It is shown how a communication abstraction called the port ensemble can simplify the handling of boundary conditions and the efficient porting of programs. A port ensemble provides an explicit interface between computation and communication descriptions, thus separating the communication structure from the details of local computation and from the compiler. Port ensembles structure ports, symbolic names to and from which process can write and read values. To simplify the expression of boundary conditions, ports can be bound not only to ports on other processors, but also to nonexistent neighbors (along the edge of the computation) using special objects that represent and implement constants, variables, and arbitrary functions. Port ensembles also provide direct access to the communication structure, which simplifies changing the structure to one appropriate for a new target architecture.
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