>The cartridge logic necessary for this could be pretty simple: First, it
must
>boot up the C64 in Ultimax mode (/EXROM=H, /GAME=L), and after a write to
the
>I/O area (either I/O1 or I/O2) change this to normal EXROM mode (/EXROM=L,
/GAME=H). This can be done with a simple LS74 flipflop I guess (/RES and
I/O1
>as set/reset inputs, /EXROM and /GAME as outputs Q and /Q). The /ROML and
>/ROMH signals must be combined to select the single EPROM, to make the mode
>switch possible from inside the ROM, and this can be done with two diodes.
I
>like this much better than modifying a Supergames cartridge.
I used the SuperGames cart as it takes 27xxx EPROMS and had the 74175 latch
(and have spares). The upper ROM only has RESET/IRQ/NMI vectors, all
directed to $9FE0
The patch at $9FE0 (in the low ROM), just pokes the latch to change memory
config, then JMP(8000).
I had to split the pad between GAME/EXROM as these are joined by default,
and add a wire to connect to the 74175.
I also added LEDs (via a 7404) for some additional diagnostics: RED for +5V,
YELLOW for RESET high (released), GREEN for ROML (verify that cart is being
started/accessed, PLA works), dual colour for Phi2 CLOCK (so as to verify it
actually toggles).
Nothing published /written up about it yet....
- Nick
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