October 31, 2000 – The Last Multics Machine

On this day in engineering history, the Canadian Department of National Defense shut down the world's last Multics computer system. Multics, an acronym for "multiplexed information and computing service", was a mainframe timesharing operating system (OS) that began as research project, but became an important milestone in the computer revolution. Multics was the first OS to use a hierarchical file system, and one of the first operating systems to use the now-standard practice of per-process stacks in the kernel. Multics was also one of the first computer systems written in a high-level programming language (PL/I), and possibly the first such system to emphasize built-in computer security. Fans of Multics also note its role in the subsequent development of UNIX and Linux.

Planning and Development

Planning for Multics began in 1964. Fernando (Corby) Corbató, a professor at the Massachusetts Institute of Technology (MIT), teamed up with computer scientists from General Electric and Bell Laboratories as part of Project MAC, a research program funded by the Advanced Research Projects Agency (ARPA) in the U.S. Department of Defense (DOD). In a paper submitted to the 1965 Fall Joint Computer Conference at MIT, Professor Corbató joined V.A. Vyssotsky of Bell Labs in predicting that "the Multics system will undoubtedly open up large classes of new uses not only in science and engineering, but also in other areas such as business and education." The researchers also posited that "multiple access systems" such as Multics would have "significant social consequences".

Initially, Multics was built with tools provided by MIT's compatible time-sharing system (CTSS), an early time-sharing OS with a robust file system. The first version of Multics was developed for the GE-645, a 36-bit mainframe computer that included multiple security levels and instructions for handling virtual memory. By modifying the addressing scheme to use 18-bit segments (files) and 18-bit addresses, the computer's designers dramatically increased its theoretical addressable memory size. By using small subroutines called operators for short but standard code-sequences, the Multics compiler optimized code density and preserved the system's main memory. Dynamic linking, another Multics innovation, enabled applications to use the latest version of any called routine.

A Virtual Memory Operating System

Although these Multics features were important, the memory savings provided by sharing code between processes created an even stronger effect. Virtual memory enabled programs to address a space far larger than physical memory. Multics machines could therefore handle larger and more complex applications than other computer systems. Power, however, came at a price. Because a virtual memory OS must perform more tasks, applications that use virtual memory do not perform as well as applications that are 100% memory resident.

Multics in the Marketplace

General Electric sold several Multics GE-645s to research facilities, but sold its entire computer business to Honeywell in 1970. A year earlier, Bell Labs had also ended its involvement with Multics - a decision that freed up resources for the development of its new UNIX operating system several years later. Subsequent versions of Multics ran on Honeywell machines, including one owned by the Canadian Department of National Defense. Only 81 Multics sites were ever installed world-wide, each representing multi-million dollars expenditure and serving hundreds of users. The system was used by education, auto and aircraft manufacturers, and military installations.

On the day before the last Multics machine was turned off, Professor Corbató addressed a letter to all "Multicians", as fans of and contributors to Multics are still known. "It is hard to believe that the last instance of Multics hardware is about to be extinguished," Corbató wrote, but "the influence of Multics has probably achieved immortality through the admirable evolution of Unix and more recently Linux."

Resources:

http://en.wikipedia.org/wiki/Multics

http://www.multicians.org/

http://www.multicians.org/corby-letter.html

http://www.multicians.org/fjcc1.html

http://en.wikipedia.org/wiki/GE-645