Multics

Multics (Multiplexed Information and Computing Service) was an extraordinarily influential early time-sharing operating system.

Novel ideas

Multics was an early operating system that implemented a single level store for data access, discarding the clear distinction between files (called segments in Multics) and process memory.

Related Topics:
Single level store - File - Process - Memory

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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.

Related Topics:
Address space - CPU - Disk

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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.

Related Topics:
POSIX - Mmap - File system

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(One disadvantage of this in practice, due only to the particular 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 256K 36-bit words, roughly equal to 1 megabyte by today's standards, and therefore extra code had to be used to work on files larger than this, called multi-segment files. In the days before large databases and, later, huge bitmap graphics, this limit was rarely encountered.)

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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.

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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.

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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.

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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.

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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.

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Multics also supported extremely aggressive on-line reconfiguration; CPUs, 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.

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(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, without ever having shut it down.)

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Since multiple CPUs were thus supported, it was thus one of the earliest multi-processor systems.

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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.

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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 (a multi-level refinement of the concept of master mode), break-ins became very rare.

Related Topics:
Security engineering - Security rings - Master mode

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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 or directory could have multiple names (typically a long and short form); and symbolic links between directories were also supported.

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It was the first to use the now-standard concept of having per-process stacks in the kernel, with a separate stack for each security ring. It was also one of the first written in a high level language; PL/I was used (the Burroughs B5000 system, which used ALGOL, preceded Multics).

Related Topics:
Process - Stack - Kernel - PL/I - Burroughs B5000 - ALGOL

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