Compatible Time-Sharing System - Implementation

Implementation

CTSS used a modified IBM 7094 mainframe computer that had two 32,768 36-bit word banks of core memory instead of the normal one; users had access to 27K of the total 32K, with the remaining 5K reserved for the monitor. One bank was reserved for the time-sharing supervisory program, the other for user programs. Processor allocation scheduling was controlled by a multilevel feedback queue. It also had some special memory management hardware, a clock interrupt and the ability to trap certain instructions. Input-output hardware was mostly standard IBM peripherals. These included six data channels connecting to:

• printers, punched card readers and punches
• IBM 729 tape drives, an IBM 1301 disk storage, later upgraded to an IBM 1302, with 38 million word capacity
• an IBM 7320 drum memory with 186K words that could load a 32K memory bank in one second (later upgraded to 1/4 second)
• two custom high speed vector graphics displays
• an IBM 7750 transmission control unit capable of supporting up to 112 teleprinter terminals, including IBM 1050 Selectrics and Teletype Model 35s. Some of the terminals were located remotely and the system could be accessed using the public Telex and TWX networks