The updating of only some memory locations in a multiple computer environment in which at least one applications program (50) executes simultaneously on a plurality of computers M1, M2 .... Mn each of which has a local memory, is disclosed. Memory locations (A, B, D, E, X) in said local memory are categorized into two groups. The first group of memory locations (X1, X2, ... Xn, A1, A2, ... An) are each accessible by other computers. The second group of memory locations (B, E) are each accessible only by the computer having the local memory including the memory location. Changes to the contents of memory locations in the first group only are transmitted to all other computers. A demotion mechanism is disclosed to demote memory locations in the first group into the second group in the event that application program execution means that a memory location in said first group is no longer referenced by another memory location in another computer.