ford, and Fred Young. We also thank John Corman for keeping our hardware running. We have drawn from many sources in the design of the Thoth Machine described in Section 2. The main influence has been other operating systems, particularly: Brinch-Hansen's RC 4000 system, Multics, Data General's RTOS, Honeywell's GCOS, and BeU Laboratories' UNIX.A note on the name: in Egyptian mythology [9], Thoth ruled Egypt for 3226 years. He was endowed with complete knowledge and wisdom, inventing all arts and sciences including arithmetic, geometry, astronomy, soothsaying, magic, medicine, drawing, and writing. In some stories, creation was accomplished by the sound of his voice. After his death, Thoth went to the skies where he became god of letters, god of wisdom, messenger for the gods, upholder of justice, and searcher after truth. He measured time, divided the world, kept divine archives, and was patron of history. When Egyptians died, Thoth weighed their hearts and proclaimed them "guilty" or "not guilty." He then revealed the magic formulae needed to traverse the underworld in safety. References(Note. References [3] and [5] are not cited in the text.) 1. Braga, R.S.C. Eh ref. manual.Synchronization of concurrent processes requires controlling the relative ordering of events in tbe processes. A new synchronization mechanism is proposed, using abstract objects called eventcounts and sequencers, tbat allows processes to control tbe ordering of events directly, ratber than using mutual exclusion to protect manipulations of shared variables that control ordering of events. Direct control of ordering seems to simplify correctness arguments and also simplifies implementation in distributed systems. Tbe mechanism is defined formally, and tben several examples of its use are given. Tbe relationship of tbe mecbanism to protection mechanisms in the system is explained; in particular, eventcounts are shown to be applicable to situations wbere confinement of information matters. An implementation of eventcounts and sequencers in a system witb sbared memory is described.

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The design of computer systems to be concurrently used by multiple, independent users requires a mechanism that allows programs to synchronize their use of shared resources. Many such mechanisms have been developed and used in practical applications. Most of the currently favored mechanisms, such as semaphores and monitors are based on the concept of mutual exclusion . In this paper, we describe an alternative synchronization mechanism that is not based on the concept of mutual exclusion, but rather on observing the sequencing of significant events in the course of an asynchronous computation. Two kinds of objects are described, an eventcount, which is a communication path for signalling and observing the progress of concurrent computations, and a sequencer, which assigns an order to events occurring in the system.

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