RISC/os is a discontinued a UNIX operating system developed by MIPS Computer Systems, Inc. from 1985 to 1992, for their computer workstations and servers, such as the MIPS M/120 server or MIPS Magnum workstation. It was also known as UMIPS or MIPS OS.RISC/os was based largely on UNIX System V with additions from 4.3BSD UNIX, ported to the MIPS architecture.