I got the switches wired up and working, with a little help from Richard Ottosen. I tested the debouncer, which worked. Then I went for broke and loaded the complete Elf system into the FPGA. After a bit more debugging, it partially works.
What doesn’t work is load mode, so I can’t yet toggle in a program. If I include an 1802 program in the FPGA BlockRAM initialization, it executes just fine. I’ve run a program that makes the display count, free-running, and one that counts when the input button is pressed.
I’m a bit surprised that load mode doesn’t work since it worked fine in the simulation. Obviously the simulation is different in some ways than the real thing, so I’ll have to investigate those differences.