UNICOS is a range of Unix and later Linux operating system (OS) variants developed by Cray for its supercomputers.
UNICOS was originally introduced in 1985 with the Cray-2 system and later ported to other Cray models.
The operating system revamp was part of a larger movement inside Cray Research to modernize their corporate software: including rewriting their most important Fortran compiler (cft to cft77) in a higher-level language (Pascal) with more modern optimizations and vectorizations.
One of the sites that ran very early versions of UNICOS was Bell Labs, where Unix pioneers including Dennis Ritchie ported parts of their Eighth Edition Unix (including STREAMS input/output (I/O)) to UNICOS.
They also experimented with a guest facility within UNICOS, allowing the stand-alone version of the OS to host itself.