This article is about the MIT Project MAC operating system. CTSS may also stand for the Cray Time Sharing System, a separate system developed for Cray supercomputers or the Cambridge Time Sharing System developed for IBM mainframes.

CTSS, which stood for the Compatible Time-Sharing System, was one of the first time-sharing operating systems; it was developed at Project MAC at MIT. CTSS was first published, as well as operated in a time-sharing environment, in 1961; in addition, it was the system with the first computerized text formatting utility, and one of the very first to have inter-user electronic mail.

Although CTSS was not an influential operating system in its technical detail, it was very influential in showing that time-sharing was viable, in the new applications for computers which were first instantiated there, and because of its successor, Multics, which all modern operating systems are intellectually descended from.

In addition to e-mail and text processing, Louis Pouzin created a facility called RUNCOM for CTSS. This took a collection of commands contained in a file and executed them; this is the direct ancestor of the Unix shell script. It also allowed parameter substitution.

CTSS used a modified IBM 7094 mainframe computer that had two 32,768 word banks of core memory instead of the normal one. One bank was reserved for the time-sharing supervisory program, the other for user programs. 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, punch 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 Model 35 Teletypes. Some of the terminals were located remotly and the system could be accessed using the public Telex and TWX networks

CTSS was compatible with the Fortran Monitor System, an older batch computing system that ran on the 7094 computer before CTSS was invented. FMS could run in the background nearly as efficiently as on a 7094 without an OS at all. Running in the background, FMS had access to some tape units and the user 32K bank of core memory.

Multics, which was also developed by Project MAC, was started in the 1960s as a successor to CTSS, for future use in multiple-access computing. Multics, infamously, was the operating system that led to the development of Unix in 1970.

ITS, the Incompatible Timesharing System, another early, revolutionary, and influential MIT time-sharing system, was produced by people who disagreed with the direction taken by Multics; the name was a hack on CTSS, as the name of Unix was later a hack on Multics.

See also


  • F. J. Corbató, M. M. Daggett, R. C. Daley, An Experimental Time-Sharing System (http://larch-www.lcs.mit.edu:8001/~corbato/sjcc62/) (IFIPS 1962)
  • Robert M. Fano, The MAC System: A Progress Report (http://www.lcs.mit.edu/publications/pubs/pdf/MIT-LCS-TR-012.pdf) (MIT Project MAC, 1964) describes the usage of CTSS
  • Jerome H. Saltzer, CTSS Technical Notes (http://www.lcs.mit.edu/publications/pubs/pdf/MIT-LCS-TR-016.pdf) (MIT Project MAC, 1965) describes the internals of CTSS in some detail
  • Jerome H. Saltzer, Manuscript Typing and Editing (http://web.mit.edu/Saltzer/www/publications/AH.9.01.html) (MIT Computation Center, 1964) describes the world's first computerized text formatting system

External links

  • John McCarthy, Reminiscences on the History of Time Sharing (http://www-formal.stanford.edu/jmc/history/timesharing/timesharing.html) presents the origin of the concept of time-sharing
  • The IBM 7094 and CTSS (http://www.multicians.org/thvv/7094.html) a fascinating personal memoir of a system programmer on CTSS
  • The Origin of the Shell (http://www.multicians.org/shell.html) describes the evolution of RUNCOM into the modern shell
  • CTSS Source (http://www.piercefuller.com/library/ctss.html?id=ctss) in Paul Pierce's collection.da:CTSS

de:Compatible Time-Sharing System


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