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Tag Wiki 'Compatible Time-sharing System'.

The Compatible Time-Sharing System ( CTSS) was the first general purpose . Compatible Time Sharing referred to time sharing which was compatible with ; it could offer both time sharing and batch processing 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 New Information Center Will Open at MIT Today - The New York Times, November 11, 1968 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.

said in the 1954 summer session at MIT that "By time sharing, a big computer could be used as several small ones; there would need to be a reading station for each user".Backus, John, Computer Advanced Coding Techniques , MIT 1954, page 16-2. The first known description of computer time-sharing. Computers at that time, like IBM 704, were not powerful enough to implement such system, but at the end of 1958, MIT's Computation Center nevertheless added a typewriter input to its 704 with the intent that a programmer or operator could "obtain additional answers from the machine on a time-sharing basis with other programs using the machine simultaneously". Progress Report Number 4 of the Research and Educational Activities in Machine Computation by the Cooperating Colleges of New England, December, 1958

In June 1959, Christopher Strachey published a paper "Time Sharing in Large Fast Computers" at the UNESCO Information Processing Conference in Paris, where he envisaged a programmer a program at a console (like a ) connected to the computer, while another program was running in the computer at the same time.F. J. Corbató, et al., The Compatible Time-Sharing System A Programmer's Guide (MIT Press, 1963) . Describe the system and its commandsJohn McCarthy, Reminiscences on the History of Time Sharing (Stanford University 1983). Debugging programs was an important problem at that time, because with batch processing, it then often took a day from submitting a changed code, to getting the results. John McCarthy wrote a memo about that at MIT, after which a preliminary study committee and a working committee were established at MIT, to develop time sharing. The committees envisaged many users using the computer at the same time, decided the details of implementing such system at MIT, and started the development of the system.

Experimental Time Sharing System
By July, 1961 Progress Report Number 9 of the Research and Educational Activities in Machine Computation by the Cooperating Colleges of New England, July, 1961 a few time sharing commands had become operational on the Computation Center's IBM 709, and in November 1961, Fernando J. Corbató demonstrated at MIT what was called the Experimental Time-Sharing System. On May 3, 1962, F. J. Corbató, M. M. Daggett and R. C. Daley published a paper about that system at the Spring Joint Computer Conference. Robert C. Daley, Peter R. Bos and at least 6 other programmers implemented the operating system, partly based on the Fortran Monitor System.

The system used an IBM 7090, modified by Herbert M. Teager, with added 3 Flexowriters for user consoles, and maybe a timer. Each of the 3 users had two tape units, one for the user's file directory, and one for dumping the core (program in memory). There was also one tape unit for the system commands, there were no disk drives. The memory was 27 k words (36-bit words) for users, and 5 k words for the supervisor (operating system). The input from the consoles was written to the buffers in the supervisor, by , and when a was received, the control was given to the supervisor, which dumped the running code to the tape and decided what to run next. The console commands implemented at the time were login, logout, input, edit, fap, mad, madtrn, load, use, start, skippm, listf, printf, xdump and xundump.

This became the initial version of the Compatible Time-Sharing System. This was apparently the first ever public demonstration of ; there are other claims, but they refer to special-purpose systems, or with no known papers published. The "compatibility" of CTSS was with background jobs run on the same computer, which generally used more of the compute resources than the time-sharing functions.


The first version of the DOTSYS translation software ran on CTSS and could output to a BRAILLEMBOSS braille page printer. DOTSYS on CTSS was first demonstrated on August 18, 1966 as part of a feasibility study where teletypesetter tape, in the form of news, was converted to Grade 2 Braille. The following month the feasibility of converting textbook information on teletypesetter tape to error-free Grade 2 Braille was successfully demonstrated. As MIT CTSS was an academic system, a research vehicle and not a system for commercial computing, two years later a version of DOTSYS stripped of CTSS dependencies for software portability was used on an IBM 709 at the American Printing House for the Blind to print the first braille edition of a book produced from teletypesetter input, only a few weeks after the ink-print version. The following year, on CTSS, a demonstration of printing mathematical tables in braille was shown. A short FORTRAN II program was written to produce a conversion table from inches to millimeters in braille via the BRAILLEMBOSS braille page printer.

The Intrex Retrieval System ran on CTSS. Intrex was an experimental, pilot-model machine-oriented bibliographic storage and retrieval system with a database that stored a catalog of roughly 15,000 journal articles. It was used to develop and test concepts for library automation. A deployment of three BRISC consoles for testing at the MIT Engineering Library showed that it was preferred over two other systems, ARDS and DATEL.

  • The original ran on CTSS.
  • CTSS was the first computer system to implement login.
  • CTSS had one of the first computerized text editing and formatting utilities, called TYPSET and RUNOFF (the successors of MEMO, MODIFY and DITTO).
  • CTSS had one of the first inter-user messaging implementations, pioneering .Tom Van Vleck's memoir of The History of Electronic Mail
  • CTSS had one of the first instant messaging systems similar to write.
    (2023). 9783863096236, University of Bamberg Press. .
  • MIT Computation Center staff member created for CTSS a command called , which executed a list of commands contained in a file. RUNCOM also provided for parameter substitution. He later created a design for the shell that was implemented by which in turn inspired Unix .
    (2023). 9780132465885, Pearson Education.
  • CTSS had an implementation of the text editor QED, the predecessor of ed, vi, and vim, with regular expressions added by .


CTSS used a modified IBM 7090 mainframe computer that had two 32,768 (32K) 36-bit-word banks of instead of the default configuration which provides only one.
(1988). 9780201187601
One bank was reserved for the time-sharing supervisory program, the other for user programs. CTSS had a protected-mode kernel, the supervisor's functions in the A-core (memory bank A) could be called only by software interrupts, like in the modern operating systems. Causing memory-protection interrupts were used for software interrupts. Processor allocation scheduling with a quantum time unit 200 ms, 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.

Supervisor subroutines
  • RDFLXA – Read an input line from console
  • WRFLX – Write an output line to console
  • DEAD – Put the user into dead status, with no program in memory
  • DORMNT – Put the user into dormant status, with program in memory
  • GETMEM – Get the size of the memory allocation
  • SETMEM – Set the size of the memory allocation
  • TSSFIL – Get access to the CTSS system files on the disk
  • USRFIL – Change back to user's own directory
  • GETBRK – Get the instruction location counter at quit

Programming languages
CTSS at first had only an assembler, FAP, and a compiler, MAD. Also, Fortran II code could be translated into MAD code by using MADTRN. Later half of the system was written in MAD. Later there were other programming languages including , LISP 1.5 and a version of .

File system
Each user had their own directory, and there were also shared directories for groups of people with the same "problem number". Each had two names, the second indicating its type as did the extension in later system. At first, each file could have one of four modes: temporary, permanent, read-only class 1, and read-only class 2. Read-only class 1 allowed the user to change the mode of the file. Files could also be symbolically linked between directories. A directory listing by listf:
5/20/63		MAIN	MAD	P	15
5/17/63 	DPFA	SYMTB	P	1
5/17/63 	DPFA	BSS	P	1
5/17/63 	DPFA	FAP	P	2

Input-output hardware was mostly standard IBM . These included six data channels connecting to:
  • Printers, 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-word memory bank in one second (later upgraded to 0.25 seconds)
  • Two custom high-speed vector graphics displays
  • An IBM 7750 transmission control unit capable of supporting up to 112 terminals, including IBM 1050 and Model 35s. Some of the terminals were located remotely, and the system could be accessed using the public networks.

CTSS was described in a paper presented at the 1962 Spring Joint Computer Conference, and greatly influenced the design of other early time-sharing systems.

witnessed CTSS and the design of the Titan Supervisor was inspired by that.

(2023). 9780387218212
(2023). 9780387218212

wrote in 1977 that UNIX could be seen as a "modern implementation" of CTSS. , which was also developed by Project MAC, was started in the 1960s as a successor to CTSS – and in turn inspired the development of Unix in 1969. One of the technical terms inherited by these systems from CTSS is daemon.

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

(2023). 9781449388393, O'Reilly Media.
CTSS and ITS file systems have a number of design elements in common. Both have an M.F.D. (master file directory) and one or more U.F.D. (user file directories). Neither of them have nested directories (sub-directories). Both have file names consisting of two names which are a maximum of six-characters long. Both support linked files.

See also
  • PLATO (computer system)
  • Timeline of operating systems
  • Time-sharing system evolution

Further reading

External links

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