Incompatible Timesharing System
ITS, the Incompatible Timesharing System, was an early, revolutionary, and influential MIT time-sharing operating system; it was developed principally by the Artificial Intelligence Laboratory at MIT, with some help from Project MAC. ITS development was initiated by those who disagreed with the direction taken by MIT's Project MAC's CTSS (which was the majority of the AI Lab in the 60s), particularly such decisions as the inclusion of rigid system security. ITS was initially developed for the Digital Equipment Corporation PDP-6 computer, and later for the PDP-10 where the it saw the majority of it's use and development.
It was the first system with many revolutionary features. For example, it had the first device-independent graphics terminal output.
- Sophisticated process management and IPC facilities were provided. Among other features to facilitate IPC, ITS provided an advanced software interrupt facility and PCLSRing (non-blocking, safely interruptible system calls). User processes were organized by a tree hierarchy.
- Initially there were no passwords, and a user could work on ITS without even logging on! Logging on was polite though, so people knew when you were connected.
- All users could bring the system down, (a message was broadcast to say who was doing it).
- All files were editable by all users.
- All users could talk with instant messaging on anothers terminal, or they could SHOUT for help.
- Users could see what was happening on anothers terminal. The one being watched was informed & could kill the viewer's session, this facility was later disabled in an interesting way. It looked like the session was killed, but actually it wasn't.
- Tourists (guest users either at MIT AI Lab terminals or later over the ARPAnet) were permitted, later there was a policy published on tourist access (see links below).
- Jobs could be frozen and the status of the registers examined. The job could then be restarted without any problems. This facility is now in one of the GNU tools.
- The GNU INFO help system used in Linux, some versions of UNIX, and EMACS was started on ITS.
Among other interesting features, ITS top-level command interpreter was the PDP-10 machine language debugger (DDT).
Further Reading
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