In the early-1970s, while working for TRW Data Systems (founded by Larry and Doug Michaels, who later founded the early commercial Unix provider Santa Cruz Operations) I led development of a proprietary OS for the Datapoint 2200.
The OS ran on a descrete implementation of the 8008 processor after Intel had encountered tech and yield issues. The OS supported drivers for keyboard, CRT, disk and tape drives, telecommunications controlling dozens of specialized remote terminals, and a hashed access database, With dynamic overlays it all fit within 16 KB of RAM.