Incompatible Timesharing System

Incompatible Timesharing System

Developer	MIT Artificial Intelligence Laboratory and Project MAC
Written in	Assembly language
Working state	Historic
Available in	English
Platforms	PDP-6, PDP-10
Official website	{{#property:P856}}

ITS (Incompatible Timesharing System), is an early 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 (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.

History

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.

By simplifying their system compared to Multics, ITS's authors were able to quickly produce a functional operating system for their lab. 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.

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 of the OS itself

ITS introduced many revolutionary features:

• 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 the virtual device mechanism, ITS 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 machine could perform the same operations with files on other ITS machines as if they were 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 designed into or added to 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. The pioneering computer game Zork was also originally written on 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, including online documentation and source code
• 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 after FBI Director J. Edgar Hoover). This facility was later disabled in an interesting way: it looked like the remote session was killed, but it was not.
• Tourists—guest users either at MIT AI Lab terminals, or over the ARPAnet—were tolerated and occasionally encouraged to actively join the ITS community. Informal policy on tourist access was later formalized in a written policy. Ease of access, with or without a guest account, allowed interested parties to informally explore and experiment with the operating system, application programs, and "hacker" culture. Working copies of documentation and source code could be freely consulted or updated by anybody on the system.

The wide-open ITS philosophy and collaborative community were the direct forerunner of the open source, open design, and Wiki movements.

Miscellaneous

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 the fact that six SIXBIT encoded characters fit into a single 36-bit PDP-10 word. "TURIST" may also have been a pun on Alan Turing, a pioneer of theoretical computer science. The less-complimentary term "LUSER" was also applied to guest users, especially those who repeatedly engaged in clueless or vandalous behavior.

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.

Original developers

• Richard Greenblatt
• Stewart Nelson
• Tom Knight

References

• documents a very early version of the system
• documents a somewhat later, but still fairly early, version
• An Introduction to ITS for the MACSYMA User

External links

• ITS System Documentation
• SV: An ITS system running online and open for logins
• UP: Public ITS system operated by the Update Computer Club at Uppsala University
• KLH10: Ken Harrenstien's PDP-10 emulator
• instructions allowing ITS to run on the SIMH PDP-10 emulator.
• Jargon File Entry
• ITS bibliography