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Multics (Multiplexed Information and Computing Service) was an experimental operating system developed initially by a cooperative project between MIT, Bell Labs, and General Electric, beginning in 1965.

Multics was developed initially for the GE-645 mainframe, a 36-bit system. It was one of the first operating systems written in a high level language; PL/I was used. (The B5000 system, which used Burroughs Algol, preceded Multics.)

Multics was especially interesting for lacking a clear distinction between files and memory. All files were treated as virtual memory; to read or write to them, an application simply mapped it into the address space[?]. (In POSIX terminology, it was as if every file was mmap()ed.) The disadvantage of this was that the size of segments on the GE 645 was limited to 1 megabyte, and therefore extra code had to be used to work on files larger than this. (In the days before huge bitmap graphics, this limit was rarely encountered.)

Multics was notable for its early emphasis on computer security by design, and Multics was possibly the very first operating system to be designed as a secure system from the ground up. In spite of this, Multics security was broken, not once, but repeatedly. This led to further work on computer security that prefigured modern security engineering techniques.

Bell Labs pulled out of the project in 1969. Some of the people who had worked on it there went on to create the UNIX system, and the UNIX system shows the influence of Multics in some areas, especially the naming of commands. The name "UNIX" is itself a pun on "Multics".

Honeywell bought GE's computer division, released a better hardware base, and continued system development until 1985. About 80 multi-million-dollar sites were installed, at universities, industry, and government sites. The French university system had quite a few in the early 1980s. After Honeywell stopped supporting Multics, users migrated to other systems including the UNIX system.

The last Multics machine was shut down on October 31, 2000.


  • M. D. Schroeder, D. D. Clark, J. H. Saltzer, D. H. Wells. Final Report of the Multics Kernel Design Project. MIT/LCS/TR-196, 1978.
  • Paul A. Karger, Roger R. Schell. Thirty Years Later: Lessons from the Multics Security Evaluation. IBM white paper.

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