Limits on the power of concurrentwrite parallel machines

The computation of specific functions using the most general form of concurrent-read-concurrent-write parallel RAM is considered. It is shown that such a machine can compute any function of Boolean inputs in time log n- log log n + 0 ( 1) given a polynomial number of processors and memory cells and that this bound is tight for integer addition. Despite this evidence of the power of the model we show that a very simple function, namely parity, requires time Q (Jiogn) to compute given a polynomial bound on the number of processors, independent of the number of memory cells. 13 ” 1988 Academic Press, Inc 1