I've started work on an emulator for my 6502-on-a-backplane with a TMS9918.
I started out by writing a TMS9918A emulator in C. Created that in a separate repo since others might find it useful: https://github.com/visrealm/vrEmuTms9918
Then I found existing code to emulate the 6502 and the AY-3-8910. Cobbled it together with rubber bands and sticky tape to produce a working emulator for my machine. I've updated the 6502 code to include instruction timing information which allows me to run the emulator at the same speed as my real machine (currently 4MHz).
I've started building out an embedded debugger/disassembler for it which can be docked to the right-side of the display window. Making use of the label map files produced by ACME assembler, I can step through the code with (most) labels. Very handy.
I've also added some other debugging regions to see the contents of RAM, the state of the TMS9918 registers and VRAM contents.
All source code and schematics: https://github.com/visrealm/hbc-56