Multics

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Multics (Multiplexed Information and Computing Service) was an extremely influential early time-sharing operating system. The project was started in 1964. The last known running Multics installation was shut down on October 30, 2000 at the Canadian Department of National Defense in Halifax, Nova Scotia, Canada.[5]

Contents

Overview

Initial planning and development for Multics started in 1964. Originally it was a cooperative project led by MIT (with Fernando Corbató) along with General Electric and Bell Labs. Bell Labs dropped out in 1969 and in 1970 GE's computer business including Multics was taken over by Honeywell.

Multics was conceived as a commercial product for GE and became one for Honeywell, but not a very successful one. Due to its many novel and valuable ideas Multics had a great impact in the computer field even though it was then much derided by its critics.[6]

It had numerous features intended to result in high availability so that it would produce a computing utility similar to the telephone and electricity services. Modular hardware structure and software architecture were used to achieve this. The system could grow in size by simply adding more of the appropriate resource—computing power, main memory, disk storage, etc. Separate access control lists on every file provided flexible information sharing and complete privacy when needed. It had a number of standard mechanisms to allow engineers to analyse the performance of the system as well as a number of adaptive performance optimisation mechanisms.

Novel ideas

Multics implemented a single level store for data access, discarding the clear distinction between files (called segments in Multics) and process memory. The memory of a process consisted solely of segments which were mapped into its address space. To read or write to them, the process simply used normal CPU instructions, and the operating system took care of making sure that all the modifications were saved to disk. In POSIX terminology, it was as if every file was mmap()ed; however, in Multics there was no concept of process memory, separate from the memory used to hold mapped-in files, as Unix has. All memory in the system was part of some segment, which appeared in the file system; this included the temporary scratch memory of the process, its kernel stack, etc.

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