Multics ("MULTiplexed Information and Computing Service") is an influential early time-sharing operating system based on the concept of a single-level memory.

The system can grow in size by simply adding more of the appropriate resource, be it computing power, main memory, or disk storage.

Separate access control lists on every file provide flexible information sharing, but complete privacy when needed.

[7] Its most lasting effect on the computer industry was to inspire the creation of Unix, which carried forward many Multics features, but was able to run on less-expensive hardware.

Another major new idea of Multics was dynamic linking, in which a running process can make external routines available by adding the segments containing them to its address space.

Equally importantly, with the appropriate settings in the Multics security facilities, the code in the other segment can gain access to data structures maintained in a different process.

Multics also supports extremely aggressive on-line reconfiguration: central processing units, memory banks, disk drives, etc.

[citation needed] A US Air Force tiger team project tested Multics security in 1973 under the codeword ZARF.

In 1975, Morrie Gasser of MITRE Corporation developed a pronounceable random word generator to address password requirements of installations such as the Air Force Data Services Center (AFDSC) processing classified information.

[21] The last known Multics installation running natively on Honeywell hardware was shut down on October 30, 2000, at the Canadian Department of National Defence in Halifax, Nova Scotia, Canada.

[24] In 2014, Multics was successfully run on current hardware using an emulator created by Multicians Harry Reed and Charles Anthony.

[27] The following is a list of programs and commands[28] for common computing tasks that are supported by the Multics command-line interface.

[30] Some common active functions are: Peter H. Salus, author of a book covering Unix's early years,[32] stated one position: "With Multics they tried to have a much more versatile and flexible operating system, and it failed miserably".

[7][better source needed] The permanently resident kernel of Multics, a system derided in its day as being too large and complex, was 135 KB of code.

High code density is a good optimisation choice for Multics as a multi-user system with expensive main memory.

The DPS-6 and GCOS was a well-regarded and reliable platform for inventory, accounting, word processing, and vertical market applications, such as banking, where it had a sizeable customer base.

In contrast, the full potential of Multics’ flexibility for even mundane tasks was not easy to comprehend in that era and its features were generally outside the skill set of contemporary business analysts.

The Multician explained that he had executed a global search and replace as the staffer was speaking, and the task was in fact completed.

[34] Influence of Multics on Unix is evident in many areas, including the hierarchical file system, redirection, the shell, and the naming of some commands.

But the internal design philosophy is quite different, focusing on keeping the system small and simple, and so correcting some perceived deficiencies of Multics because of its high resource demands on the limited computer hardware of the time.

(Garfinkel and Abelson[35] cite an alternative origin: Peter Neumann at Bell Labs, watching a demonstration of the prototype, suggested the pun name UNICS – pronounced "eunuchs" – as a "castrated Multics", although Dennis Ritchie is said to have denied this.

[37] The Prime Computer operating system, PRIMOS, was referred to as "Multics in a shoebox" by William Poduska, a founder of the company.

Stratus VOS is the most directly related descendant of Multics still in active development and production usage today.

General Motors' Multiple Console Time Sharing System (MCTS) for the Control Data Corporation STAR-100 computer was based on Multics.