Re: 6502/6510 ROM selection daughterboard

Hello!

Marko Mäkelä wrote:

> Above, I was thinking a daughterboard that would be installed in the
> 6510 socket.

The trouble with this approach is that on many motherboards the CPU is 
not socketed.

 > 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.

There is a whole document from creator of EasyFlash on how to detect the 
state of the HIROM line from external cartridge. It might be also 
possible to detect other lines as well, though I am not sure about the 
character ROM.

I was thinking of another approach - a 6502 running side by side on the 
cartridge, executing the same code as the main CPU. IIRC, 6502 and 6510 
are cycle-compatible, so this could be done easily. So one could easily 
detect writes to $01 and set the flags appropriately without playing 
crudely with the PLA lines. And 6502 CPUs can still be had from Rockwell 
for about $3. Plus, this might make the cartridge compatible with C128 
in C64 mode.

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

I decoded the PLA equations from the truth table some time ago, can supply.

> 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.

No, any of these methods cannot replace character ROM.

Regards,
Michau.

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