I've spent quite a bit of time so far this weekend trying to interface to the MCP23017 I hope to use to interface to the address bus. The ATMega328p documentation on the TWI (Two Wire Interface) are a little dense, yet missing some useful details.

Anyway I have something working on a breadboard now to prove the concept of controlling the external pins.

The thinking is I can use one of these and the similar MCP23008 to interface to the address and data busses, freeing up MCU pins and also allowing me to use a smaller, newer and cheaper AVR.

I think one of the next tasks is going to be to rework the comms protocol to make it less string-based. This is mostly to make it easier to control via software rather than via a serial terminal app. This would make it easier to upload code to the board than my current copying and pasting of hex files into a console!