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77-450: Multics (" MULTiplexed Information and Computing Service ") is an influential early time-sharing operating system based on the concept of a single-level memory . Nathan Gregory writes that Multics "has influenced all modern operating systems since, from microcomputers to mainframes." Initial planning and development for Multics started in 1964, in Cambridge, Massachusetts . Originally it

154-493: A command window , but rather from a Graphical User Interface . Some programs may add their directory to the front of the PATH variable's content during installation, to speed up the search process and/or override OS commands. In the DOS era, it was customary to add a PATH {program directory}; %PATH% or SET PATH = {program directory}; %PATH% line to AUTOEXEC.BAT . When a command

231-466: 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

308-490: A large number of papers about Multics, and various components of it; a fairly complete list is available at the Multics Bibliography page and on a second, briefer 1994 Multics bibliography (text format). The most important and/or informative ones are listed below. Time-sharing Developed during the 1960s, its emergence as the prominent model of computing in the 1970s represented a major technological shift in

385-482: 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 stacks in the 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 ), after

462-487: 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

539-456: A much more versatile and flexible operating system, and it failed miserably". 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. The permanently resident kernel of Multics, a system derided in its day as being too large and complex, was 135 KB of code. The first MIT GE-645 had 512 kilowords of memory (2 MiB),

616-524: A multilevel refinement of the concept of master mode . 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 , and file names can be of almost arbitrary length and syntax. A given file or directory can have multiple names (typically

693-492: 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 STAR-100 computer was based on Multics. The protection architecture of Multics, restricting

770-655: A particular set of tasks and operated by control panels, the operator manually entering small programs via switches in order to load and run a series of programs. These programs might take hours to run. As computers grew in speed, run times dropped, and soon the time taken to start up the next program became a concern. Newer batch processing software and methodologies, including batch operating systems such as IBSYS (1960), decreased these "dead periods" by queuing up programs ready to run. Comparatively inexpensive card punch or paper tape writers were used by programmers to write their programs "offline". Programs were submitted to

847-458: A process first attempts to begin execution in them. Since different processes can use different 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 libraries (DLLs) ;

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924-449: 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 (in another process), a user's process simply performs 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 can then modify data maintained and used in

1001-599: A second deployment of CTSS was installed on an IBM 7094 that MIT has purchased using ARPA money. This was used to support Multics development at Project MAC . JOSS began time-sharing service in January 1964. Dartmouth Time-Sharing System (DTSS) began service in March 1964. Throughout the late 1960s and the 1970s, computer terminals were multiplexed onto large institutional mainframe computers ( centralized computing systems), which in many implementations sequentially polled

1078-417: 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. The scope of this disconnect was concretized by an anecdote conveyed by Paul Stachour, CNO/CSC: When American Telephone and Telegraph was changing its name to just AT&T in 1983,

1155-459: 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

1232-415: A transcribed 2007 interview with Peter Seibel 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

1309-409: 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 MiB of procedure code, which was fairly large by

1386-413: Is called by putting the active function name and the arguments to the active function in square brackets [ and ] . 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 echo [working_dir] is processed, the active function working_dir is run; it returns the full path of the working directory , which

1463-489: Is entered in a command shell or a system call is made by a program to execute a program, the system first searches the current working directory and then searches the path, examining each directory from left to right, looking for an executable filename that matches the command name given. Executable programs have filename extensions of EXE or COM , and batch scripts have extensions of BAT or CMD . Other executable filename extensions can be registered with

1540-465: Is one of the earliest multiprocessor systems. Multics is the first major operating system to be designed as a secure system from the outset. Despite this, early versions of Multics were compromised repeatedly. 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-oriented security ,

1617-433: 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 kilowords , just over 1  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 kiloword limit was rarely encountered in practice, because at

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1694-447: 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 uniplexed as opposed to the multiplexed of Multics, further underscoring

1771-479: Is sometimes included by users as well, allowing programs residing in the current working directory to be executed directly. System administrators as a rule do not include it in $ PATH in order to prevent the accidental execution of scripts residing in the current directory, such as may be placed there by a malicious tarbomb . In that case, executing such a program requires specifying an absolute ( /home/userjoe/bin/script.sh ) or relative path ( ./script.sh ) on

1848-432: Is substituted into the command, so that the 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. Some common active functions are: Peter H. Salus , author of a book covering Unix's early years, stated one position: "With Multics they tried to have

1925-715: Is time-sharing". For DEC, for a while the second largest computer company (after IBM), this was also true: Their PDP-10 and IBM's 360/67 were widely used by commercial timesharing services such as CompuServe, On-Line Systems, Inc. (OLS), Rapidata and Time Sharing Ltd. The advent of the personal computer marked the beginning of the decline of time-sharing. The economics were such that computer time went from being an expensive resource that had to be shared to being so cheap that computers could be left to sit idle for long periods in order to be available as needed. Although many time-sharing services simply closed, Rapidata held on, and became part of National Data Corporation . It

2002-404: The %PATH% variable is specified as a list of one or more directory names separated by semicolon ( ; ) characters. The Windows system directory (typically C:\WINDOWS\system32 ) is typically the first directory in the path, followed by many (but not all) of the directories for installed software packages. Many programs do not appear in the path as they are not designed to be executed from

2079-499: The PATH -string are not meant to be escaped, making it impossible to have directories with : in their name. The /bin , /usr/bin , and /usr/local/bin directories are typically included in most users' $ PATH setting (although this varies from implementation to implementation). The superuser also typically has /sbin and /usr/sbin entries for easily executing system administration commands. The current directory ( . )

2156-642: The Burroughs MCP system written in 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,

2233-561: The IBM 2741 ) with two different seven-bit codes. They would connect to the central computer by dial-up Bell 103A modem or acoustically coupled modems operating at 10–15 characters per second. Later terminals and modems supported 30–120 characters per second. The time-sharing system would provide a complete operating environment, including a variety of programming language processors, various software packages, file storage, bulk printing, and off-line storage. Users were charged rent for

2310-537: The UK . By 1968, there were 32 such service bureaus serving the US National Institutes of Health (NIH) alone. The Auerbach Guide to Timesharing (1973) lists 125 different timesharing services using equipment from Burroughs , CDC , DEC , HP , Honeywell , IBM , RCA , Univac , and XDS . In 1975, acting president of Prime Computer Ben F. Robelen told stockholders that "The biggest end-user market currently

2387-449: The telephone and electricity 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 lists on every file provide flexible information sharing, but complete privacy when needed. Multics has a number of standard mechanisms to allow engineers to analyze

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2464-411: The 1.0 release of the emulator, and adds a few new features, including command line recall and editing using the video system. The following is a list of programs and commands for common computing tasks that are supported by the Multics command-line interface . The Multics shell language supports "active functions", which are similar to commands, but which return a string value. An active function

2541-422: The 1970s, Ted Nelson 's original " Xanadu " hypertext repository was envisioned as such a service. Time-sharing was the first time that multiple processes , owned by different users, were running on a single machine, and these processes could interfere with one another. For example, one process might alter shared resources which another process relied on, such as a variable stored in memory. When only one user

2618-550: 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 phonemes , 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. A descendant of this generator

2695-575: The United States. In 2006, 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. The last known Multics installation running natively on Honeywell hardware was shut down on October 30, 2000, at the Canadian Department of National Defence in Halifax, Nova Scotia, Canada. In 2006 Bull HN released

2772-527: The University of Illinois in early 1961. Bitzer has long said that the PLATO project would have gotten the patent on time-sharing if only the University of Illinois had not lost the patent for two years. The first interactive , general-purpose time-sharing system usable for software development, Compatible Time-Sharing System , was initiated by John McCarthy at MIT writing a memo in 1959. Fernando J. Corbató led

2849-412: 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 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. The literature contains

2926-708: The already existing Discretionary access control that Multics already possessed. 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). The SCOMP and it's STOP operating system would eventually evolved via XTS-200 and XTS-300 into current XTS-400 offering of secure operating systems. 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

3003-430: The change in the meaning of the term time-sharing a source of confusion and not what he meant when he wrote his paper in 1959. There are also examples of systems which provide multiple user consoles but only for specific applications, they are not general-purpose systems. These include SAGE (1958), SABRE (1960) and PLATO II (1961), created by Donald Bitzer at a public demonstration at Robert Allerton Park near

3080-419: The command line. When a command name is specified by the user or an exec call is made from a program, the system searches through $ PATH , examining each directory from left to right in the list, looking for a filename that matches the command name. Once found, the program is executed as a child process of the command shell or program that issued the command. On DOS, OS/2, and Windows operating systems,

3157-705: The computer much the same way that the average household buys power and water from utility companies." Christopher Strachey , who became Oxford University's first professor of computation, filed a patent application in the United Kingdom for "time-sharing" in February 1959. He gave a paper "Time Sharing in Large Fast Computers" at the first UNESCO Information Processing Conference in Paris in June that year, where he passed

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3234-440: The computer's resources, such as when a large JOSS application caused paging for all users. The JOSS Newsletter often asked users to reduce storage usage. Time-sharing was nonetheless an efficient way to share a large computer. As of 1972 DTSS supported more than 100 simultaneous users. Although more than 1,000 of the 19,503 jobs the system completed on "a particularly busy day" required ten seconds or more of computer time, DTSS

3311-528: The concept on to J. C. R. Licklider . This paper was credited by the MIT Computation Center in 1963 as "the first paper on time-shared computers". The meaning of the term time-sharing has shifted from its original usage. From 1949 to 1960, time-sharing was used to refer to multiprogramming without multiple user sessions. Later, it came to mean sharing a computer interactively among multiple users. In 1984 Christopher Strachey wrote he considered

3388-632: The concept, but did not use the term, in the 1954 summer session at MIT . Bob Bemer used the term time-sharing in his 1957 article "How to consider a computer" in Automatic Control Magazine and it was reported the same year he used the term time-sharing in a presentation. In a paper published in December 1958, W. F. Bauer wrote that "The computers would handle a number of problems concurrently. Organizations would have input-output equipment installed on their own premises and would buy time on

3465-469: 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 on-line reconfiguration : central processing units , memory banks, disk drives, etc. can be added and removed while the system continues operating. At the MIT system, where most early software development

3542-405: The designers' rejections of Multics' complexity in favor of a more straightforward and workable approach for smaller computers. (Garfinkel and Abelson 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.) Ken Thompson, in

3619-409: The development of the system, a prototype of which had been produced and tested by November 1961. Philip M. Morse arranged for IBM to provide a series of their mainframe computers starting with the IBM 704 and then the IBM 709 product line IBM 7090 and IBM 7094 . IBM loaned those mainframes at no cost to MIT along with the staff to operate them and also provided hardware modifications mostly in

3696-438: The earliest days of personal computers, many were in fact used as particularly smart terminals for time-sharing systems. DTSS's creators wrote in 1968 that "any response time which averages more than 10 seconds destroys the illusion of having one's own computer". Conversely, timesharing users thought that their terminal was the computer, and unless they received a bill for using the service, rarely thought about how others shared

3773-752: 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

3850-490: The form of RPQs as prior customers had already commissioned the modifications. There were certain stipulations that governed MIT's use of the loaned IBM hardware. MIT could not charge for use of CTSS. MIT could only use the IBM computers for eight hours a day; another eight hours were available for other colleges and universities; IBM could use their computers for the remaining eight hours, although there were some exceptions. In 1963

3927-511: The history of computing. By allowing many users to interact concurrently with a single computer, time-sharing dramatically lowered the cost of providing computing capability, made it possible for individuals and organizations to use a computer without owning one, and promoted the interactive use of computers and the development of new interactive applications . The earliest computers were extremely expensive devices, and very slow in comparison to later models. Machines were typically dedicated to

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4004-400: The idea of a search path. The early Unix shell only looked for program names in /bin , but by Version 3 Unix the directory was too large and /usr/bin , and a search path, became part of the operating system. On POSIX and Unix-like operating systems, the $ PATH variable is specified as a list of one or more directory names separated by colon ( : ) characters. Directories in

4081-409: The operations team, which scheduled them to be run. Output (generally printed) was returned to the programmer. The complete process might take days, during which time the programmer might never see the computer. Stanford students made a short film humorously critiquing this situation. The alternative of allowing the user to operate the computer directly was generally far too expensive to consider. This

4158-471: 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. Finally by slashing prices, MIT managed to lure users back to Multics in 1978. In 1974 Honeywell entered into a development contract with the Air Force (with MIT as a sub-contractor) to develop

4235-410: 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. 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

4312-402: 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 . In POSIX terminology, it is as if every file were mmap () 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

4389-404: 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

4466-529: The rise of microcomputing in the early 1980s, time-sharing became less significant, because individual microprocessors were sufficiently inexpensive that a single person could have all the CPU time dedicated solely to their needs, even when idle. However, the Internet brought the general concept of time-sharing back into popularity. Expensive corporate server farms costing millions can host thousands of customers all sharing

4543-648: The same common resources. As with the early serial terminals, web sites operate primarily in bursts of activity followed by periods of idle time. This bursting nature permits the service to be used by many customers at once, usually with no perceptible communication delays, unless the servers start to get very busy. Genesis In the 1960s, several companies started providing time-sharing services as service bureaus . Early systems used Teletype Model 33 KSR or ASR or Teletype Model 35 KSR or ASR machines in ASCII environments, and IBM Selectric typewriter -based terminals (especially

4620-424: The shell — a separate process that you can replace with some other process". Dennis Ritchie wrote that the design of UNIX was influenced by CTSS . 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

4697-409: The source code for MR12.5, the final 1992 Multics release, to MIT. Most of the system is now available as free software with the exception of some optional pieces such as TCP/IP . In 2014, Multics was successfully run on current hardware using an emulator created by Multicians Harry Reed and Charles Anthony. The 1.0 release of the emulator is available as of 2017. Release 12.6f of Multics accompanies

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4774-424: The staffer was speaking, and the task was in fact completed. The design and features of Multics influenced the Unix operating system, which was originally written by two Multics programmers, Ken Thompson and Dennis Ritchie . Influence of Multics on Unix is evident in many areas, including the hierarchical file system , redirection , the shell , and the naming of some commands. But the internal design philosophy

4851-434: 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

4928-434: The system as well. Once a matching executable file is found, the system spawns a new process that runs it. The PATH variable makes it easy to run commonly used programs located in their own folders. If used unwisely, however, the value of the PATH variable can slow down the operating system by searching too many locations, or invalid locations. Invalid locations can also stop services from running altogether, especially

5005-724: The terminal, a charge for hours of connect time, a charge for seconds of CPU time, and a charge for kilobyte-months of disk storage. Common systems used for time-sharing included the SDS 940 , the PDP-10 , the IBM 360 , and the GE-600 series . Companies providing this service included GE 's GEISCO , the IBM subsidiary The Service Bureau Corporation , Tymshare (founded in 1966), National CSS (founded in 1967 and bought by Dun & Bradstreet in 1979), Dial Data (bought by Tymshare in 1968), AL/COM , Bolt, Beranek, and Newman (BBN) and Time Sharing Ltd. in

5082-469: The terminals to see whether any additional data was available or action was requested by the computer user. Later technology in interconnections were interrupt driven, and some of these used parallel data transfer technologies such as the IEEE 488 standard. Generally, computer terminals were utilized on college properties in much the same places as desktop computers or personal computers are found today. In

5159-416: The time, one megabyte of memory was prohibitively expensive. Another major new idea of Multics was 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

5236-474: Was a cooperative project led by 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 635 systems with an early timesharing system known as "Mark I" and intended to offer the 645 with Multics as a larger successor. Bell withdrew from

5313-402: Was able to handle the jobs because 78% of jobs needed one second or less of computer time. About 75% of 3,197 users used their terminal for 30 minutes or less, during which they used less than four seconds of computer time. A football simulation, among early mainframe games written for DTSS, used less than two seconds of computer time during the 15 minutes of real time for playing the game. With

5390-526: Was added to Multics during Project Guardian. 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 6180 machines, which included security improvements including hardware support for protection rings . Bell Labs pulled out of the project in 1969; some of

5467-429: Was because users might have long periods of entering code while the computer remained idle. This situation limited interactive development to those organizations that could afford to waste computing cycles: large universities for the most part. The concept is claimed to have been first described by Robert Dodds in a letter he wrote in 1949 although he did not use the term time-sharing . Later John Backus also described

5544-531: 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 . Multics implements 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 consists solely of segments that were mapped into its address space . To read or write to them,

5621-477: 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 supports multiple CPUs; it

5698-540: 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 DPS 6 running 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

5775-503: Was primarily driven by the time-sharing industry and its customers. Time-sharing in the form of shell accounts has been considered a risk. Significant early timesharing systems: PATH (variable) PATH is an environment variable on Unix-like operating systems , DOS , OS/2 , and Microsoft Windows , specifying a set of directories where executable programs are located. In general, each executing process or user session has its own PATH setting. Multics originated

5852-479: Was still of sufficient interest in 1982 to be the focus of "A User's Guide to Statistics Programs: The Rapidata Timesharing System". Even as revenue fell by 66% and National Data subsequently developed its own problems, attempts were made to keep this timesharing business going. Beginning in 1964, the Multics operating system was designed as a computing utility , modeled on the electrical or telephone utilities. In

5929-539: Was using the system, this would result in possibly wrong output - but with multiple users, this might mean that other users got to see information they were not meant to see. To prevent this from happening, an operating system needed to enforce a set of policies that determined which privileges each process had. For example, the operating system might deny access to a certain variable by a certain process. The first international conference on computer security in London in 1971

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