The address decoding of IO peripherals is managed inside C128 by IC
74LS138 named U3. Probably the best way (fastest in signal timing) is a
solution also recommended in the MixSID project by replacing this IC with a
GAL with such a functionality. Described here:
https://github.com/hbekel/MixSID/tree/master/firmware/c128
But if you do not like cutting pins on the board as we do not like, NOBOMI
created a piggy back solution for U3 with 74HCT541. This uses a little bit of
“force” to push the signal where we want to, but all is in
the working range
of participating ICs.
Shorten all pins on 74HCT541 with the exception of pins 10 and 17. Wire
(solder) together all pins 2-10 and 11-18. Piggy back place 74HCT541 on top
of U3 and solder the pin 10 of 74HCT541 to the pin 8 of U3. Then solder the
pin 17 of 74HCT541 to the pin 15 of U3. Connect VCC (the pin 20 of
74HCT541 to the pin 16 of U3) and connect G2 (the pin 19 of 74HCT541 to
the pin 14 of U3) by short wires. Finally connect 64/128 signal from the
MMU pin 47 to the pin 1 of 74HCT541.
