[saati]

The original UNIX literally swapped processes, as in write all their memory to disk and read another program's state from disk to memory, it could only run as many processes as many times the swap was bigger than core, this is a wholly unacceptable design nowadays.

[inkyoto]

It also supported overlays on the PDP-11, although not in the beginning. I do not think that anybody makes use of the overlays anymore.