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Incompatible Timesharing System

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About ITS

ITS, the Incompatible Timesharing System, is an operating system for the PDP-10 computer family. It was created by hackers at MIT in the 1960s. The MIT site was shut down in 1990, but enthusiasts continue to operate ITS systems to this day.

Some notable ITS features:

  • Hosted the first versions of Emacs, Zork, Macsyma, Maclisp, Scheme, and multi-player Maze War
  • Virtual memory
  • User-space device drivers
  • Networking: TCP/IP, ARPAnet, Chaosnet
  • Transparent network file system
  • Terminal-independent text output
  • Graphical workstations

There is a mailing list for discussion about ITS. Go to for more information.

About this project

This repository contains source code, tools, and scripts to build an ITS system from scratch.

The goals are:

  • To provide an automated build from start to end. No user intervention is necessary.

  • To check which programs have source code, and which programs are missing source code.

  • To ensure that we know how to build all programs.

  • To update programs with the latest bug fixes and enhancements.

  • To be able to set configurable options across the whole system, such as host name, IP address, hardware devices.

The build currently runs on the SIMH and KLH10 emulators. Of course, we'd be delighted to test this on a real KS10.


To build ITS with this repository, you need some tools installed; see the table below. Ensure all submodules are checked out, and then type make EMULATOR=simh, make EMULATOR=pdp10-ka, make EMULATOR=pdp10-kl, make EMULATOR=pdp10-ks, or make EMULATOR=klh10. This will leave built files in the out directory, some of which are disk images with ITS installed.

Emulator Dependencies
klh10 git, c compiler, make, expect, curses, autoconf
simh git, c compiler, make, expect, curses, autoconf, sdl2
pdp10-ka git, c compiler, make, expect, curses, autoconf, sdl2, sdl2-image, sdl2-net, gtk3
pdp10-kl git, c compiler, make, expect, curses, autoconf, sdl2, sdl2-image, gtk3
pdp10-ks git, c compiler, make, expect, curses, autoconf, sdl2

The repo comes with a handy script to install required dependencies. After cloning the repo change to the build folder. For example if you cloned into ~/its use the following commands

cd ~/its/build
sudo chmod +x

the script takes 2 parameters to run: install_<os> and EMULATOR=<emnulatorname> To install the dependencies

  • on MacOS for the PDP-10 KA run
    • ./ install_osx EMULATOR=pdp10-ka
  • on Ubuntu run
    • ./ install_linux EMULATOR=pdp10-ka
  • on FreeBSD run
    • ./ install_freebsd EMULATOR=pdp10-ka

Once the script completes navigate back to the ~/its folder with cd .. and run make clean all EMULATOR=pdp10-ka


To start ITS, type ./start. If you see KLH10#, type go and Enter. If you see the DSKDMP prompt, type its, press Enter, and then ESCG. If you use the pdp10-kl emulator there is no prompt and you need to type ESCL ITS, press Enter, and then ESCG. Eventually, you will see SYSTEM JOB USING THIS CONSOLE. You are now ready to log in, so type CTRLZ. See doc/ for a list of useful commands.

Alternatively to logging in directly in the system console window, which will always be displaying daemon messages, you may prefer to use a seperate terminal session for logging in. To do this just run telnet localhost <port> from another shell window and press CTRLZ to log in there. The port should be determined as per this list:

Port Type Emulator
10000 TK10 pdp10-ka
10002 Datapoint kludge pdp10-ka
10003 Morton box pdp10-ka
10004 DZ11 simh, pdp10-ks
10007 DTE2 pdp10-kl
10008 DL10 (pdp10-kl in the future?)

To shut down ITS, type :lock and then 5down. Log yourself out to avoid the 5 minute grace period: type :logout. When ITS writes SHUTDOWN COMPLETE, it's safe to stop the emulator. Press CTRL\ to escape to the simulator command prompt and quit to close it. If you had a separate telnet user session running you can similary press CTRL[ and then exit telnet.

ITS can optionally use some additional peripheral devices. To attach a simulated GT40 graphics terminal, type ./start gt40. If you run the KA10 emulator, you can use the Knight TV raster display by typing ./start tv11 tvcon. On a TV, type F1 instead of CTRLZ.

Here is an overview of the repository:

  • bin - PDP-10 binary files necessary to bootstrap the system.
  • build - build scripts.
  • conf - configuration for building ITS.
  • doc - documentation, most of which appear in the INFO system.
  • src - source code for ITS and all programs.
  • tools - build tools.
  • out - build output.
  • user - optional user files installed in ITS.

Terminal Emulators

Several emulators for terminals and peripheral processors are built along with ITS. They can be started conveniently with the start script, or separately. Not all terminal emulators are set up to work with all PDP-10 emulators by default.

Name Description Type klh10 pdp10-ka pdp10-kl pdp10-ks simh
type340 Type 340 vector no yes no no no
gt40 GT40 PDP-11 vector no yes no yes yes
imlac Imlac PDS-1 vector no yes no no no
simh_imlac Imlac PDS-1 vector no yes no no no
tv11 Knight TV PDP-11 cpu no yes no no no
tvcon Knight TV console raster no yes no no no
datapoint Datapoint 3300 text no yes no no no
vt52 VT52 text no yes yes yes yes
tek Tektronix 4010 vector no yes yes no no


See the doc subdirectory for documentation.

There are some short introductions for beginners:

A list of known ITS machines.

How ITS is built.


Some major applications:

  • Adventure, game
  • C10, C compiler
  • CLU, progamming language.
  • DDT, debugger
  • Emacs, editor
  • Logo, interpreter
  • Mac Hack VI and Tech II, chess programs
  • Maclisp, interpreter and compiler
  • Muddle, interpreter
  • Macsyma, symbolic math
  • Maze War, game
  • Midas, assembler
  • Muscom and musrun, for playing music
  • PDP-11 simulator
  • Scheme, interpreter
  • Spacewar!, game
  • Stanford University Drawing System
  • TECO, editor
  • TOPS-10 and WAITS emulator

There is a detailed list of all installed programs.