This 6809 system includes a nice little mechanism to allow interrupt service routines to be able to return to 6809 or 6309 mode user code. But also, it’s multi-process, multi-user, has user and kernel modes, even has a TCP/IP stack.
It’s got a homebrew MMU too:
My new design has a DATBOX in the form of dual ported RAM that will store the maps for 64 processes all at once. Context switching is very cheap, disable/enable a latch now. Furthermore the CPU board has full crash protection. No user process is able to bring down the OS.
And if I read correctly, there are also several high level language compilers and other tools, although I’m not sure they are part of this repo.
This system was recently mentioned in Anyone know of these systems?: