From LQWiki

TOPS-20 is (was) one of the operating systems that controlled a series of 36-bit computers made by Digital Equipment Corporation (now defunct). These machines served as timesharing computers in the days when economics prohibited the notion of each person having one or more personal machines in his or her immediate environment. The last of these machines was the KL10 processor, which occupied a space 5ft high, 12ft wide, and 20in deep.

TOPS-20 provided a very large number of the features that we have only recently begun to have available on personal computers. Its file system supported different levels of access for owner, group, and world; its virtual memory controls managed shared code pages with copy-on-write, so that programs could be used by many users simultaneously without consuming vast amounts of memory. Its scheduler could readily manage a few hundred concurrent processes, and inter-process communication facilities were as good as today's.

The principal drawback to this operating system was that it was coded primarily by hand in assembly language; as a result, it was difficult to maintain, and nearly impossible to make major changes, even to add new features.