I guess I didn't elucidate my ideas properly. My point was _if_ you wanted to write an asm OS and learn from it, following in the steps of the masters might be more productive and enlightening. The PDP11 is sufficiently different from modern architectures, the scheduler as written in Lions relies on undefined CPU behaviour to work, that it wouldn't be a straight re-write; there is plenty to learn here. It would mean however that you would avoid plenty of unnecessary dead ends.