View source for Multics

{{Short description|Time-sharing operating system}}
{{Use mdy dates|date=February 2015}}
{{Infobox OS
| name = Multics
| logo = Multics-logo.svg
| logo_size = 128px
| screenshot = Multics-Login.png
| caption = Login screen for the BAN.AI public Multics server
| developer = [[Massachusetts Institute of Technology|MIT]], [[General Electric|GE]], [[Honeywell International, Inc.|Honeywell]], [[Bell Labs]]
| prog_language = 
| programmed_in = [[PL/I]], [[Assembly language]]<ref name="Mpl1" />
| family = <!-- [[Unix]] ? -->
| working_state = Mature, current, historic, emulator available
| source_model = [[Open source]]
| released = {{Start date and age|1969}}
| latest_release_version = 12.8
| latest_release_date = {{Start date and age|2023|08|10}}
| latest_test_version = 
| latest_test_date = 
| marketing_target = 
| language = [[English language|English]]
| updatemodel = 
| package_manager = 
| supported_platforms = [[GE-645]], [[Honeywell 6000 series]] machines ([[Honeywell 6180]], Series-60/Level-68, DPS-8/M mainframes)
| kernel_type = [[Monolithic kernel]]
| ui = [[Command-line interface]]
| license = Originally [[proprietary software|proprietary]],<br />Free software Multics License since 2007<ref>{{cite web|url=http://opensource.org/licenses/Multics|title=Multics License (Multics) - Open Source Initiative|website=opensource.org|date=February 5, 2008 |access-date=April 11, 2018}}</ref><ref name="myths">{{cite web|url=http://www.multicians.org/myths.html#source|title=Myths about Multics|website=www.multicians.org|access-date=April 11, 2018}}</ref>
| preceded by = [[Compatible Time-Sharing System]]
| influenced = [[Unix]], [[GEORGE (operating system)|GEORGE 3]], [[ICL VME]], [[PRIMOS]], [[Domain/OS]], [[Stratus VOS]]
| website = {{URL|multics-wiki.swenson.org}}
}}
'''Multics''' ("'''Multiplexed Information and Computing Service'''") is an influential early [[time-sharing]] [[operating system]] based on the concept of a [[single-level store|single-level memory]].<ref name="Ritchie84">Dennis M. Ritchie, "The Evolution of the Unix Time-sharing System", Communications of the ACM, Vol. 17, 1984, pp. 365-375.</ref><ref>{{cite web|url=http://tenex.opost.com/hbook.html|title=Origins and Development of TOPS-20|author=Dan Murphy|year=1996|orig-year=1989}}</ref> It has been written that Multics "has influenced all modern operating systems since, from microcomputers to mainframes."<ref>{{cite book |last1=Gregory |first1=Nathan |title=The Tym Before |date=May 2018 |publisher=Lulu.com |isbn=9781387824755 |page=66 |url=https://books.google.com/books?id=kn9eDwAAQBAJ&q=page%2066 |access-date=Mar 29, 2020}}</ref>

Initial planning and development for Multics started in 1964, in [[Cambridge, Massachusetts]]. Originally it was a cooperative project led by [[Massachusetts Institute of Technology|MIT]] ([[Project MAC]] with [[Fernando Corbató]]) along with [[General Electric]] and [[Bell Labs]]. It was developed on the [[GE 645]] computer, which was specially designed for it; the first one was delivered to MIT in January 1967. GE offered their earlier [[GE 635]] systems with the [[Dartmouth Time-Sharing System]] which they called "Mark I" and intended to offer the 645 with Multics as a larger successor. Bell withdrew from the project in 1969 as it became clear it would not deliver a working system in the short term. Shortly thereafter, GE decided to exit the computer industry entirely and sold the division to [[Honeywell]] in 1970. Honeywell offered Multics commercially, but with limited success.

Multics has numerous features intended to ensure high availability so that it would support a [[computing utility]] similar to the [[telephone]] and [[electricity]] [[public utility|utilities]]. Modular hardware structure and software architecture are used to achieve this. 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 list]]s on every file provide flexible information sharing, but complete privacy when needed. Multics has 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.

Due to its many novel and valuable ideas, Multics has had a significant influence on [[computer science]] despite its faults.<ref name="Myths about Multics">{{cite web|url=http://www.multicians.org/myths.html|title=Myths about Multics|website=www.multicians.org|access-date=April 11, 2018}}</ref> 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. Unix was developed at Bell to allow their Multics team to continue their research using smaller machines, first a [[PDP-7]] and ultimately the [[PDP-11]].

==Novel ideas==
[[File:Multics-book.png|thumb|upright|Multics Commands reference manual]]Multics implements a [[single-level store]] for data access, discarding the clear distinction between [[computer file|file]]s (called ''segments'' in Multics) and ''[[computer process|process]] [[computer memory|memory]]''. The memory of a process consists solely of segments that were mapped into its [[address space]]. To read or write to them, the process simply uses normal [[central processing unit]] (CPU) instructions, and the operating system takes care of making sure that all the modifications were saved to [[disk storage|disk]]. In [[POSIX]] terminology, it is as if every file were <code>[[mmap]]()</code>ed; however, in Multics there is no concept of ''process memory'', separate from the memory used to hold mapped-in files, as [[Unix]] has. ''All'' memory in the system is part of ''some'' segment, which appears in the [[file system]]; this includes the temporary scratch memory of the process, its kernel stack, etc.

Segments are limited to 256 [[kiloword]]s, just over 1&nbsp;[[megabyte|MB]], because Multics hardware had 18-bit word addresses for the content of a segment. Larger files are "multisegment files" and are handled differently. The 256&nbsp;kiloword limit was rarely encountered in practice, because at the time, one megabyte of memory was prohibitively expensive.

Another major new idea of Multics was [[shared library|dynamic linking]], in which a running process can make external routines available by adding the segments containing them to its address space. This allows applications to always use the latest version of any external routine, since those routines are kept in other segments, which are dynamically linked only when a process first attempts to begin execution in them. Since different processes can use different [[PATH (variable)|search rules]], different users can end up using different versions of external routines. 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. Dynamic linking in Multics does not require special [[Dynamic-link library|dynamic-link libraries (DLLs)]]; a program can dynamically link to any executable segment to which it has access rights.

Thus, to interact with an application running in part as a [[daemon (computing)|daemon]] (in another process), a user's process simply performs a normal [[procedure call|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 can then modify data maintained and used in the daemon. When the action necessary to commence the request is completed, a simple procedure return instruction returns control of the user's process to the user's code.

Multics also supports extremely aggressive [[hot-swapping|on-line reconfiguration]]: [[central processing unit]]s, memory banks, disk drives, etc. can be added and removed while the system continues 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 for 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 is one of the earliest multiprocessor systems.

Multics is the first major operating system to be designed as a [[computer security|secure system]] from the outset.<ref name="JHSProt">{{cite report |chapter-url=https://web.mit.edu/Saltzer/www/publications/TRs+TMs/Multics/TR-123.pdf |title=Introduction to Multics", MAC TR-123 |last=Saltzer |first=Jerome H. |date=February 1974 |chapter=Protection and the Control of Information Sharing in Multics |pages=2–41 |publisher=Massachusetts Institute of Technology and Honeywell Information Systems Inc. |location=Cambridge, MA |access-date=March 16, 2022 |quote=}}</ref> Despite this, early versions of Multics were compromised repeatedly.<ref>{{cite web|url=http://www.multicians.org/security.html|title=How the Air Force cracked Multics Security|author=Tom Van Vleck|year=2002|author-link=Tom Van Vleck}}</ref> This led to further work that made the system more secure, and prefigured modern [[security engineering]] techniques. Break-ins became very rare once the second-generation hardware base was adopted; it has hardware support for [[ring (computer security)|ring-oriented security]], a multilevel refinement of the concept of [[kernel mode|master mode]].{{Citation needed|date=October 2007}} A US Air Force tiger team project tested Multics security in 1973 under the codeword ZARF. On 28 May 1997, the American National Security Agency declassified this use of the codeword ZARF.

Multics is the first operating system to provide a [[hierarchical file system]],<ref>{{cite web|url=http://www.multicians.org/mgf.html#filesystem|title=Multics Glossary -F-|website=www.multicians.org|access-date=April 11, 2018}}</ref><ref>R. C. Daley and P. G. Neumann, [http://dl.acm.org/citation.cfm?id=1463915 "A general-purpose file system for secondary storage"], AFIPS '65 (Fall, part I) Proceedings of the November 30 – December 1, 1965</ref> and file names can be of almost arbitrary length and syntax. A given file or directory can have multiple names (typically a long and short form), and symbolic links between directories are also supported. Multics is the first to use the now-standard concept of per-[[process (computing)|process]] [[Stack (data structure)|stack]]s{{disputed inline|Kernel stacks|reason=What about MCP|date=November 2021}} in the [[kernel (operating system)|kernel]], with a separate stack for each security ring. It is also the first to have a [[command processor]] implemented as ordinary user code – an idea later used in the [[Unix shell]]. It is also one of the first written in a high-level language (Multics [[PL/I]]),<ref name="Mpl1">R. A. Freiburghouse, [http://www.multicians.org/pl1-raf.html "The Multics PL/1 Compiler"], General Electric Company, Cambridge, Massachusetts, 1969.</ref><ref>[https://lwn.net/Articles/881431/ Brian Kernighan on the origins of Unix] LWN.net. 2022.</ref> after the [[Burroughs MCP]] system written in [[Executive Systems Problem Oriented Language|ESPOL]], an expanded version of [[ALGOL]].

The deployment of Multics into secure computing environments also spurred the development of innovative supporting applications. 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. To avoid guessable passwords, the AFDSC decided to assign passwords but concluded the manual assignment required too much administrative overhead. Thus, a random word generator was researched and then developed in PL/I. Instead of being based on [[phoneme]]s or individual letters or clarities, the system employed phonemic segments (second order approximations of English) and other rules to enhance pronounceability and randomness, which was statistically modeled against other approaches.<ref>{{cite web |title=A Random Word Generator for Pronounceable Passwords |url= https://apps.dtic.mil/sti/pdfs/ADA017676.pdf |archive-url= https://web.archive.org/web/20220408145811/https://apps.dtic.mil/sti/pdfs/ADA017676.pdf |url-status= live |archive-date= April 8, 2022 |date=November 1975 |publisher=Electronic Systems Division, Air Force Systems Command, USAF |location=Bedford, MA |id= ESD-TR-75-97 |access-date=8 March 2021}}</ref> A descendant of this generator was added to Multics during Project Guardian.<ref>{{cite web |title= Password Generator |url=https://multicians.org/thvv/gpw-js.html |last= Van Vleck |first=Tom |access-date= 8 March 2021}}</ref>

==Project history==
In 1964, Multics was developed initially for the [[GE-645]] mainframe, a [[36-bit]] system. GE's computer business, including Multics, was taken over by Honeywell in 1970; around 1973, Multics was supported on the [[Honeywell 6000 series|Honeywell 6180]] machines, which included security improvements including hardware support for [[protection ring]]s.

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. Multics development continued at MIT and General Electric. At MIT in 1975, use of Multics was declining and did not recover by 1976 to prior levels.<ref>{{cite journal |author=<!--Staff writer(s); no by-line.--> |date=December 1975 |title=Report of the President |url=https://libraries.mit.edu/archives/mithistory/presidents-reports/1975.pdf |journal=Massachusetts Institute of Technology Bulletin |volume=111 |issue=4 |pages=355 |access-date=April 12, 2022 |quote=Use of the Multics service, operated on the Honeywell 6180 system, declined...a decrease of 15 percent. |archive-date=April 13, 2022 |archive-url=https://web.archive.org/web/20220413015059/https://libraries.mit.edu/archives/mithistory/presidents-reports/1975.pdf |url-status=dead }}</ref><ref>{{cite journal |author=<!--Staff writer(s); no by-line.--> |date=November 1976 |title=Report of the President |url=https://libraries.mit.edu/archives/mithistory/presidents-reports/1975.pdf |journal=Massachusetts Institute of Technology Bulletin |volume=112 |issue=4 |pages=392 |access-date=April 12, 2022 |quote=... the demise of several Multics projects which failed to obtain funding. |archive-date=April 13, 2022 |archive-url=https://web.archive.org/web/20220413015059/https://libraries.mit.edu/archives/mithistory/presidents-reports/1975.pdf |url-status=dead }}</ref> Finally by slashing prices, MIT managed to lure users back to Multics in 1978.<ref>{{cite web |author=<!--Staff writer(s); no by-line.--> |date=1978 |title=Report of the President and the Chancellor |url=https://libraries.mit.edu/archives/mithistory/presidents-reports/1978.pdf |publisher=Massachusetts Institute of Technology |page=450 |access-date=April 12, 2022 |quote=This new pricing proved so attractive to many people, however, that Multics users and usage increased considerably. |archive-date=April 13, 2022 |archive-url=https://web.archive.org/web/20220413021106/https://libraries.mit.edu/archives/mithistory/presidents-reports/1978.pdf |url-status=dead }}</ref>

In 1974 Honeywell entered into a development contract with the Air Force (with MIT as a sub-contractor) to develop a [[security kernel]] for Multics. This would involve reducing the size of the Multics hardcore by moving specific components of the supervisor out of Ring 0. One of the initial steps after carrying out a security evaluation was the implementation of a [[multilevel security]] framework within Multics called AIM (Access Isolation Mechanism). This provided [[mandatory access control]] which could be enabled to supplement the already existing [[discretionary access control]] that Multics already possessed.<ref name="MDS">{{cite web |title=Multics Data Security |url=https://multicians.org/multics-data-security.html |website=multicians.org}}</ref> The resulting Project Guardian ran until termination in 1976; whilst most of its changes were not added to Multics, some parts of the project such as the proposed Secure Front End Processor was productized by Honeywell as SCOMP (Secure Communications Processor).<ref name="scomp">{{cite journal |last1=Fraim |first1=Lester J. |title=Scomp: A Solution to the Multilevel Security Problem |journal=Computer |date=July 1983 |volume=16 |issue=7 |pages=26–34 |doi=10.1109/MC.1983.1654440 |bibcode=1983Compr..16g..26F }}</ref> The SCOMP and its STOP operating system eventually evolved via XTS-200 and XTS-300 into current [[XTS-400]] offering of secure operating systems.<ref>{{cite web |title=STOP OS™ Version 7.3.1 Security Target |url=https://www.commoncriteriaportal.org/files/epfiles/BAE%20STOP%207%20Security%20Target%201.08.pdf |publisher=Common Criteria}}</ref>

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, such as 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]]; it was 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|Bull SAS]] released the source code of Multics versions MR10.2, MR11.0, MR12.0, MR12.1, MR12.2, MR12.3, MR12.4 & MR12.5 under a [[free software]] license.<ref>{{Cite web |title=Multics |url=https://stuff.mit.edu/afs/athena/reference/multics-history/ |access-date=2022-07-25 |website=stuff.mit.edu}}</ref>

The last known Multics installation running [[native (computing)|natively]] on Honeywell hardware was shut down on October 30, 2000, at the Canadian [[Department of National Defence (Canada)|Department of National Defence]] in Halifax, Nova Scotia, Canada.<ref name="urlMultics Dates">{{cite web
| url = http://www.multicians.org/chrono.html
| title = Multics History Dates
| quote = Shutdown of DND-H (17:08Z 10/30/00)
| access-date = 2015-09-13
}}</ref>

===Current status===
In 2006 Bull HN released the source code for MR12.5, the final 1992 Multics release, to MIT.<ref>{{cite web|last1=Van Vleck|first1=Tom|url=http://web.mit.edu/multics-history/#community|title=Open Source for Multics|website=Multicians.org|access-date=April 11, 2016}}</ref> Most of the system is now available as [[free software]] with the exception of some optional pieces such as [[TCP/IP]].<ref>{{cite web|first=Charles|last=Anthony|title=(email) Re: [dps8m-developers] Multiprocessor and/or networked Multics|url=https://sourceforge.net/p/dps8m/mailman/message/34310157/|website=Sourceforge.net|access-date=April 11, 2016}}</ref>

In 2014, Multics was successfully run on current hardware using an emulator created by Multicians Harry Reed and Charles Anthony.<ref>{{cite web|title=RingZero - Multics reborn|url=http://ringzero.wikidot.com/start|website=WikidotCom|access-date=April 11, 2015}}</ref> The 1.0 release of the emulator is available {{As of|2017|lc=y}}.<ref>{{cite web|title=Multics Simulator|url=http://multicians.org/simulator.html|access-date=July 9, 2017}}</ref> 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.<ref>{{cite web|title=Installing Multics|url=http://multics-wiki.swenson.org/index.php/Main_Page#Installing_Multics|access-date=May 19, 2020}}</ref>

==Commands==
The following is a list of programs and [[command (computing)|commands]]<ref>{{cite book
| last       = Honeywell Bull, Inc.
| title       = Multics Commands and Active Functions (AG92-06)
| url       = http://www.bitsavers.org/pdf/honeywell/large_systems/multics/AG92-06B_multicsCmds_Nov87.pdf
| access-date       = March 30, 2023
| date       = Feb 1985
}}</ref> for common computing tasks that are supported by the Multics [[command-line interface]].<ref>{{cite web| url = https://www.multicians.org/unix.html| title = Unix and Multics}}</ref><ref name="multics-commands">{{cite web| url = https://www.multicians.org/multics-commands.html| title = Multics Commands}}</ref>

=== File and directory access commands ===
* {{mono|[[cd (command)|change_wdir]]}} ({{mono|cwd}}) - change the [[working directory]]
* {{mono|[[mkdir|create_dir]]}} ({{mono|cd}}) - create a directory
* {{mono|[[copy (command)|copy]]}} ({{mono|cp}}) - copy files
* {{mono|[[ls|list]]}} ({{mono|ls}}) - list files and directories
* {{mono|[[cat (Unix)|print]]}} ({{mono|pr}}) - print the contents of a text file
* {{mono|[[pwd|print_wdir]]}} ({{mono|pwd}}) - print the [[working directory]]
* {{mono|[[mv (Unix)|move]]}} ({{mono|mv}}) - move a file or directory to a different directory
* {{mono|rename}} ({{mono|rn}}) - rename a file without moving it

=== Text editors ===
* {{mono|edm}} - a simple [[line editor]]
* {{mono|emacs}} - [[Multics Emacs]]
* {{mono|[[TECO (text editor)|teco]]}}

=== Document formatters ===
* {{mono|[[TYPSET and RUNOFF#Multics|runoff]]}} ({{mono|rf}})

=== Compilers, assemblers, and interpreters ===
{{Div col|colwidth=10em}}
* {{mono|alm}}
* {{mono|[[APL (programming language)|apl]]}}
* {{mono|[[BASIC|basic]]}}
* {{mono|[[C (programming language)|c]]}}<ref>{{cite book |title=Multics C User's Guide |date=November 1987 |publisher=Honeywell Bull |url=https://bitsavers.org/pdf/honeywell/large_systems/multics/HH07-01_C_UsersGuide_Nov87.pdf |access-date=March 26, 2024}}</ref>
* {{mono|[[COBOL|cobol]]}}
* {{mono|[[fortran]]}} ({{mono|ft}})
* {{mono|[[Pascal (programming language)|pascal]]}}
* {{mono|[[PL/I|pl1]]}}
{{Div col end}}

=== Email ===
{{Div col|colwidth=10em}}
* {{mono|mail}} ({{mono|ml}})
* {{mono|read_mail}} ({{mono|rdm}})
* {{mono|send_mail}} ({{mono|sdm}})
{{Div col end}}

=== On-line documentation ===
{{Div col|colwidth=10em}}
* {{mono|[[help (command)#Multics|help]]}}
{{Div col end}}

=== Scripting ===
* {{mono|[[echo (command)|echo]]}}
* {{mono|[[if (command)|if]]}}

=== Data processing ===
* {{mono|gcos}} ({{mono|gc}}) - simulator to run [[General Comprehensive Operating System|GCOS]] programs
* {{mono|sort}}, {{mono|merge}} - sorting and merging of text and binary files

=== Login sessions ===
* {{mono|[[logout (command)|logout]]}}
* {{mono|[[who (Unix)|who]]}}

=== Active functions ===
The Multics [[shell (computing)|shell]] language supports "active functions", which are similar to commands, but which return a string value.  An active function is called by putting the active function name and the arguments to the active function in square brackets {{mono|[}} and {{mono|]}}. The string returned by the active function is substituted into the command in place of the call to the active function.  For example, when the command {{mono|echo [working_dir]}} is processed, the active function {{mono|working_dir}} is run; it returns the full path of the [[working directory]], which is substituted into the command, so that the {{mono|echo}} command prints the working directory.

Some programs can act either as commands or as active functions; when run as a command, its result is printed, and when run as an active function, its result is returned as a string.<ref name="multics-commands" />

Some common active functions are:

* {{mono|[[Floor and ceiling functions|ceil]]}} - returns the smallest integer greater than or equal to the argument
* {{mono|[[Floor and ceiling functions|floor]]}} - returns the largest integer less than or equal to the argument
* {{mono|home_dir}} ({{mono|hd}}) - returns the [[home directory]]
* {{mono|[[ltrim]]}}
* {{mono|[[rtrim]]}}
* {{mono|[[Truncation|trunc]]}} - returns the integer part of the argument
* {{mono|working_dir}} ({{mono|wd}}) - returns the [[working directory]]

==Retrospective observations==
[[Peter H. Salus]], author of a book covering Unix's early years,<ref name=Salus>{{cite book|last=Salus|first=Peter H.|title=A quarter century of UNIX|year=1994|publisher=Addison-Wesley Pub. Co.|location=Reading, Mass.|isbn=978-0-201-54777-1|edition=Reprinted with corrections Jan. 1995.}}</ref> stated one position: "With Multics they tried to have a much more versatile and flexible operating system, and it failed miserably".<ref name="bbc-40yr-8205976">{{cite news|title=40 years of Unix|url=http://news.bbc.co.uk/2/hi/technology/8205976.stm | work=BBC News | date=August 20, 2009 | first = Mark | last = Ward | access-date = April 27, 2010}} Quoting Peter Salus.<span class="plainlinks"></span></ref> This position, however, is said to have been discredited in the computing community because many of Multics' technical innovations are used in modern commercial computing systems.<ref name="Myths about Multics"/>{{Better source needed|reason=Citation is a Multics enthusiast site, not representative of the entire computing community|date=June 2022}}

The permanently resident kernel of Multics, a system derided in its day as being too large and complex, was 135&nbsp;KB of code.{{citation needed |reason=KB is kilobytes yet the GE 645 was not a byte-oriented machine|date=March 2022}} The first [[MIT]] [[GE 645|GE-645]] had 512 kilowords of memory (2&nbsp;MiB), a truly enormous amount at the time, and the kernel used a moderate portion of Multics main memory.

The entire system, including the operating system and the complex PL/I [[compiler]], user commands, and subroutine libraries, consists of about 1500 source modules. These average roughly 200 lines of source code each, and compile to a total of roughly 4.5&nbsp;MiB of procedure code, which was fairly large by the standards of the day.

Multics compilers generally optimise more for [[code density]] than CPU performance, for example using small sub-routines called ''operators'' for short standard code sequences, which makes comparison of object code size with modern systems less useful. High code density is a good optimisation choice for Multics as a [[multi-user]] system with expensive main memory.

During its commercial product history, it was often commented internally that the Honeywell Information Systems (HIS) (later Honeywell-Bull) sales and marketing staff were more familiar with and comfortable making the business case for Honeywell's other computer line, the [[Honeywell Level 6|DPS 6]] running [[General Comprehensive Operating System|GCOS]]. 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.{{citation needed|date=March 2021}} The scope of this disconnect was concretized by an anecdote conveyed by Paul Stachour, CNO/CSC:
<blockquote>
When American Telephone and Telegraph was changing its name to just AT&T in 1983, a staffer from Honeywell’s legal department showed up and asked a Multician if he could arrange to have the name changed in all of their computerized documents. When asked when the process could be completed, the Multician replied, "It's done." The staffer repeated that he needed ''hundreds perhaps thousands'' of documents updated. The Multician explained that he had executed a global search and replace as the staffer was speaking, and the task was in fact completed.{{citation needed|date=March 2021}}</blockquote>

==Influence on other projects==

===Unix===
The design and features of Multics influenced the [[Unix]] operating system, which was originally written by two Multics programmers, [[Ken Thompson]] and [[Dennis Ritchie]].<ref name="Seibel" /> Influence of Multics on Unix is evident in many areas, including the [[hierarchical file system]], [[redirection (computing)|redirection]], the [[shell (computing)|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.

The name ''Unix'' (originally ''Unics'') is itself a [[pun]] on ''Multics''. The ''U'' in Unix is rumored to stand for ''[[wikt:uniplex|uniplex]]ed'' as opposed to the ''[[wikt:multiplex|multiplex]]ed'' of Multics, further underscoring the designers' rejections of Multics' complexity in favor of a more straightforward and workable approach for smaller computers. (Garfinkel and Abelson<ref>Garfinkel, Simson and Abelson, Harold. Architects of the Information Society: Thirty-Five Years of the Laboratory for Computer Science at MIT. MIT Press, 1999. {{ISBN|978-0262071963}}</ref> 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.<ref>{{cite newsgroup|author=Karn, Phil|title=Origins of unix|date=1981-10-28|newsgroup=fa.unix-wizards|message-id=4743@Aucbvax.UUCP|url=http://article.olduse.net/4743@Aucbvax.UUCP|access-date=2014-04-11}}</ref>)

Ken Thompson, in a transcribed 2007 interview with Peter Seibel<ref name="Seibel">Peter Seibel. [[Coders at Work]]: Reflections on the Craft of Programming. APress Publications, 2007. {{ISBN|978-1-4302-1948-4}}</ref> refers to Multics as "overdesigned and overbuilt and over everything. It was close to unusable. They [Massachusetts Institute of Technology] still claim it's a monstrous success, but it just clearly wasn't". On the influence of Multics on Unix, Thompson stated 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".

Dennis Ritchie wrote that the design of UNIX was influenced by [[Compatible Time-Sharing System|CTSS]].<ref>{{cite conference |last=Ritchie |first=Dennis M. |author-link=Dennis Ritchie |year=1977 |title=The Unix Time-sharing System: A retrospective |conference=Tenth Hawaii International Conference on the System Sciences |url=https://www.nokia.com/bell-labs/about/dennis-m-ritchie/retro.pdf |quote=a good case can be made that UNIX is in essence a modern implementation of MIT’s CTSS system}}</ref>

===Other operating systems===
The [[Prime Computer]] operating system, [[PRIMOS]], was referred to as "Multics in a shoebox" by [[William Poduska]], a founder of the company. Poduska later moved on to found [[Apollo Computer]], whose AEGIS and later [[Domain/OS]] operating systems, sometimes called "Multics in a matchbox", extends the Multics design to a networked graphics workstation environment.

The [[Stratus VOS]] operating system of Stratus Computer (now [[Stratus Technologies]]) is 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 are 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.

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

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 [[International Computers Limited|ICL]]'s [[ICL VME|VME]] operating system.

The [[Edinburgh Multiple Access System]] (EMAS) draws particularly on the one-level store concept used by Multics, providing access to files only by mapping them into memory. All memory space is associated with a segment.
<!--See the "External Links" section of this article for more information on the influence of Multics on other HW & SW systems-->

==See also==
{{Portal|Free and open-source software}}
* [[Time-sharing system evolution]]
* [[Peter J. Denning]]
* [[Jack B. Dennis]]
* [[Robert Fano]] – director of Project MAC at MIT (1963–1968)
* [[Robert M. Graham (computer scientist)]]
* [[J. C. R. Licklider]] – director of Project MAC at MIT (1968–1971)
* [[Peter G. Neumann]]
* [[Elliott Organick]]
* [[Louis Pouzin]] – introduced the term ''shell'' for the command language used in Multics
* [[Jerome H. Saltzer]]
* [[Roger R. Schell]]
* [[Glenda Schroeder]] – implemented the first [[command line]] [[user interface]] [[shell (computing)|shell]] and proposed the first [[email]] system with Pouzin and Crisman
* [[Victor A. Vyssotsky]]

==References==
{{Reflist|30em}}

==Further reading==
The literature contains a large number of papers about Multics, and various components of it; a fairly complete list is available at the [http://www.multicians.org/biblio.html Multics Bibliography] page and on a second, briefer [http://web.mit.edu/srz/www/multics-bibliography.html 1994 Multics bibliography] (text format). The most important and/or informative ones are listed below.
* [[Fernando J. Corbató|F. J. Corbató]], [[Michael Schroeder|V. A. Vyssotsky]], [http://www.multicians.org/fjcc1.html ''Introduction and Overview of the Multics System''] ([[AFIPS]] 1965) is a good introduction to the system.
* F. J. Corbató, C. T. Clingen, [[J. H. Saltzer]], [http://www.multicians.org/f7y.html ''Multics – The First Seven Years''] (AFIPS, 1972) is an excellent review, written after a considerable period of use and improvement over the initial efforts.
* J. J. Donovan, [[Stuart Madnick|S. Madnick]], [http://catalog.loc.gov/cgi-bin/Pwebrecon.cgi?v1=6&ti=1,6&Search_Arg=Donovan%2C%20John%20J.&Search_Code=NAME%40&CNT=100&PID=fahdDtIh7Y1ODWe5X_fO2_UUafeDT&SEQ=20090914123346&SID=1 Operating Systems], is a fundamental read on operating systems.
* J. J. Donovan, [http://catalog.loc.gov/cgi-bin/Pwebrecon.cgi?v1=5&ti=1,5&Search_Arg=Donovan%2C%20John%20J.&Search_Code=NAME%40&CNT=100&PID=fahdDtIh7Y1ODWe5X_fO2_UUafeDT&SEQ=20090914123346&SID=1 Systems Programming], is a good introduction into systems programming and operating systems.

===Technical details===
* [[Jerome H. Saltzer]], ''[https://web.archive.org/web/20060918161127/http://www.lcs.mit.edu/publications/specpub.php?id=691 Introduction to Multics]'' (MIT Project MAC, 1974) is a considerably longer introduction to the system, geared towards actual users.
* [[Elliott Organick|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.
* [[Victor A. Vyssotsky|V. A. Vyssotsky]], [[Fernando J. Corbató|F. J. Corbató]], R. M. Graham, ''[http://www.multicians.org/fjcc3.html Structure of the Multics Supervisor]'' (AFIPS 1965) describes the basic internal structure of the Multics kernel.
* [[Jerome H. Saltzer]], ''[https://web.archive.org/web/20011127053649/http://ncstrl.mit.edu/Dienst/UI/2.0/Describe/ncstrl.mit_lcs/LCS-TR-30?abstract=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, [[Peter G. Neumann|P. G. Neumann]], ''[http://www.multicians.org/fjcc4.html 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, ''[http://www.multicians.org/rjf.html The Multics Input/Output System]''. Describes the lower levels of the I/O implementation.
* A. Bensoussan, C. T. Clingen, R. C. Daley, ''[http://www.multicians.org/multics-vm.html The Multics Virtual Memory: Concepts and Design]'', ([[Association for Computing Machinery|ACM]] [[SOSP]], 1969) describes the Multics memory system in some detail.
* [[Paul Green (engineer)|Paul Green]], ''[https://web.archive.org/web/20010305133541/http://ftp.stratus.com/pub/vos/multics/pg/mvm.html 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.

===Security===
* Paul A. Karger, Roger R. Schell, ''[http://csrc.nist.gov/publications/history/karg74.pdf Multics Security Evaluation: Vulnerability Analysis]'' (Air Force Electronic Systems Division, 1974) describes the classic attacks on Multics security by a "[[tiger team]]".
* [[Jerome H. Saltzer]], Michael D. Schroeder, ''[http://cap-lore.com/CapTheory/ProtInf/ The Protection of Information in Computer Systems]'' (Proceedings of the [[IEEE]], September 1975) describes the fundamentals behind the first round of security upgrades; another classic paper.
* [[Michael Schroeder|M. D. Schroeder]], D. D. Clark, [[J. H. Saltzer]], D. H. Wells. ''[https://web.archive.org/web/20011124185621/http://ncstrl.mit.edu/Dienst/UI/2.0/Describe/ncstrl.mit_lcs/LCS-TR-196?abstract=saltzer Final Report of the Multics Kernel Design Project]'' (MIT LCS, 1978) describes the security upgrades added to produce an even more improved version.
* Paul A. Karger, Roger R. Schell, ''[http://www.acsac.org/2002/papers/classic-multics.pdf Thirty Years Later: Lessons from the Multics Security Evaluation]'' (IBM, 2002) is an interesting retrospective which compares actual deployed security in today's hostile environment with what was demonstrated to be possible decades ago. It concludes that Multics offered considerably stronger security than most systems commercially available in 2002.

==External links==
{{Commons category|Multics}}
* [https://www.multicians.org/ multicians.org] is a comprehensive site with a lot of material
** [http://www.multicians.org/papers.html Multics papers online]
** [http://www.multicians.org/mga.html Multics glossary]
** [http://www.multicians.org/myths.html Myths] discusses numerous myths about Multics in some detail, including the myths that it failed, that it was big and slow, as well as a few understandable misapprehensions
** [http://www.multicians.org/security.html Multics security]
** [http://www.multicians.org/unix.html Unix and Multics]
** [http://www.multicians.org/general.html Multics general info and FAQ] Includes extensive overview of other software systems influenced by Multics
* [https://dps8m.gitlab.io/dps8m/ Open source emulator for the GE Large Systems / Honeywell / Bull 600/6000‑series mainframe computers]
* [http://purl.umn.edu/41295 Honeywell, Inc., MULTICS records, 1965–1982]. [[Charles Babbage Institute]], University of Minnesota. Multics development records include the second MULTICS System Programmers Manual; MULTICS Technical Bulletins that describe procedures, applications, and problems, especially concerning security; and returned "Request for Comments Forms" that include technical papers and thesis proposals.
* [https://swenson.org/multics_wiki/ Current Multics development home page]
* [http://web.mit.edu/multics-history/ Official historical source code archive at MIT]
* {{webarchive |url=http://webarchive.loc.gov/all/20011130013712/http://www.mit.edu:8001/afs/net/user/srz/www/multics.html |title=Link page to various Multics information |date=2001-11-30}}
* {{webarchive |url=https://web.archive.org/web/20020219203621/http://ftp.stratus.com:80/pub/vos/multics/multics.html |title=Multics repository at Stratus Computer |date=2002-02-19}}
* [http://www.vaxman.de/historic_computers/multics/multics.html Multics at Universitaet Mainz]
* [http://bitsavers.org/pdf/honeywell/large_systems/multics/ Various scanned Multics manuals] 
* {{webarchive |url=https://web.archive.org/web/20210114010400/http://www.cbi.umn.edu/iterations/haigh.html |title=Multicians.org and the History of Operating Systems |date=2021-01-14}}, a critical review of Multicians.org, plus a capsule history of Multics.

{{Multics}}
{{Honeywell|state=autocollapse}}
{{Time-sharing operating systems}}
{{Authority control}}

[[Category:Multics| ]]
[[Category:1969 software]]
[[Category:AT&T computers]]
[[Category:Bell Labs]]
[[Category:Discontinued operating systems]]
[[Category:Free software operating systems]] <!-- cite: http://stuff.mit.edu/afs/athena/reference/multics-history/source/Multics/ -->
[[Category:General Electric mainframe computers]]
[[Category:Honeywell mainframe computers]]
[[Category:Massachusetts Institute of Technology software]]
[[Category:Time-sharing operating systems]]
[[Category:Mainframe computer software]]