Last Updated on: 30th April 2024, 07:22 am

Web site: (not active)
Origin: USA
Category: PDP-6/PDP-10
Desktop environment: CLI
Architecture: Digital Equipment Corporation (DEC) PDP-6/PDP-10
Based on: Independent
Media: Install
The last version | Released: 1990

ITS (Incompatible Timesharing System) – an OS developed by hackers at MIT’s AI lab from 1967 to the early 1980’s for the DEC PDP-6 and PDP-10 family. It was famous for being the antithesis of MIT’s security-obsessed CTSS (Compatible Time-Sharing System), by being very hackable and so user rights were a matter of social respect and ethics alone, rather than the dictates of the system, which often limited performance and capacity of the system.

The Incompatible Time Sharing (ITS) operating system was originally written for the PDP-6 at the MIT Artificial Intelligence Laboratory during the late 1960’s. At MIT during the 1970’s ITS ran on three KA-10’s and one KL-10, providing service to researchers in the AI Laboratory and in MIT’s Laboratory for Computer Science. Many well-known PDP-10 programs, such as the Emacs editor, the MacLisp dialect of Lisp, the Macsyma symbolic algebra system, and the Midas assembler, were originally developed under ITS. More than twenty years later ITS still has a small, but devoted, user community. It now runs on a handful of KS-10 systems around the world.

The designers of ITS were not primarily interested in doing research in operating systems. They held a pragmatic view of ITS as a tool for enabling the AI Lab’s small user community to get the most out of their PDP-6/10 and its peripherals (robot arms, TV cameras, etc.). This is not to say that the designers were ignorant of the state of the art in operating systems, but they were unlikely to publish papers about their experiences. As a result, many of their pioneering ideas remain largely unknown outside the ITS user community.

Among other features, ITS boasts a convenient symbolic system call mechanism that allows calls to be invoked by name and to pass values in arbitrary locations, a flexible system for delivering software interrupts to processes, device-independent display terminal support, a process organization that allows a single command processor to control multiple sub-processes, and a powerful mechanism for implementing devices in software that enabled ITS to have what was probably the first network file system. Many of these features have since been adopted or rediscovered by other operating systems, but one of ITS’s major design principles, called “PCLSRing”, has never found its way into any other operating system.

It is my belief that PCLSRing is an important idea that does not deserve to be neglected merely because it has never been adequately explained in print. Therefore, in this paper I have endeavored to explain what PCLSRing is, what its benefits are for both the users and the implementors of ITS, and how it is implemented.

The reader should not expect to completely understand the description of the implementation of PCLSRing on first reading. Many of the points are subtle, and can probably only be fully appreciated by someone who tries to implement PCLSRing for himself.

ITS was driven by the hacker ethic. Richard Stallman, founder of the GNU project, got the idea of free software from his work with ITS.

Party source from Alan Bawden’s paper; and Lars Brinkhoff corrections.

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