> the PDP-11 disk unit throwing itself around violently when somebody started nroff.

That was the overlays… the early days substitute for the virtual memory proper. If I remember correctly, the first PDP-11 to have the true virtual memory and a MMU was PDP-11/70 which had 18-bit wide hardware addresses.

The 11/70 had a 22-bit wide memory bus and an 18-bit wide unibus which was mapped to the high 256kb of the 4M physical address space.

You are correct. I have long forgotten about the mapped memory and was thinking about Unibus device registers having long 18-bit addresses. The 11/70 also had separate instruction and data spaces which did not exist in earlier models.

Many years ago the team I worked out got some shiny new Sun 4 machines and we soon ran out of disk space - solution was to plug a cabinet full of a few old huge disks that I think came from some kind of ancient mini into a Sun 3 and then NFS the new storage onto the Sun 4s.

It was truly bizarre to watch the cabinet judder and shake as we accessed stuff - I think there were only 2 disks in it and it was mostly full!

Although the 11/70 had primitive mmu support, V7 ran on the /45 and /55 and did not use anything besides I/D separation.