RISC/os is a discontinued UNIX operating system developed by MIPS Computer Systems, Inc. from 1985 to 1992, for their computer workstations and servers, including such models as the MIPS M/120 server and MIPS Magnum workstation. It was also known as UMIPS or MIPS OS.
RISC/os was mainly based on UNIX System V with additions from 4.3BSD UNIX, ported to the MIPS architecture. It was a "dual-universe" operating system, meaning it had separate, switchable runtime environments that were compatible with either System V Release 3 or 4.3BSD.