Incompatible Timesharing System

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The Incompatible Timesharing System (usually ITS) was an early time-sharing operating system; initially for the PDP-6, and later for PDP-10's. It was developed at MIT in the Artificial Intellegence Lab, after Multics was done by Project MAC. It first became operational in July, 1967, after a very short design and implementation period, starting earlier that year.

The KL10 MC machine at MIT

The earliest versions ran on the PDP-6, using the base and bounds memory management hardware native to that machine. Later versions ran on KA10s which were modified with MIT-designed and built paging hardware (which that generation of PDP-10 did not have). It later ran on the KL10 and KS10 models as well, in both cases running custom microcode that emulated the operation of the MIT paging box.

ITS was one of the first OS's connected to the ARPANET, and it was on an ITS system that the first versions of Emacs, Zork, MACLISP, and Scheme were created.

During much of its operational lifetime, ITS ran on only a handful of machines:

  • The AI Lab PDP-6, serial number 2. (The Dynamic Modeling group also had a PDP-6, not much used.)
  • Three KA10s: AI, DM, ML. Their serial numbers were 8, 144, and 198.
  • One KL10: MC, serial number 1038.

Due to failing hardware, the PDP-6 was shut down in the late 1970s, and physically removed in the early 1980s. The KA10s followed shortly after, but some were replaced with KS10s. By 1990 all MIT machines were shut down permanently.

Output from the PEEK program

In modern times, ITS runs on a KS10 at the Living Computers museum; it was previously MIT-AI. The museum also has the KL10 MIT-MC in storage. ITS also runs on several software emulators.

Some information on installing & images can be found here.


  • 1967 - First version for PDP-6
  • 1969 - Ported to KA10
  • 1969 - Second PDP-6 at Dynamic Modeling group
  • 1970 - Second KA10 at Dynamic Modeling group
  • 1971 - Connected to ARPANET, version ~670
  • 1972 - Third KA10 at Mathlab group
  • 1975 - Ported to KL10, version 915
  • 1978 - PDP-6 support dropped, version 1115
  • 1984 - All KA10 machines shut down, first KS10 arrives
  • 1985 - Ported to KS10, version 1488, three more KS10s
  • 1986 - KS ITS distributed outside MIT (SI, FU, PM, DX)
  • 1988 - KL10 machine shut down
  • 1990 - All MIT KS10 machines shut down; last MIT version 1644
  • 1992 - First run on KS10 software simulator
  • 2001 - First simulator available on Internet
  • 2010 - First run on an FPGA PDP-10 implementation
  • 2017 - First run on KA10 software simulator

Notable contributions

The Artificial Intelligence PDP-6 and PDP-10 were host to many influential programs: TECO (adopted form the RLE PDP-1), Logo (coming from BBN), Mac Hack VI, MACLISP, Scheme, EMACS, and was used to bootstrap the LISP machine hardware and software.

The Dynamic Modeling PDP-10 was used to develop Muddle, MAZE, the first C compiler outside Bell Labs, CLU, and Zork.

The Mathlab PDP-10 was used to develop Macsyma, but was later supplanted by a dedicated Macsyma Consortium KL10.

Early artifacts

The earliest scrap of ITS code on record is a screenshot from the 340 display showing the IMEMR subroutine (which still exists in the 1990 version). The dating is unknown.

The next is ITS version 724 from April 1972. It's a complete binary core image including Salvager and Exec DDT. It runs on Richard Cornwell's KA10 simulator with Systems Concepts DC-10 disks. It announces itself as "ITS 724" rather than "AI ITS 724", so it's likely from before ML code was added.

The earliest source code version is ITS 785 from 1973. It has support for the AI machine and the ML machine.

See also

External links