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 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
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