There was more than one OS. Supercomputer manufacturers (not just Cray) were basically hardware companies, and operating systems were an afterthought. Software

There was more than one OS. Supercomputer manufacturers (not just Cray) were basically hardware companies, and operating systems were an afterthought. Software was written on other systems and loaded in from the outside. UNICOS was a regular System V Unix port, to give users the sort of rich experience of a modern operating system.