Multics ( Multiplexed Information and Computing Service) is an influential early time-sharing operating system, based around the concept of a single-level memory.Dennis M. Ritchie, "The Evolution of the Unix Time-sharing System", Communications of the ACM, Vol. 17, 1984, pp. 365-375. Virtually all modern operating systems were heavily influenced by Multics – often through Unix, which was created by some of the people who had worked on Multics – either directly (Linux, macOS) or indirectly (Windows NT).
Multics was conceived as a commercial product for General Electric, and became one for Honeywell, albeit not very successfully. Due to its many novel and valuable ideas, Multics had a significant impact on computer science despite its faults.
Multics had numerous features intended to ensure high availability so that it would support a computing utility similar to the telephone and electricity public utility. 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, be it computing power, main memory, or disk storage. Separate access control lists on every file provided flexible information sharing, but complete privacy when needed. Multics had a number of standard mechanisms to allow engineers to analyze the performance of the system, as well as a number of adaptive performance optimization mechanisms.
One disadvantage of this was that the size of segments was limited to 256 kilowords, just over 1 MiB. This was due to the particular hardware architecture of the machines on which Multics ran, having a 36-bit word size and index registers (used to address within segments) of half that size (18 bits). Extra code had to be used to work on files larger than this, called multisegment files. In the days when one megabyte of memory was prohibitively expensive, and before large databases and later huge bitmap graphics, this limit was rarely encountered.
Another major new idea of Multics was shared library, in which a running process could request that other segments be added to its address space, segments which could contain code that it could then execute. This allowed applications to automatically use the latest version of any external routine they called, since those routines were kept in other segments, which were dynamically linked only when a process first tried to begin execution in them. Since different processes could use different search rules, different users could end up using different versions of external routines automatically. Equally importantly, with the appropriate settings on the Multics security facilities, the code in the other segment could then gain access to data structures maintained in a different process.
Thus, to interact with an application running in part as a daemon (in another process), a user's process simply performed a normal procedure call instruction to a code segment to which it had dynamically linked (a code segment that implemented some operation associated with the daemon). The code in that segment could then modify data maintained and used in the daemon. When the action necessary to commence the request was completed, a simple procedure return instruction returned control of the user's process to the user's code.
The single-level store and dynamic linking are still not available to their full power in other widely used operating systems, despite the rapid and enormous advance in the computer field since the 1960s. They are becoming more widely accepted and available in more limited forms, for example, dynamic linking.
Multics also supported extremely aggressive hot-swapping: central processing units, memory banks, disk drives, etc. could be added and removed while the system continued operating. At the MIT system, where most early software development was done, it was common practice to split the multiprocessor system into two separate systems during off-hours by incrementally removing enough components to form a second working system, leaving the rest still running the original logged-in users. System software development testing could be done on the second system, then the components of the second system were added back to the main user system, without ever having shut it down. Multics supported multiple CPUs; it was one of the earliest multiprocessor systems.
Multics was the first major operating system to be designed as a secure system from the outset.Jerome H. Saltzer, "Protection and the Control of Information Sharing in Multics", in "Introduction to Multics", MAC TR-123, Project MAC, Cambridge, February 1974; pg. 2-41. Despite this, early versions of Multics were broken into repeatedly. This led to further work that made the system much more secure and prefigured modern security engineering techniques. Break-ins became very rare once the second-generation hardware base was adopted; it had hardware support for ring-oriented security, a multilevel refinement of the concept of kernel mode.
Multics was the first operating system to provide a hierarchical file system,R. C. Daley and P. G. Neumann, "A general-purpose file system for secondary storage", AFIPS '65 (Fall, part I) Proceedings of the November 30 – December 1, 1965 and file names could be of almost arbitrary length and syntax. A given file or directory could have multiple names (typically a long and short form), and symbolic links between directories were also supported. Multics was the first to use the now-standard concept of per-process stacks in the kernel, with a separate stack for each security ring. It was also the first to have a command processor implemented as ordinary user code – an idea later used in the Unix shell. It was also one of the first written in a high-level language (Multics PL/I), after the Burroughs MCP system written in ALGOL.R. A. Freiburghouse, "The Multics PL/1 Compiler", General Electric Company, Cambridge, Massachusetts, 1969.
Honeywell continued system development until 1985. About 80 multimillion-dollar sites were installed, at universities, industry, and government sites. The French university system had several installations in the early 1980s. After Honeywell stopped supporting Multics, users migrated to other systems like Unix.
In 1985, Multics was issued certification as a B2 level secure operating system using the Trusted Computer System Evaluation Criteria from the National Computer Security Center (NCSC) a division of the NSA, the first operating system evaluated to this level.
Multics was distributed from 1975 to 2000 by Groupe Bull in Europe, and by Bull HN Information Systems Inc. in the United States. In 2006, Groupe Bull Multics versions MR10.2, MR11.0, MR12.0, MR12.1, MR12.2, MR12.3, MR12.4 & MR12.5. Multics history MIT
In 2014 Multics was successfully run on current hardware using a simulator. The 1.0 release of the simulator is now available. Release 12.6f of Multics accompanies the 1.0 release of the emulator, and adds a few new features, including command line recall and editing using the video system.
The permanently resident kernel of this powerful multiprocessor mainframe computing utility, much derided in its day as being too large and complex, was only 135 KB of code. In comparison, a Linux system in 2007 might have occupied 18 MB.
The entire system, including the operating system and the complex PL/1 compiler, user commands, and subroutine libraries, consisted of about 1500 source modules. These averaged roughly 200 lines of source code each, and compiled to produce a total of roughly 4.5 MiB of procedure code, which was fairly large by the standards of the day.
Multics compilers generally optimised more for code density than CPU performance, for example using small sub-routines called operators for short standard code-sequences, making direct comparison of object code size with more modern systems less useful. High code density was a good optimisation choice for a multi-user system with expensive main memory, such as Multics.
The name Unix (originally Unics) is itself a pun on Multics. The U in Unix is rumored to stand for as opposed to the of Multics, further underscoring the designers' rejections of Multics' complexity in favor of a more straightforward and workable approach for smaller computers. (Garfinkel and AbelsonGarfinkel, Simson and Abelson, Harold. Architects of the Information Society: Thirty-Five Years of the Laboratory for Computer Science at MIT. MIT Press, 1999. 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 claimed to have denied this.)
Ken Thompson, in a transcribed 2007 interview with Peter SeibelPeter Seibel. Coders at Work: Reflections on the Craft of Programming. APress Publications, 2007. refers to Multics as "…overdesigned and overbuilt and over everything. It was close to unusable. They Massachusetts still claim it’s a monstrous success, but it just clearly wasn't." He admits, however, that "the things that I liked enough (about Multics) to actually take were the hierarchical file system and the shell—a separate process that you can replace with some other process."
The Stratus VOS operating system of Stratus Computer (now Stratus Technologies) was very strongly influenced by Multics, and both its external user interface and internal structure bear many close resemblances to the older project. The high-reliability, availability, and security features of Multics were extended in Stratus VOS to support a new line of fault tolerant computer systems supporting secure, reliable transaction processing. Stratus VOS is the most directly-related descendant of Multics still in active development and production usage today.
The protection architecture of Multics, restricting the ability of code at one level of the system to access resources at another, was adopted as the basis for the security features of ICL's ICL VME operating system.