Multics

Multics ("MULTiplexed Information and Computing Service") is an influential early time-sharing operating system based on the concept of a single-level memory. It has been written 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 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 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 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 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 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, 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 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 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 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); 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 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.