The use of monitors for describing concurrency has been much discussed in the literature. When monitors are used in real sy ¢ stems of any size, however, a number of problems arise which have not been adequately dealt with: the semantics of nested monitor calls; the various ways of defining the meaning of WAIT; priority scheduling; handling of timeouts, aborts and other exceptional conditions; interactions with process creation and destruction; monitoring large numbers of small objects. These problems are addressed by the facilities described here for concurrent programming in Mesa. Experience with several substantial applications gives us some confidence in the validity of our solutions.

The Pilot operating system provides a single-user, single-language environment for higher level software on a powerful personal computer. Its features include virtual memory, a large "flat" file system, streams, network communication facilities, and concurrent programming support. Pilot thus provides rather more powerful facilities than are normally associated with personal computers. The exact facilities provided display interesting similarities to and differences from corresponding facilities provided in large multi-user systems. Pilot is implemented entirely in Mesa, a highlevel system programming language. The modularization of the implementation displays some interesting aspects in terms of both the static structure and dynamic interactions of the various components.

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In this paper we describe restartable atomic sequences, an optimistic mechanism for implementing simple atomic operations (such as Test-And-Set ) on a uniprocessor. A thread that is suspended within a restartable atomic sequence is resumed by the operating system at the beginning of the sequence, rather than at the point of suspension. This guarantees that the thread eventually executes the sequence atomically . A restartable atomic sequence has significantly less overhead than other software-based synchronization mechanisms, such as kernel emulation or software reservation. Consequently, it is an attractive alternative for use on uniprocessors that do no support atomic operations. Even on processors that do support atomic operations in hardware, restartable atomic sequences can have lower overhead. We describe different implementations of restartable atomic sequences for the Mach 3.0 and Taos operating systems. These systems' thread management packages rely on atomic operations to implement higher-level mutual exclusion facilities. We show that improving the performance of low-level atomic operations, and therefore mutual exclusion mechanisms, improves application performance.

In this paper we describe