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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.


Initial planning and development for Multics started in 1964. Originally it was a cooperative project led by MITmarker (with Fernando Corbató) along with General Electric and Bell Labsmarker. 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.

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 file (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.

One disadvantage of this was that the size of segments was limited to 256 kibiwords, giving just over 1 MiB. This was due to the particular hardware architecture of the machines Multics ran on, which had a 36 bit word size (not using the 8-bit byte). Extra code had to be used to work on files larger than this, called multisegment files. But 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 dynamic linking, 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 which it had dynamically linked to (a code segment which 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 (e.g., dynamic linking).

Multics also supported extremely aggressive on-line reconfiguration; 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 machine, then the components of the second system were added back onto 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 ground up. 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 master mode.

Multics was the first operating system to provide a hierarchical file system, and filenames 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 stack in the kernel, with a separate stack for each security ring. It was also one of the first written in a high level language (Multics PL/1), just after the Burroughs MCP system written in ALGOL.

Project history

Multics was developed initially for the GE-645 mainframe, a 36-bit system; later, it was supported on the Honeywell 6180 series machines. "With Multics they tried to have a much more versatile and flexible operating system, and it failed miserably," said Dr Peter H. Salus, author of the definitive history of Unix's early years.

Bell Labs pulled out of the project in 1969; some of the people who had worked on it there went on to create the Unix system. Superficial influence of Multics on Unix is evident in many areas, including the naming of commands (such as "ls" to "list segments" or files). But the design philosophy was quite different, focusing on keeping the system small and simple, and so correcting some perceived deficiencies of Multics because it facilitated high resource demands on the limited computers of the time. The name Unix (originally Unics) is itself a hack on Multics. The U in UNIX is rumored to stand for uniplexed as opposed to the multiplexed of Multics, further underscoring the designers' rejections of Multics' complexity in favor of a more straightforward and workable approach for smaller computers.

Honeywell bought GE's computer division, released a better hardware base, and continued system development until 1985. About 80 multimillion dollar sites were installed, at universities, industry, and government sites. The Frenchmarker university system had quite a few in the early 1980s. After Honeywell stopped supporting Multics, users migrated to other systems, including Unix. The last Multics machine was shut down on October 30, 2000 at the Canadian Department of National Defence.

Multics was distributed in 1975 to 2000 by Groupe Bull in Europe, and by Bull HN Information Systems Inc in the US. In 2006 Bull SAS open sourced Multics versions MR10.2, MR11.0, MR12.0, MR12.1, MR12.2, MR12.3, MR12.4 & MR12.5.

Retrospective observations

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 kiB of code. The first MITmarker GE-645 had 512 kibiwords of memory (2 MiB), a truly enormous amount at the time, and the kernel only used a moderate portion of Multics main memory.

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.

See also

Further reading

The literature contains a large number of papers about Multics, and various components of it; a fairly complete list is available here. The most important and/or informative ones are listed below.

Technical details

  • Jerome H. Saltzer, Introduction to Multics (MIT Project MAC, 1974) is a considerably longer introduction to the system, geared towards actual users.
  • Elliott I. Organick, The Multics System: An Examination of Its Structure (MIT Press, 1972) is the standard work on the system, although it documents an early version, and some features described therein never appeared in the actual system.
  • V. A. Vyssotsky, F. J. Corbató, R. M. Graham, Structure of the Multics Supervisor (AFIPS 1965) describes the basic internal structure of the Multics kernel.
  • Jerome H. Saltzer, Traffic Control in a Multiplexed Computer System (MIT Project MAC, June 1966) is the original description of the idea of switching kernel stacks; one of the classic papers of computer science.
  • R. C. Daley, P. G. Neumann, A General Purpose File System for Secondary Storage (AFIPS, 1965) describes the file system, including the access control and backup mechanisms.
  • R. J. Feiertag, E. I. Organick, The Multics Input/Output System. Describes the lower levels of the I/O implementation.
  • A. Bensoussan, C. T. Clingen, R. C. Daley, The Multics Virtual Memory: Concepts and Design, (ACM SOSP, 1969) describes the Multics memory system in some detail.
  • Paul Green, Multics Virtual Memory - Tutorial and Reflections is a good in-depth look at the Multics storage system.
  • Roger R. Schell, Dynamic Reconfiguration in a Modular Computer System (MIT Project MAC, 1971) describes the reconfiguration mechanisms.



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