IntroductionThe multiprogramming system developed by Regnecentralen for the RC 4000 computer is a general tool for the design of operating systems. It allows the dynamic creation of a hierarchy of processes in which diverse strategies of program scheduling and resource allocation can be implemented.For the designer of advanced information systems, a vital requirement of any operating system is that it allow him to change the mode of operation it controls; otherwise his freedom of design can be seriously limited. Unfortunately, this is precisely what present operating systems do not allow. Most of them are based exclusively on a single mode of operation, such as batch processing, priority scheduling, real-time scheduling, or conversational access.When the need arises, the user often finds it hopeless to modify an operating system that has made rigid assumptions in its basic design about a specific mode of operation. The alternative-to replace the original operating system with a new one-is in most computers a serious, if not impossible, * P. Brinch Hansen, The nucleus of a multiprogramming system, Communications of the ACM 13, 4 (April 1970), 238-242.