A shared variable is an abstraction of persistent interprocess communication. Processors execute operations, often concurrently, on shared variables to exchange information among themselves. The behavior of operation executions is required to be “consistent” for effective interprocess communication. Consequently, a consistency specification of a shared variable describes some guarantees on the behavior of the operation executions. A Read/Write shared variable has two operations: a Write stores a specified value in the variable and a Read returns a value from the variable. For Read/Write variables, a consistency specification describes what values Reads may return. Using an intuitive notion of illegality of Reads, we propose a framework that facilitates specifying a large variety of Read/Write variables.

Shared registers are basic objects used as communication mediums in asynchronous concurrent computation. A concurrent timestamp system is a higher typed communication object, and has been shown to be a powerful tool to solve many concurrency control problems. It has turned out to be possible to construct such higher typed objects from primitive lower typed ones. The next step is to find efficient constructions. We propose a very efficient wait-free construction of bounded concurrent timestamp systems from 1-writer shared registers. This finalizes, corrects, and extends a preliminary bounded multiwriter construction proposed by the second author in 1986. That work partially initiated the current interest in wait-free concurrent objects, and introduced a notion of discrete vector clocks in distributed algorithms.

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A simple wait-free construction of 1-writer multireader multivalued atomic variable from multireader regular variables is presented in this paper. A key point of the construction is the use of an elegant forwarding technique to overcome the new-old inversion property inherent in regular variables. Another construction, using a different forwarding technique, is also given. This technique is a refinement of one proposed in the literature. Formal correctness proofs for both the constructions are short and easy to follow.