I wish there was more time to work on this, but at least I could take a simple step of moving the extended Basic ROM in the cartridge port into the FPGA, along with the ROM page selection logic. Thus now the only thing requiring the cartridge port to be occupied at all is the GROM section, which needs to move over to the FPGA next.
Obviously having cartridge functionality in the expansion bus is probably not exactly proper behavior, but I don't have the extended basic cartridge. I suppose the FPGA could have a sense mode, to see if there is a cartridge driving the databus, and if not, then take over and become the cartridge.
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