In computing, time-sharing is the sharing of a computing resource
among many users by means of multiprogramming and multi-tasking at the
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, and promoted
the interactive use of computers and the development of new
1.1 Batch processing
1.4.1 Rise and Fall
184.108.40.206 Rapidata as an example
1.5 The computer utility
2 Notable time-sharing systems
3 See also
5 Further reading
6 External links
Main article: 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
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. 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. 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, and later by
Bob Bemer in his 1957
article "How to consider a computer" in Automatic Control
Magazine. In a paper published in December 1958 by W. F.
Bauer, 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
Implementing a system able to take advantage of this was initially
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
and later on an additionally modified
IBM 709 (one of the first
computers powerful enough for time-sharing). 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
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 not lost the patent for 2 years. JOSS
began time-sharing service in January 1964.
The first commercially successful time-sharing system was the
Dartmouth Time Sharing System.
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". Conversely, timesharing
users thought that their terminal was the computer.
With the rise of microcomputing in the early 1980s, time-sharing
became less significant, 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
In the 1960s, several companies started providing time-sharing
services as service bureaus. Early systems used
Teletype Model 33
Teletype Model 33 KSR
or ASR or Teletype Model 35 KSR or ASR machines in
IBM Selectric typewriter-based terminals (especially the
with two different seven-bit codes. 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
Common systems used for time-sharing included the SDS 940, the PDP-10,
IBM 360. Companies providing this service included GE's
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
Bolt, Beranek, and Newman (BBN). By 1968, there were 32 such service
bureaus serving the US
National Institutes of Health
National Institutes of Health (NIH) alone.
The Auerbach Guide to Timesharing (1973) lists 125 different
timesharing services using equipment from Burroughs, CDC, DEC, HP,
Honeywell, IBM, RCA, Univac, and XDS.
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 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 were widely used 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
held on, and became part of National Data Corporation. It was
still of sufficient interest in 1982 to be the focus of "A User's
Guide to Statistics Programs: The Rapidata Timesharing System".
Even as revenue fell by 66% and National Data subsequently
developed its own problems, attempts were made to keep this
timesharing business going.
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. 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
Notable time-sharing systems
Time-sharing system evolution
Significant early timesharing systems:
Allen-Babcock RUSH (Remote Users of Shared Hardware) Time-sharing
IBM S/360 hardware (1966) → Tymshare
Unix (1971) → UC Berkeley BSD
Time-sharing System → Massachusetts General Hospital
PDP-1D → MUMPS
BBN TENEX → DEC TOPS-20, Foonly FOONEX, MAXC OS at PARC, Stanford
Low Overhead Timesharing System (LOTS)
Berkeley Timesharing System at UC Berkeley
Project Genie →
Scientific Data Systems
Scientific Data Systems
SDS 940 (Tymshare, BBN, SRI, Community Memory)
→ BCC 500 → MAXC at PARC
Time-sharing MCP →
HP 3000 MPE
Cambridge Multiple Access System was developed for the Titan, the
prototype Atlas 2 computer built by
Ferranti for the University of
Cambridge. This was the first time-sharing system developed
outside the United States, and which influenced the later development
Compower Ltd., a wholly owned subsidiary of the National Coal Board
(later British Coal Corporation) in the UK. Originally National Coal
Board (NCB) Computer Services, it became Compower in 1973 providing
computing and time-share services to internal NCB users and as a
commercial service to external users. Sold to Philips C&P
(Communications and Processing) in August 1994.
CompuServe, also branded as Compu-Serv, CIS.
Compu-Time, Inc., on
Honeywell 400/4000, started in 1968 in Ft
Lauderdale, Florida, moved to Daytona Beach in 1970.
CDC MACE, APEX → Kronos → NOS → NOS/VE
Dartmouth Time Sharing System (DTSS) → GE
Time-sharing → GEnie
Time-sharing Monitor →
TOPS-10 → BBN TENEX → DEC
TSS-8 → RSTS-11,
RSX-11 → VAX/VMS
HP 2000 Time-Shared BASIC
HP 3000 series
IBM CALL/360, CALL/OS - using
IBM System/360 Model 50
IBM CP-40 →
CP/CMS → VM/CMS
IBM TSO for
OS/MVT → for OS/VS2 → for
MVS → for z/OS
TSS/360 → TSS/370
International Timesharing Corporation on dual CDC 3300 systems.
MIT CTSS →
MIT / GE / Bell Labs) → Unix
Time-sharing System for the DEC
PDP-1 → ITS
McGill University MUSIC →
Michigan Terminal System, on the
IBM S/360-67, S/370, and successors.
Michigan State University
Michigan State University CDC SCOPE/HUSTLER System
National CSS VP/CSS, on
IBM 360 series; originally based on IBM's
Oregon State University
Oregon State University OS-3, on
CDC 3000 series.
Prime Computer PRIMOS
JOSS → JOSS-2 → JOSS-3
RCA TSOS →
Unisys VMOS → VS/9
Service in Informatics and Analysis (SIA), on
CDC 6600 Kronos.
System Development Corporation
Time-sharing System, on the AN/FSQ-32.
Stanford ORVYL and WYLBUR, on
Time-sharing System → SAIL → WAITS
Time Sharing Ltd. (TSL) on DEC
PDP-10 systems → Automatic Data
Processing (ADP), first commercial time-sharing system in Europe and
first dual (fault tolerant) time-sharing system.
Tone (TSO-like, for VS1), a non-
Time-sharing product, marketed by
Software Co; TSO required VS2.
Tymshare SDS-940 → Tymcom X → Tymcom XX
EXEC 8 → OS 1100 → OS 2200
UC Berkeley CAL-TSS, on CDC 6400.
XDS UTS → CP-V →
The Heralds of Resource Sharing, a 1972 film.
History of CP/CMS, IBM's virtual machine operating system (CP) that
supported time-sharing (CMS).
IBM M44/44X, an experimental computer system based on an
IBM 7044 used
to simulate multiple virtual machines.
IBM System/360 Model 67, the only
IBM S/360 series mainframe to
support virtual memory.
Multiseat configuration, multiple users on a single personal computer.
Project MAC, a
DARPA funded project at
MIT famous for groundbreaking
research in operating systems, artificial intelligence, and the theory
TELCOMP, an interactive, conversational programming language based on
JOSS, developed by BBN in 1964.
Timeline of operating systems
VAX (Virtual Address eXtension), a computer architecture and family of
computers developed by DEC.
^ a b DEC TIMESHARING (1965), by Peter Clark, The DEC Professional,
VOLUME 1, Number 1
IBM advertised, early 1960s, with a headline: "This man is sharing a
$2 million computer"
^ Eisenson, Arthur; and Yager, Heather (1967). Ellis D. Kropotchev
Silent Film. Stanford University, 1967. This student-produced film
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
^ Backus, John, Computer Advanced Coding Techniques,
MIT 1954, page
16-2. The first known description of computer time-sharing.
^ Bemer, Bob (March 1957). "Origins of Timesharing". bobbemer.com.
Retrieved June 24, 2016.
^ Middleburg, C.A. (2010). "Searching Publications on Operating
Systems". arXiv:1003.5525 [cs.OS].
^ 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.
^ "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
Clist and Rexx.
^ a b McCarthy, John. "REMINISCENCES ON THE HISTORY OF TIME SHARING".
stanford.edu. stanford.edu. Retrieved 12 March 2017.
^ Brian Dear, Chapter 4 -- The Diagram, The Friendly Orange Glow,
Pantheon Books, New York, 2017; pages 71-72 discuss the development of
time-sharing and the University of Illinois loss of the patent.
^ 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.
^ Kemeny, John G.; Kurtz, Thomas E. (11 October 1968). "Dartmouth
Time-Sharing". Science. 162: 223–228.
^ "TRANSCRIPTS OF 1974 National Computer Conference Pioneer Day
Session". Dartmouth Time Sharing System. Dartmouth College.
IBM 2741 Communication Terminal (PDF). IBM. p. 12.
^ "Information Technology Corporate Histories Collection". Computer
History Museum. Retrieved on 2013-11-29 from
^ a b c d Auerbach Guide to Time Sharing (PDF). Auerbach Publishers,
Inc. 1973. Retrieved 2013-11-29.
^ Computerworld, June 11, 1975, p. 35
^ 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?
^ p.1425, Encyclopedia of Computer Science, Litton Educational
- "I worked for RapiData Timesharing for about a year circa 1969..."
^ someone else: "I worked there for almost 2 years 1977 to 1979."
^ NDC started in 1967, and paralleled Rapidata; see Bloomberg's
^ Bruce Bosworth, ISBN 978-089529-1-677
^ Computerworld, Oct. 6, 1986, p.179, "Rapidata revenue was $11
million ... in 1986, down from ... ($31 million in 1982)."
^ 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
^ National Data Corp became NDC-Health Corp in 2001
^ As for a place in history, Rapidata is listed in 'The AUERBACH Guide
to Time Sharing (1973)'
^ 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.
^ "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.
^ Hartley, D. F. (1968), The Cambridge multiple-access system: user's
reference manual, Cambridge: Cambridge Univ. Press,
^ "Time Sharing", James Miller. Retrieved 30 November 2013.
Nelson, Theodor (1974). Computer Lib: You Can and Must Understand
Computers Now; Dream Machines: "New Freedoms Through Computer
Screens— A Minority Report". Self-published.
ISBN 0-89347-002-3. pp. 56–57.
"Time Sharing Supervisor Programs", notes comparing the supervisor
programs of CP-67, TSS/360, the
Michigan Terminal System
Michigan Terminal System (MTS), and
Multics by Michael T. Alexander, Advanced Topics in Systems
Programming (1970, revised 1971), University of Michigan Engineering
"The Computer Utility As A Marketplace For Computer Services", Robert
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,
"Mainframe Computers: The Virtues of Sharing", Revolution: The First
2000 Years of Computing, Computer History Museum Exhibition, January
"Mainframe Computers: Timesharing as a Business", Revolution: The
First 2000 Years of Computing, Computer History Museum Exhibition,
Loadable kernel module
Process control block
Multilevel feedback queue
Shortest job next
Memory management and
General protection fault
Storage access and
Virtual file system
Virtual tape library
Classic Mac OS
CP/CMS family relationships
→ derivation >> strong
influence > some influence/precedence
>> CP-40/CMS → CP[-67]/CMS
VM/370 → VM/SE versions → VM/SP versions → VM/XA versions
VM/ESA → z/VM
> TSO for MVT → for OS/VS2 → for
MVS → ... → for z/OS
MULTICS and most other time-sharing pla