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In computing, time-sharing is the sharing of a computing resource among many users by means of multiprogramming and multi-tasking at the same time.[1]

Its introduction in the 1960s and emergence as the prominent model of computing in the 1970s represented a major technological shift in the history of computing.

By allowing a large number of users to interact concurrently with a single computer, time-sharing dramatically lowered the cost of providing computing capability, made it possible for individuals and organizations to use a computer without owning one,[2] and promoted the interactive use of computers and the development of new interactive applications.


Batch processing

The earliest computers were extremely expensive devices, and very slow in comparison to later models. Machines were typically dedicated to a particular set of tasks and operated by control panels, the operator manually entering small programs via switches in order to load and run a series of programs. These programs might take hours, or even weeks, to run. As computers grew in speed, run times dropped, and soon the time taken to start up the next program became a concern. Batch processing methodologies evolved to decrease these "dead periods" by queuing up programs so that as soon as one program completed, the next would start.

To support a batch processing operation, a number of comparatively inexpensive card punch or paper tape writers were used by programmers to write their programs "offline". When typing (or punching) was complete, the programs were submitted to the operations team, which scheduled them to be run. Important programs were started quickly; how long before less important programs were started was unpredictable[citation needed]. When the program run was finally completed, the output (generally printed) was returned to the programmer. The complete process might take days, during which time the programmer might never see the computer.

The alternative of allowing the user to operate the computer directly was generally far too expensive to consider. This was because users might have long periods of entering code while the computer remained idle. This situation limited interactive development to those organizations that could afford to waste computing cycles: large universities for the most part. Programmers at the universities decried the behaviors that batch processing imposed, to the point that Stanford students made a short film humorously critiquing it.[3] They experimented with new ways to interact directly with the computer, a field today known as human–computer interaction.


Unix time-sharing at the University of Wisconsin, 1978.

Time-sharing was developed out of the realization that while any single user would make inefficient use of a computer, a large group of users together would not. This was due to the pattern of interaction: Typically an individual user entered bursts of information followed by long pauses but a group of users working at the same time would mean that the pauses of one user would be filled by the activity of the others. Given an optimal group size, the overall process could be very efficient. Similarly, small slices of time spent waiting for disk, tape, or network input could be granted to other users.

The concept is claimed to have been first described by John Backus in the 1954 summer session at MIT,[4] and later by Bob Bemer in his 1957 article "How to consider a computer" in Automatic Control Magazine.[5][6] In a paper published in December 1958 by W. F. Bauer,[7] he wrote that "The computers would handle a number of problems concurrently. Organizations would have input-output equipment installed on their own premises and would buy time on the computer much the same way that the average household buys power and water from utility companies."

Implementing a system able to take advantage of this was initially difficult.[1][8][9] Batch processing was effectively a methodological development on top of the earliest systems. Since computers still ran single programs for single users at any time, the primary change with batch processing was the time delay between one program and the next. Developing a system that supported multiple users at the same time was a completely different concept. The "state" of each user and their programs would have to be kept in the machine, and then switched between quickly. This would take up computer cycles, and on the slow machines of the era this was a concern. However, as computers rapidly improved in speed, and especially in size of core memory in which users' states were retained, the overhead of time-sharing continually decreased, relatively speaking.

The first project to implement a time-sharing system was initiated by John McCarthy at MIT in 1959, initially planned on a modified IBM 704, and later on an additionally modified IBM 709 (one of the first computers powerful enough for time-sharing).[9] One of the deliverables of the project, known as the Compatible Time-Sharing System or CTSS, was demonstrated in November 1961. CTSS has a good claim to be the first time-sharing system and remained in use until 1973. Another contender for the first demonstrated time-sharing system was PLATO II, created by Donald Bitzer at a public demonstration at Robert Allerton Park near the University of Illinois in early 1961. But this was a special purpose system. Bitzer has long said that the PLATO project would have gotten the patent on time-sharing if only the University of Illinois had known how to process patent applications faster, but at the time university patents were so few and far between, they took a long time to be submitted. JOSS began time-sharing service in January 1964.[10]

The first commercially successful time-sharing system was the Dartmouth Time Sharing System.[11]


Throughout the late 1960s and the 1970s, computer terminals were multiplexed onto large institutional mainframe computers (centralized computing systems), which in many implementations sequentially polled the terminals to see whether any additional data was available or action was requested by the computer user. Later technology in interconnections were interrupt driven, and some of these used parallel data transfer technologies such as the IEEE 488 standard. Generally, computer terminals were utilized on college properties in much the same places as desktop computers or personal computers are found today. In the earliest days of personal computers, many were in fact used as particularly smart terminals for time-sharing systems.

The Dartmouth Time Sharing System's creators wrote in 1968 that "any response time which averages more than 10 seconds destroys the illusion of having one's own computer".[12] Conversely, timesharing users thought that their terminal was the computer.[13] With the rise of microcomputing in the early 1980s, time-sharing faded into the background because individual microprocessors were sufficiently inexpensive that a single person could have all the CPU time dedicated solely to their needs, even when idle. However, the Internet brought the general concept of time-sharing back into popularity. Expensive corporate server farms costing millions can host thousands of customers all sharing the same common resources. As with the early serial terminals, web sites operate primarily in bursts of activity followed by periods of idle time. This bursting nature permits the service to be used by many customers at once, usually with no perceptible communication delays, unless the servers start to get very busy.

Time-sharing business

In the 1960s, several companies started providing time-sharing services as service bureaus. Early systems used Teletype Model 33 KSR or ASR or Teletype Model 35 KSR or ASR machines in ASCII environments, and IBM Selectric typewriter-based terminals (especially the IBM 2741) with two different seven-bit codes.[14] They would connect to the central computer by dial-up Bell 103A modem or acoustically coupled modems operating at 10–15 characters per second. Later terminals and modems supported 30–120 characters per second. The time-sharing system would provide a complete operating environment, including a variety of programming language processors, various software packages, file storage, bulk printing, and off-line storage. Users were charged rent for the terminal, a charge for hours of connect time, a charge for seconds of CPU time, and a charge for kilobyte-months of disk storage.

Common systems used for time-sharing included the SDS 940, the PDP-10, and the IBM 360. Companies providing this service included GE's GEISCO, IBM subsidiary The Service Bureau Corporation, Tymshare (founded in 1966), National CSS (founded in 1967 and bought by Dun & Bradstreet in 1979), Dial Data (bought by Tymshare in 1968), and Bolt, Beranek, and Newman (BBN). By 1968, there were 32 such service bureaus serving the US National Institutes of Health (NIH) alone.[15] The Auerbach Guide to Timesharing (1973) lists 125 different timesharing services using equipment from Burroughs, CDC, DEC, HP, Honeywell, IBM, RCA, Univac, and XDS.[16]

An example of a UK-based time-sharing bureau business is OLS Computer Services (UK) Ltd. Previously Leasco Response and later acquired by On-Line Systems of Pittsburgh, PA, the company operated four HP-2000 TSB (Time shared Basic) systems from its Knightsbridge (later Islington) headquarters offering off-the-shelf business packages as well as raw time to universities. In addition to its HP estate a DEC PDP-10 operated as a front-end processor (FEP) connecting UK users to 16 DEC PDP-11s based in the US. Connectivity was provided via leased Westrex ASR 33 or Data Dynamics 390 punched-tape enabled teletype machines connected via Post Office (ex GPO) type 2 modems or acoustic couplers connecting telephone handsets operating at up to 110cps.

Rise and Fall

In 1975, it was said about one of the major super-mini computer manufacturers[17] that "The biggest end-user market currently is time-sharing." For DEC, for a while the second largest computer company (after IBM), this was also true: Their PDP-10 and IBM's 360/67[18] were widely used[19] by commercial timesharing services such as CompuServe, On-Line Systems (OLS), and Rapidata.

Rapidata as an example

Although many time-sharing services simply closed, Rapidata[20][21] held on, and became part of National Data Corporation.[22] It was still of sufficient interest in 1982 to be the focus of "A User's Guide to Statistics Programs: The Rapidata Timesharing System".[23] Even as revenue fell by 66%[24] and National Data subsequently developed its own problems, attempts were made to keep this timesharing business going.[25][26][27]

The computer utility

Beginning in 1964, the Multics operating system was designed as a computing utility, modeled on the electrical or telephone utilities. In the 1970s, Ted Nelson's original "Xanadu" hypertext repository was envisioned as such a service. It seemed as the computer industry grew that no such consolidation of computing resources would occur as timesharing systems. In the 1990s the concept was, however, revived in somewhat modified form under the banner of cloud computing.


Time-sharing was the first time that multiple processes, owned by different users, were running on a single machine, and these processes could interfere with one another.[28] For example, one process might alter shared resources which another process relied on, such as a variable stored in memory. When only one user was using the system, this would result in possibly wrong output - but with multiple users, this might mean that other users got to see information they were not meant to see.

To prevent this from happening, an operating system needed to enforce a set of policies that determined which privileges each process had. For example, the operating system might deny access to a certain variable by a certain process.

The first international conference on computer security in London in 1971 was primarily driven by the time-sharing industry and its customers.

Notable time-sharing systems

Significant early timesharing systems:[16]

See also


  1. ^ a b DEC TIMESHARING (1965), by Peter Clark, The DEC Professional, VOLUME 1, Number 1
  2. ^ IBM advertised, early 1960s, with a headline: "This man is sharing a $2 million computer"
  3. ^ Eisenson, Arthur; and Yager, Heather (1967). Ellis D. Kropotchev Silent Film. Stanford University, 1967. This student-produced film from Stanford University is a humorous spoof of the trials and tribulations of a college hacker condemned to use batch processing. Originally created by Arthur Eisenson and Gary Feldman, the film gives the viewer a feel for the process of computer programming in the 1960s. Original music by Heather Yager. Computer History Museum, Object ID 102695643. Retrieved on 2013-11-29 from http://www.computerhistory.org/revolution/punched-cards/2/211/2253.
  4. ^ Backus, John, Computer Advanced Coding Techniques, MIT 1954, page 16-2. The first known description of computer time-sharing.
  5. ^ Bemer, Bob (March 1957). "Origins of Timesharing". bobbemer.com. Retrieved June 24, 2016. 
  6. ^ Middleburg, C.A. "Searching Publications on Operating Systems". arXiv:1003.5525Freely accessible. 
  7. ^ Bauer, W. F., Computer design from the programmer's viewpoint (Eastern Joint Computer Conference, December 1958) One of the first descriptions of computer time-sharing.
  8. ^ "There were no command files supported." - the commands to compile and then 'link' a program had to be typed in each time. As the article adds: "No CCL (Concise Command Language)" referring to the DEC world's equivalent of Clist and Rexx.
  9. ^ a b McCarthy, John. "REMINISCENCES ON THE HISTORY OF TIME SHARING". stanford.edu. stanford.edu. Retrieved 12 March 2017. 
  10. ^ J. C. Shaw (1964). "JOSS: a designer's view of an experimental on-line computing system". Proceeding AFIPS '64 (Fall, part I) Proceedings of the October 27-29, 1964, fall joint computer conference, part I. pp. 455–464. 
  11. ^ dtss.dartmouth.edu/history.php
  12. ^ Kemeny, John G.; Kurtz, Thomas E. (11 October 1968). "Dartmouth Time-Sharing". Science. 162: 223–228. 
  13. ^ "TRANSCRIPTS OF 1974 National Computer Conference Pioneer Day Session". Dartmouth Time Sharing System. Dartmouth College. 
  14. ^ IBM 2741 Communication Terminal (PDF). IBM. p. 12. 
  15. ^ "Information Technology Corporate Histories Collection". Computer History Museum. Retrieved on 2013-11-29 from http://www.computerhistory.org/corphist/view.php?s=stories&id=136.
  16. ^ a b c d Auerbach Guide to Time Sharing (PDF). Auerbach Publishers, Inc. 1973. Retrieved 2013-11-29. 
  17. ^ Computerworld, June 11, 1975, p. 35
  18. ^ One Two-page IBM print ad was headlined "100 or more people can use IBM's new time-sharing computer at the same time." Originals were/are? on e-Bay
  19. ^ p.1425, Encyclopedia of Computer Science, Litton Educational Publishing, Inc.
  20. ^ https://groups.google.com/forum/#!topic/alt.folklore.computers/aE4TwORruB8 - "I worked for RapiData Timesharing for about a year circa 1969..."
  21. ^ someone else: "I worked there for almost 2 years 1977 to 1979." alt.folklore.computers/aE4TwORruB8/EdpKfFAlBncJ
  22. ^ NDC started in 1967, and paralleled Rapidata; see Bloomberg's http://www.bloomberg.com/research/stocks/private/snapshot.asp?privcapId=290092
  23. ^ Bruce Bosworth, ISBN 978-089529-1-677
  24. ^ Computerworld, Oct. 6, 1986, p.179, "Rapidata revenue was $11 million ... in 1986, down from ... ($31 million in 1982)."
  25. ^ Computerworld, Aug.25,1986, p.5, "National Data Corp. said it is close to reaching an agreement with a buyer of its Rapidata timesharing division. In May, National Data said it would close down ..."
  26. ^ National Data Corp became NDC-Health Corp in 2001 (bizjournals.com/atlanta/stories/2001/10/29/daily25.html)
  27. ^ As for a place in history, Rapidata is listed in 'The AUERBACH Guide to Time Sharing (1973)' http://bitsavers.informatik.uni-stuttgart.de/pdf/auerbach/GuideToTimesharing_Jan73.pdf
  28. ^ Silberschatz, Abraham; Galvin, Peter; Gagne, Greg (2010). Operating system concepts (8th ed.). Hoboken, N.J.: Wiley & Sons. p. 591. ISBN 978-0-470-23399-3. 
  29. ^ "A Brief Description of Privacy Measures in the RUSH Time-Sharing System", J.D. Babcock, AFIPS Conference Proceedings, Spring Joint Computer Conference, Vol. 30, 1967, pp. 301-302.
  30. ^ Hartley, D. F. (1968), The Cambridge multiple-access system: user's reference manual, Cambridge: Cambridge Univ. Press, ISBN 978-0901224002 
  31. ^ "Time Sharing", James Miller. Retrieved 30 November 2013.

Further reading

External links

  • "Time Sharing Supervisor Programs", notes comparing the supervisor programs of CP-67, TSS/360, the Michigan Terminal System (MTS), and Multics by Michael T. Alexander, Advanced Topics in Systems Programming (1970, revised 1971), University of Michigan Engineering Summer Conference.
  • "The Computer Utility As A Marketplace For Computer Services", Robert Frankston's MIT Master's Thesis, 1973.
  • Reminiscences on the Theory of Time-Sharing by John McCarthy, 1983.
  • Origins of timesharing by Bob Bemer.
  • "40 years of Multics, 1969-2009", an interview with Professor Fernando J. Corbató on the history of Multics and origins of time-sharing, 2009.
  • "Mainframe Computers: The Virtues of Sharing", Revolution: The First 2000 Years of Computing, Computer History Museum Exhibition, January 2011.
  • "Mainframe Computers: Timesharing as a Business", Revolution: The First 2000 Years of Computing, Computer History Museum Exhibition, January 2011.
> IBM M44/44X
>> CP-40/CMSCP[-67]/CMS  VM/370 → VM/SE versions → VM/SP versions → VM/XA versions → VM/ESAz/VM
> TSS/360
> TSO for MVT → for OS/VS2 → for MVS → ... → for z/OS
>> MULTICS and most other time-sharing platforms
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