected the combination to use, the 
>>custom ROM would write to the CPLD-implemented registers to select the 
>>combination and to hide the CPLD register from the 6510 address space.
>Yes, this is almost exactly what I have in mind. Only plus additional 
>trigger line so that the menu (or whatever for other systems) is being 
>displayed only upon request. Otherwise it would pass through to normal 
>cold boot, using the last stored on-board ROM combination.
>>I wrote something like this in the Vic Flash Plugin.
>How did you deal with the need for VIC to "see" CHAROM on the 

I did not. The Vic Flash Plugin is similar to what you are looking for, 
but not exactly the same. It only expands the memory, does not replace 
any of the internal memory. The KERNAL, CHAROM and BASIC and the 
internal RAM remain the same.

Above, I was thinking a daughterboard that would be installed in the 
6510 socket. But, I think that it might be possible to implement a 
KERNAL&BASIC replacement cartridge for the C64. As noted earlier this 
week, you cannot directly observe writes to the 6510 on-chip I/O 
register at 0 and 1. But, maybe you could observe the state of the 3 
bank switching lines by observing the address lines and the CS signals 
on the cartridge port. Then, by actively controlling the -GAME or -EXROM 
lines on the 6510 half of the AEC, you could effectively replace the 
internal ROMs with the cartridge ROM.

This approach would require some careful studying of the PLA truth 
table, and there might be some glitches.

I am not sure that the CHAROM could be replaced in this way. I don't 
think it can be replaced with a 6510 daughterboard either, at least not 
without redirecting some PLA lines.

>Now to the second part - both of you Ruud and Marko got the idea and 
>know right away how to do it from the technical POV but this is also 
>the point where I don't feel like I could do it myself. At least not in 
>a reasonable time. Therefore if someone would be willing to make it 
>according to my specs, I am ready not only to buy it right away but 
>alternatively I can also fund some development / return costs, ...
>And of course the eternal hall of fame awaits for those who dare.. :-)

Good luck. :-) I am too busy with my day job at the moment. Even in a 
less busy period, it feels like a professional software developer cannot 
work on software in spare time. Things were much easier when I was a 
researcher, writing software only as part of my job. I feel a bit quilty 
about this too, because I would like to teach programming to my kids.  
Somehow I just lack the power, do not have much capacity to learn 
something new.

Best regards,


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Received on 2012-06-03 11:00:06

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