They didn't (not for AMOS at least, the UNIMOS capable machines had an external MMU).

"AMOS is also a strict real-memory operating system, which is to say there's no MMU, and programs were expected to be fully position-independent and run wherever the monitor ended up loading them. This makes it fast, but also makes it possible for jobs to stomp on other jobs, and it was not uncommon for busy systems to crash on a regular basis."