Incompatible Timesharing System
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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: make, C compiler, and expect. For KA10, you also need SDL or SDL2 for the Type 340 display. Ensure all submodules are checked out, and then type make EMULATOR=simh, make EMULATOR=sims, or make EMULATOR=klh10. This will leave built files in the out directory, one of which is a disk image with ITS installed.

To start ITS, type ./start. If you see KLH10#, type go and Enter. When you see the DSKDMP prompt, type its, press Enter, and then ESC g. Eventually, you will see SYSTEM JOB USING THIS CONSOLE. You are now ready to log in, so type Control-Z. See doc/ for a list of useful commands. When done, shut down orderly by typing :lock and then 5down.

ITS can optionally use some additional peripheral devices. To attach a simulated GT40 graphics terminal, type ./start gt40 when booting ITS.

To install your personal user files, add a subdirectory under user with your files inside. They will be copied to ITS. The directory name is limited to six characters, and file names must have two six-character parts separated by a period.

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.


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
  • 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
  • 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.

Network Support

Currently, networking is only supported under the KLH10 emulator. SIMH does not have the necessary support. As of this release, only the ITS monitor, host table tools, and binary host table are installed.

Currently, basic TCP network support is in the build, in addition to both a TELNET/SUPDUP server, and both TELNET and SUPDUP clients. Additionally, both an FTP server and client are included. Chaosnet TELNET and FTP (CHTN and CFTP), but this requires support and configuration in the emulator to actually use. SMTP mail inbound and outbound is included, as well as local mail delivery.

The KLH10 dskdmp.ini file has an IP address ( and gateway IP address ( configured for the ITS system. The IP address matches the address configured in SYSTEM; CONFIG > (as IMPUS3). Finally, the HOST table source (SYSHST; H3TEXT >) and binary (SYSBIN; HOSTS3 >) defined a host called DB-ITS.EXAMPLE.COM at the IP address

In order to change the IP address of the host, you can edit IP, GW, and NETMASK in the file conf/network. You can also set a Chaosnet address. After that, a full rebuild (e.g. make clean all) is required.