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One method for identifying contingency relationships in a sequence of observed behaviors studies occurrences of matching behaviors at successive event or time steps from a criterion event. This paper briefly summarizes the method and presents a computer program for generating lag conditional probabilities and associated measures for assessing statistical significance.

Three stages of the development of programming languages are examined and compared with similar stages in the development of operating systems. It is noted that not only do these stages agree but particular developments within these stages also have remarkable similarities; some of these are superficial while others are deeper. The parallels are used to note that certain developments have yet to occur in operating systems. Finally, the essential identity of purpose of programming languages and operating systems is discussed.

A project to produce a kernel-based, message-passing version of UNIX is described. The system is designed to be (1) useful in teaching operating systems, (2) easily changeable, (3) easily portable, and (4) a vehicle for studying the message-passing approach to operating systems design. The system calls normally handled by the UNIX kernel are handled by system processes, each of which operates in its own address space. Interprocess communication and process environment management is done by a small kernel. The design and implementation of the system are described. Message passing as a system design method is evaluated in this context and compared with the procedure call orientation of standard UNIX. The message-based design proved successful in creating a modular and understandable system.