Multics

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Multics (Multiplexed Information and Computing Service) - an operating system that was designed by a team of employees of the Massachusetts Institute of Technology (MIT), General Electric (whose computer division was later sold to Honeywell) and Bell Laboratories (which abandoned the development of the project in 1969). GE645

By definition, Multics was meant to be a natural extension of the CTSS system. CTSS and other early operating systems with time division have gained such success that they immediately caused the need for a quick transition to larger and better studies of this kind. When the larger computers were made available, the constructors of the CTSS system decided to introduce the facility in the form of time division. Computer services were to be delivered like electricity. Large computer systems should be connected by telephone lines with terminals in offices and homes around the world. The operating system would work continuously in a time-sharing mode with a large file system of commonly used programs and data.

The basic GE 635 computer has been converted into a new computer system GE 645, the modification mainly consisting of adding hardware implementing paged memory segmentation. The virtual address consisted of an 18-bit segment number and a 16-bit word distance from the beginning of the segment. The segments were then subdivided into 1K pages. A second chance algorithm was used to replace the pages.

The segmented virtual address space has been linked to the file system - each segment was a file. The segments were addressed by means of file names. The file system itself had a multi-level tree structure, allowing users to create their own subdirectory structures.

As in CTSS, a multi-level feedback queue was used in the Multics system for scheduling the processor. The protection was accomplished by assigning access lists to each file and applying a set of protection rings when executing processes. The system, which was almost entirely written in PL / I, was about 300,000 lines of code. It was extended to a multiprocessor system, allowing the processors to be disconnected for maintenance purposes while the system continued to operate.

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