The basic concept is that of a multiprocessor machine with global shared memory according to Flynn's MIMD rpinciple, see Fig. Each processor may execute an independent program. At most one processor can read from one cell at a time. At most one processor can write to a memory cell at a time. Several processors can read a cell at the same time.
Several processors can write toread a cell at the same time.
Thus there are four different variants of PRAM machines, depending on how these possibilities are combined. Another common variant is the CREW-PRAM concept, where processors may read in parallel form the same cell, however only one processor may write to a cell at a time. The combination ERCW is not very common, however the most flexible concept of CRCW is the object of many studies. Here both read and write access may be concurrent, and therefore the concurrent write accesses must be somehow resolved. Concurrent write is only permitted, when all processors write the same value.