left Regnecentralen's hardware group to become head of RC 4000 software development, where he led a team including Jørn Jensen, Peter Kraft and Søren Lauesen in defining a general-purpose RC 4000 multiprogramming system, with a goal to avoid developing a custom real-time control operating system for every RC 4000 installation, and to support batch processing and time-sharing as well. The resulting system was not a complete operating system, but a small kernel providing the mechanisms upon which operating systems for different purposes could be built. By the spring of 1969, a well-documented, reliable version of the RC 4000 multiprogramming system was running. Carnegie Mellon University In late 1970, Brinch Hansen moved to Pittsburgh, accepting an invitation from Alan Perlis to visit the department of computer science at Carnegie Mellon University as a research associate, while he wrote the first systematic textbook on operating system principles. During this time, at the 1971 Summer School in Marktoberdorf and a symposium in Belfast, Brinch Hansen, Tony Hoare and Dijkstra began to discuss ideas that evolved into the monitor concept. In the spring of 1972, after reading about the class concept invented by Ole-Johan Dahl and Kristen Nygaard for Simula 67, Brinch Hansen completed his text with a chapter on resource protection that proposed the first monitor notation, using shared classes. In 1973, Operating System Principles was published, becoming the first comprehensive textbook on operating systems. California Institute of Technology In July 1972, Brinch Hansen joined the faculty of the California Institute of Technology (Caltech) as an Associate Professor of computer science, where he began work on defining a programming language with concurrent processes and monitors. In April 1974, he distributed a technical report on Concurrent Pascal. A Concurrent Pascal compiler for the PDP 11/45, written by Brinch Hansen's doctoral student, Al Hartmann, was released in January 1975. Subsequently, Brinch Hansen began writing model operating systems in Concurrent Pascal, to evaluate the language. In May 1975, he completed Solo, a single-user operating system for development of Concurrent Pascal programs. Next, he rewrote the original RC 4000 real-time scheduler in Concurrent Pascal, taking three days to write it, and three hours of machine time to systematically test it. When the Caltech computer science department shifted focus toward computer engineering and away from programming Brinch Hansen decided to leave, rather than to seek tenure there. University of Southern California In 1976, Brinch Hansen chose University of