Incompatible Timesharing System

Incompatible Timesharing System (ITS) is a

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.^[3]

By simplifying their system compared to Multics, ITS's authors were able to quickly^{[clarification needed]} produce a functional operating system for their lab.^[4] 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, largely compatible, PDP-10.^[3]

Although not used as intensively after about 1986, ITS continued to operate on original hardware at MIT until 1990, and then until 1995 at Stacken Computer Club in Sweden. Today, some ITS implementations continue to be remotely accessible, via emulation of PDP-10 hardware running on modern, low-cost computers supported by interested hackers.

Significant technical features

ITS introduced many then-new features:

• The first
  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 running in user processes (which were called "jobs" in ITS).
• Using the virtual-device mechanism, ITS provided transparent
  inter-machine filesystem access. 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 resumed transparently.
• An advanced
  interrupt handling
  mechanisms.
• 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.

User environment

The environment seen by ITS users was philosophically significantly different from that provided by most operating systems at the time.^[3]

• Initially there were no
  passwords, and a user could work on ITS without logging on.^[5]
  Logging on was considered polite, though, so people knew when one was 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 that caused the system to crash was implemented and could be run by anyone, 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. A series of informal papers and technical notes documented new commands, technical issues, primitive games, mathematical puzzles, and other topics of interest to the ITS hacker community. Some were issued as more formal AI Memos, including the HAKMEM compendium.
• 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 with a placebo command: it appeared as if the remote session was killed, but it was not.^[6]
• Tourists (guest users either at MIT AI Lab terminals, or over the ARPAnet) were tolerated and occasionally invited to actively join the ITS community. Informal policy on tourist access was later formalized in a written policy.^[7] 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.
• System security, to the extent that it existed, was mostly-based on de facto "security by obscurity". Guest hackers willing to dedicate significant time and effort to learning ITS were expected to behave respectfully, and to avoid interfering with the research projects which funded the hardware and software systems. There was little of exclusive value on the ITS systems except information, much of which would eventually be published for free distribution, and open and free sharing of knowledge was generally encouraged.

The wide-open ITS philosophy and collaborative community were the direct forerunner of the free and open-source software, open-design, and Wiki movements.^[8][9][10]

Important applications developed on ITS

The

GNU‘s 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

Among other significant and influential software subsystems developed on ITS, the Macsyma symbolic algebra system, started in 1968, was the first widely-known mathematical computing environment. It was a forerunner of Maxima, MATLAB, Wolfram Mathematica, and many other computer algebra systems.

Terry Winograd's SHRDLU program was developed in ITS. The computer game Zork was also originally written on ITS.

Richard Greenblatt's Mac Hack VI was the top-rated chess program for years^{[citation needed]} and was the first to display a graphical board representation.^{[citation needed]}

Miscellaneous

The default ITS top-level

The Xerox Graphics Printer (XGP), one of the first laser printers in the world, was supported by ITS by 1974.^[14] The MIT AI Lab had one of these prototype continuous roll-fed printers for experimentation and use by its staff. By 1982, the XGP was supplemented by a Xerox Dover printer, an early sheet-fed laser printer.^[15] Although any ITS user could access the laser printers, physical access to pick up printouts was limited to staff, to control usage of printer supplies which had to be specially ordered.

CTSS and ITS file systems have a number of design elements in common. Both have an M.F.D. (master file directory) and one or more U.F.D. (user file directories). Neither of them have nested directories (sub-directories) Both have file names consisting of two names which are a maximum of six-characters long. Both support linked files.

Original developers

• Richard Greenblatt
• Stewart Nelson
• Tom Knight
• Richard Stallman

See also

• Time-sharing system evolution

References