6502/6510 ROM selection daughterboard

On Tue, May 15, 2012 at 11:45:58PM +0200, silverdr@wfmh.org.pl wrote:
>I am not sure if I made myself clear. I want to be able to have a ROM 
>replacement for the C64 (also other systems but 64 is the starting 
>point), where I can easily manage what rom image will be made visible 
>to the system at various addresses. It has to have a piece of own 
>software too. The one that will display the menus for selection of the 
>ROM images. After selecting, the images are made visible os ROM areas 
>to the system and the system is rebooted via cold vector.

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.

I wrote something like this in the Vic Flash Plugin. It decompresses the 
ROM images to write-protectable on-board RAM. You might use a simple 
EPROM for all images, or make some effort to load the ROM images from a 
SD card. But would it be handy to open the case to switch SD cards?

By the way, if you go the CPLD route, why not pick a bigger chip that 
could implement a co-processor or a REU clone or whatever?

	Marko

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