Compatible Time-Sharing System

The Compatible Time-Sharing System (CTSS) was the first general purpose time-sharing operating system. Compatible Time Sharing is time sharing which is compatible with batch processing, in that they operate concurrently. CTSS was developed at the MIT Computation Center ("Comp Center"). CTSS was first demonstrated on MIT's modified IBM 709 in November 1961. The hardware was replaced with a modified IBM 7090 in 1962 and later a modified IBM 7094 called the "blue machine" to distinguish it from the Project MAC CTSS IBM 7094. Routine service to MIT Comp Center users began in the summer of 1963 and was operated there until 1968. A second deployment of CTSS on a separate IBM 7094 that was received in October 1963 (the "red machine") was used early on in Project MAC until 1969 when the red machine was moved to the Information Processing Center and operated until July 20, 1973. CTSS ran on only those two machines; however, there were remote CTSS users outside of MIT including ones in California, South America, the University of Edinburgh and the University of Oxford.