All, in case you haven't seen it:

        https://www.ioccc.org/2018/mills/

This is a PDP-7 emulator in C, enough to run PDP-7 Unix. But the author
has written a PDP-11 emulator in PDP-7 assembly, and uses this to run
2.9BSD on the emulated PDP-7 :-)

Cheers, Warren