The Solo Operating System: A Concurrent Pascal Program

This is a description of the single-user operating system Solo written in the programming language Concurrent Pascal. It supports the development of Sequential and Concurrent Pascal programs for the PDP 11/45 computer. Input/output are handled by concurrent processes. Pascal programs can call one another recursively and pass arbitrary parameters among themselves. This makes it possible to use Pascal as a job control language. Solo is the first major example of a hierarchical concurrent program implemented in terms of abstract data types (classes, monitors and processes) with compile-time control of most access rights. It is described here from the user’s point of view as an introduction to another paper describing its internal structure.