Incompatible Timesharing System

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Incompatible Timesharing System
Company / developer MIT Artificial Intelligence Laboratory and Project MAC
Written in Assembly language
Working state Historic
Available in English
Supported platforms PDP-6, PDP-10

ITS (Incompatible Timesharing System), is an early, revolutionary, and influential 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.

ITS, and the software developed on it, were technically influential. They also contributed to the hacker culture, described in Steven Levy's book, Hackers.


ITS development was initiated in the late 1960s by those (the majority of the MIT AI Lab 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.[citation needed]

ITS was written in assembler, originally for the Digital Equipment Corporation PDP-6 computer, but the majority of ITS development and use was on the later PDP-10.

Although not used much after about 1982, ITS was run at MIT until 1990, and then until 1995 at Stacken Computer Club in Sweden. A few instances are still running today for historical interest, almost all on simulated PDP-10's.[citation needed]

Significant technical features of the OS itself[edit]

ITS introduced many revolutionary features:

  • It had the first device-independent graphics terminal output; programs generated generic commands to control screen content, which the system automatically translated into the appropriate character sequences for the particular type of terminal operated by the user.
  • A general mechanism for implementing virtual devices in software which ran in user processes (which were called "jobs" in ITS).
  • Using this mechanism, it provided transparent inter-machine filesystem access (almost certainly the first operating system to do so[citation needed]). The ITS machines were all connected to the ARPAnet, and a user on one could perform the same operations on files on other ITS machines as on local files.
  • Sophisticated process management; user processes were organized in a tree, and a superior process could control a large number of inferior processes. Any inferior process could be frozen at any point in its operation, and its state (including contents of the registers) examined; the process could then be restarted transparently.
  • An advanced software interrupt facility that allowed user processes to operate asynchronously, using complex interrupt handling mechanisms.
  • PCLSRing, a mechanism which provided what appeared (to user processes) to be quasi-atomic, safely interruptible system calls. No process could ever observe any process (including itself) in the middle of executing any system call.
  • In support of the AI Lab's robotics work, ITS also supported simultaneous real-time and time-sharing operation.

Many of these, and numerous other significant advances, were later picked up by other operating systems.

Important applications developed on ITS[edit]

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 pioneering SHRDLU program was also developed in ITS. The game Zork was also originally written on ITS.

User environment[edit]

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

  • Initially there were no passwords, and a user could work on ITS without even logging on. Logging on was considered polite, though, so people knew when you were connected.
  • To deal with a rash of incidents where users sought out flaws in the system in order to crash it, a novel approach was taken. A command was implemented which anyone could run which caused the system to crash, which took away all the fun and challenge of doing so. It did, however, broadcast a message to say who was doing it.
  • All files were editable by all users.
  • All users could talk with instant messaging on another's terminal, or they could use a command (SHOUT) to ask all active users for help.
  • Users could see what was happening on another's terminal (using a command called OS for "output spy"). A target of OS could detect and kill it using another command called JEDGAR, named for FBI Director J. Edgar Hoover). This facility was later disabled in an interesting way: it looked like the session was killed, but was not.[1]
  • Tourists - guest users either at MIT AI Lab terminals, or over the ARPAnet - were permitted. A policy was later published on tourist access.[2] The local spelling "TURIST" is an artifact of six character filename (and other identifier) limitations, which is traceable to the fact that six SIXBIT encoded characters fit in a single 36-bit PDP-10 word. "TURIST" may also have been a pun on Alan Turing[citation needed].


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.

ITS's task manager was called Peek.

The Jargon File started as a combined effort between people on the ITS machines at MIT and at SAIL.

Original developers[edit]


  1. ^ Eric S. Raymond, ed. (December 29, 2003). "OS and JEDGAR". The Jargon File (4.4.7 ed.). Retrieved December 21, 2009. 
  2. ^ "MIT AI Lab Tourist Policy". January 15, 1997. Retrieved December 21, 2009. 

External links[edit]