I'm working on some A8 (atari 8 bit) related hardware, and in the process of this I'm using an Atarimax multicart, this device if you've never met it is a cartridge that goes in your A8 and gives a menu of games. Unfortunately it's not open source (or at least, if it is, I've not found any source ...

*** EDIT ***
It works now -- once I rebuilt the whole project (including asm parts) with the --cpu=65c02 flag and rebuilt the library (I'd made a few changes to the crt0.s to put the interrupt service routine back, which I had disabled while debugging)

I shall leave this here in case someone else ...