AbstractThe National Magnetic Fusion Energy Computer Center (NMFECC) at the Lawrence Livermore National Laboratory (LLNL) has implemented a simple, yet powerful interactive operating system, the Cray Time‐Sharing System (CTSS), on a Cray‐1 supercomputer. CTSS augments the multi‐programming batch facilities normally found in supercomputer systems with many of the interactive services typical of interactive minicomputer systems. This paper gives some of the historical background leading to CTSS and gives an overview of the system that emphasizes the strong points or unusual features such as multiple channels, decentralized control of resources, priorities and program scheduling, system recovery, and on‐line docume