RISC/OS
From TechPubs Wiki
RISC/OS, also known as UMIPS was an early UNIX produced by MIPS Computer Systems, and a predecessor of both 4D1 and IRIX. Unlike GL2, RISC/OS had roots in both BSD and System V. When SGI purchased MIPS, it was succeeded by 4D1-1.x.
Versions
It was referred to as UMIPS between releases 1 through 3. 4.x and onwards were RISC/OS. The final release, based on Release 4 of System V, was released in 1991.
