compatible timesharing system

Incompatible Timesharing System

ITS, the Incompatible Timesharing System (named in comparison with the Compatible Time-Sharing System also in use at MIT), was an early, revolutionary, and influential time-sharing operating system from MIT; it was developed principally by the Artificial Intelligence Laboratory at MIT, with some help from Project MAC.

In addition to being technically influential (both in the operating system itself, as well as applications developed on it), it was one of the projects most important in the original development of the hacker culture (as documented 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 hack on the earliest MIT time-sharing operating system, the Compatible Time-Sharing System, which dated from the early 1960s.

ITS was written in assembly, and initially developed for the Digital Equipment Corporation PDP-6 computer, and later moved to the PDP-10 once it became available, where it saw the majority of its development and use.

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

Significant technical features of the OS itself

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). 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

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.

User environment

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"). The user being watched was informed, and could kill the viewer's session (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.
  • Tourists - guest users either at MIT AI Lab terminals, or over the ARPAnet - were permitted. A policy was later published on tourist access. 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.


Among numerous interesting features and oddities, the default ITS top-level command interpreter was the PDP-10 machine language debugger (DDT), whose commands looked like line noise to the uninitiated.

Its main editor for many years, TECO, was programmable in a similar-looking gibberish.

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

Original developers


See the annotated ITS bibliography on the STS Wiki for a more detailed list of references.

External links

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