Re: [Plus4] C64 and plus/4 PLAs revisited

From: Richard Atkinson (
Date: 2002-07-03 02:19:49

Marko Mäkelä wrote:
> There was some discussion on the cbm-hackers mailing list on reverse
> engineering the programmable logic arrays found in Commodore computers.
>   Simple formulae can be obtained by guessing from truth table dumps and
> verifying the guesses with a program that transforms the logic equations
> to truth tables that can be compared to truth table dumps from real chips.
> Last night, I wrote the following two programs:
> These contain the logic equations for the C64 and plus/4 PLA,
> respectively.  Thanks to William Levak for providing me with the truth
> table dumps.
> By the way, does anyone know what the output F0 of the plus/4 PLA could
> have been used for?  It is '0' whenever the inputs I15..I0 are 0111 011x
> 1001 011x, and '1' otherwise.  In other words, the signal is low
> whenever the address is $fd2x and RAS is low and PHI0 is high.

It accesses the custom chip in the V364. I think it has four registers.


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