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Initial planning and development for Multics started in 1964. Originally it was a cooperative project led by MIT (with Fernando Corbató), along with General Electric and Bell Labs. 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 it did achieve that for Honeywell, although it was not a very successful one. However, it had a powerful impact in the computer field, due to its many novel and valuable ideas; in particular, the Unix system (produced by Bell Labs personnel who had worked on Multics), the GNU project and the much later Linux kernel, are in part descended from Multics. Although it was much derided at the time by its critics [1], history has shown these complaints to be canards.
Among its many new ideas, it was the first operating system to provide a hierarchical file system, a feature that can be now found in virtually every operating system. It had numerous features intended to result in high availability, so that it would produce a computing utility , similar to the telephoneThe telephone or phone is a telecommunications device that transmits speech by means of electric signals. Generally attributed to the inventor Alexander Graham Bell, the first was built in Boston, Massachusetts, in 1876. However, an Italian inventor Anton and electricityElectricity is a property of certain subatomic particles, such as electrons and protons, that gives rise to attractive and repulsive forces between them. It is one of the four fundamental forces of nature, and is a conserved property of matter that can be services. To achieve this, in addition to being modular in software structure, the hardware was too, and the system could grow in size by simply adding more of the appropriate resource - computing power, main memory, disk storage, etc.
It was the first to have a command processor implemented as ordinary user code - an idea later used in the Unix shellA Unix shell also called "the command line", provides the traditional user interface for the Unix operating system. Users direct the operation of the computer by entering command input as text for a shell to execute. Within the Microsoft Windows suite of (although the details are different, since Multics possessed powerful mechanisms which Unix lacks). Separate Access Control Lists on every file provided flexible information sharing, but also provided complete privacy when needed. It contained 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. Even this extensive list does not encompass all of Multics' new ideas, though.
Multics was especially interesting for deliberately discarding a clear distinction between fileA file in a computer system is a stream (sequence) of bits stored as a single unit, typically in a file system on disk or magnetic tape. While a file is usually presented as a single stream, it most often is stored as multiple fragments of data at differes (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 in practice, due only to the hardware architecture of the particular machines it ran on, and not a result of the basic concept, was that the size of segments was limited to 1 megabyte, and therefore extra code had to be used to work on files larger than this.
In the days before large databases and, later, huge bitmap graphics, this limit was rarely encountered.)
This led to the second of Multics' major new ideas, 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. With this facility available, applications automatically used the latest version of any external routine they called, since those routines were kept in other segments, which were dynamically linked in only when a process first tried to begin execution in them. Since different processes, belonging to different users, 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.
It is worth noting that these two ideas, in their full power, are even now still not available in other widely used operating systems, despite the rapid and otherwise enormous advance in the computer field since the 1960s; although in more limited forms they are now becoming more widely accepted and available, e.g. dynamic linking.
Multics also supported extremely aggressive on-line reconfiguration; CPU's, memory banks, disk drives, etc. could all be added and removed while the system continued operating; being added into service, or removed from it, as required. (In fact, it was common practice at the MIT system, where most early software development was done, to split the system - which was a multi-processor 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 performed on the second machine; when testing was over, the components of the second system would be added back onto the main user system.) Since multiple CPU's were thus supported, it was thus one of the earliest multi-processor systems.
Multics was also notable for its early emphasis on computer security by design, and Multics was possibly the very first operating system to be designed as a secure system from the ground up. In spite of this, early versions of Multics were broken into, not once, but repeatedly. This led to further work on improved security that both prefigured modern security engineering techniques, and made the system much more secure; once the second-generation hardware base was adopted, with hardware support for security rings , break-ins became very rare.
In addition to having been the first operating system to provide a hierarchical file system, filenames could be of almost arbitrary length and syntax; a given file could have more than one name; and symbolic links between different file names were also supported. It was the first to use the now-standard concept of having per- process stacks in the kernel. It was also one of the first written in a high level language; PL/I was used (the B5000 system, which used Burroughs Algol, preceded Multics).
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