RC 4000 multiprogramming system

The RC 4000 Multiprogramming System is a discontinued operating system developed for the RC 4000 minicomputer in 1969. It is historically notable for being the first attempt to break down an operating system into a group of interacting programs communicating via a message passing kernel. Although RC 4000 itself was not very successful it was nevertheless extremely influential, sparking the microkernel concept that dominated operating system research through the 1970s and 1980s. The system is also known as Monitor and, somewhat confusingly, simply RC 4000 depending on the reference. For clarity, this article will use the term Monitor.

Monitor was created largely by one programmer, Per Brinch Hansen, who worked at Regnecentralen where the RC 4000 was being designed. Leif Svalgaard participated in the implementation and testing of Monitor. Brinch Hansen found that no existing operating system was suited to the new machine, and was tired of having to adapt existing systems. He felt that a better solution was to build an underlying kernel, which he referred to as the nucleus, that could be used to build up an operating system from interacting programs. Unix, for instance, uses small interacting programs for many tasks, transferring data through a system known as pipes. However a large amount of fundamental code is buried in the kernel itself, notably things like file systems and program control. Monitor would remove this code as well, making almost the entire system a set of interacting programs, reducing the kernel (nucleus) to a communications and support system only.

Monitor used a pipe-like system of shared memory as the basis of its inter-process communications. Data to be sent from one process to another was copied into an empty memory buffer, and when the receiving program was ready, back out again. The buffer was then returned to the pool. Programs had a very simple API for passing data, using an set of four methods. Client applications send data with send message and could optionally block using wait answer. Servers used a mirroring set of calls, wait message and send answer. Note that messages had an implicit "return path" for every message sent, making the semantics more like a remote procedure call than Mach's completely I/O-based system.

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