Incompatible Timesharing System

Incompatible Timesharing System
Developer MIT Artificial Intelligence Laboratory and Project MAC
Written in Assembly language
Working state Discontinued
Initial release early 1960s
Available in English
Platforms Digital PDP-6, PDP-10

ITS (Incompatible Timesharing System), is a time-sharing operating system developed principally by the MIT Artificial Intelligence Laboratory, with help from Project MAC. The name is the jocular complement of the MIT Compatible Time-Sharing System (CTSS).

ITS, and the software developed on it, were technically influential far beyond their core user community. Remote "guest" or "tourist" access was easily available via the early ARPAnet, allowing many interested parties to informally try out features of the operating system and application programs. The software environment of ITS was a major influence on the hacker culture, as described in Steven Levy's book Hackers.[1]


ITS development was initiated in the late 1960s by those (the majority of the MIT AI Lab staff at that time) who disagreed with the direction taken by Project MAC's Multics project (which had started in the mid-1960s), particularly such decisions as the inclusion of powerful system security. The name was chosen by Tom Knight as a joke on the name of the earliest MIT time-sharing operating system, the Compatible Time-Sharing System, which dated from the early 1960s.[1]

By simplifying their system compared to Multics, ITS's authors were able to quickly produce a functional operating system for their lab.[2] ITS was written in assembly language, originally for the Digital Equipment Corporation PDP-6 computer, but the majority of ITS development and use was on the later PDP-10.[1]

Although not used as intensively after about 1982, ITS continued to operate at MIT until 1990, and then until 1995 at Stacken Computer Club in Sweden.

Significant technical features

ITS introduced many then-new features:

User environment

The environment seen by ITS users was philosophically significantly different from that provided by most operating systems at the time.[1]

The wide-open ITS philosophy and collaborative community were the direct forerunner of the free and open source software, open design, and Wiki movements.[5][6][7]

Important applications developed on ITS

The EMACS ("Editor MACroS") editor was originally written on ITS; in its ITS instantiation, it was a collection of TECO programs (called "macros"). For later operating systems it was written in the common language of those systems. For example, the C language under Unix, and Zetalisp under the Lisp Machine system.

The GNU info help system was originally an EMACS subsystem, and then was later written as a complete standalone system for Unix-like machines.

Several important programming languages and systems were developed on ITS, including MacLisp (the precursor of Zetalisp and Common Lisp), Microplanner (implemented in MacLisp), MDL (which became the basis of Infocom's programming environment), and Scheme.

Among other significant and influential software subsystems which were developed on ITS, the Macsyma symbolic algebra system is probably the most important. Terry Winograd's SHRDLU program was developed in ITS. The computer game Zork was also originally written on ITS.


The default ITS top-level command interpreter was the PDP-10 machine language debugger (DDT). The usual text editor on ITS was TECO and later Emacs, which was written in TECO. Both DDT and TECO were implemented through simple dispatch tables on single-letter commands, and thus had no true syntax. The ITS task manager was called PEEK.

The local spelling "TURIST" is an artifact of six-character filename (and other identifier) limitations, which is traceable to six SIXBIT encoded characters fitting into a single 36-bit PDP-10 word. "TURIST" may also have been a pun on Alan Turing, a pioneer of theoretical computer science.[8] The less-complimentary term "LUSER" was also applied to guest users, especially those who repeatedly engaged in clueless or vandalous behavior.[9]

The Jargon File started as a combined effort between people on the ITS machines at MIT and at Stanford University SAIL. The document described much of the terminology, puns, and culture of the two AI Labs and related research groups, and is the direct predecessor of the Hacker's Dictionary.[10]

Original developers


  1. 1 2 3 4 Levy, Steven (2010). "Winners and Losers". Hackers: Heroes of the Computer Revolution - 25th Anniversary Edition (1st ed.). Sebastopol, CA: O'Reilly Media. pp. 85–102. ISBN 978-1449388393.
  2. Stuart, Brian L. (2008). Principles of Operating Systems: Design & Applications. Cengage Learning EMEA. p. 23.
  3. Eric S. Raymond, ed. (December 29, 2003). "OS and JEDGAR". The Jargon File (4.4.7 ed.). Retrieved December 21, 2009.
  4. "MIT AI Lab Tourist Policy". January 15, 1997. Retrieved December 21, 2009.
  5. Pan, Guohua; Bonk, Curtis J. (April 2007). "A Socio-Cultural Perspective on the Free and Open Source Software Movement". International Journal of Instructional Technology & Distance Learning. 4 (4). Retrieved 2014-06-02.
  6. Stallman, Richard M. Free Software, Free Society: Selected Essays of Richard M. Stallman. p. 13. Retrieved 2014-06-02.
  7. "History of OSS". Software Development for the Masses. Retrieved 2014-06-02.
  8. "turist"., LLC. Retrieved 2014-06-04.
  9. "luser"., LLC. Retrieved 2014-06-04.
  10. "The Original Hacker's Dictionary". Paul Dorish. Retrieved 2014-06-16.
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