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The paper is concerned with communication mechanisms in which a record in shared memory is maintained by a writer to provide a coherent and up-to-date data reference which may be accessed at any time by a reader. The dynamic properties of several possible designs are briefly discussed before concentrating on a fully asynchronous form called a four-slot mechanism. This takes its name from the four-element array through which data is routed. Coherence is maintained by means of an orthogonal avoidance strategy without recourse to conventional synchronisation techniques such as semaphores, rendezvous, monitors or critical sections. Various implementation options are considered including software and hardware designs. It is seen that the fully asynchronous mechanism completes the range of basic communications options available to digital system designers.