Abstract: A virtual memory system is described in which a set of conversion tables is used to translate an arbitrarily assigned programming designation called a virtual address into an actual main memory location called a real address. To avoid the necessity of translating the same address over and over again, a table called the Directory Look Aside Table (DLAT) retains current virtual to real address translations for use where particular virtual addresses are requested more than once. A counter is associated with the DLAT. Each time a translation is stored in the DLAT the present count in the counter is stored along side the translation. Each time the DLAT is invalidated the counter is stepped so that with each invalidation a new number is stored in the DLAT with the next translation. When a translation is read out of the DLAT the number stored with the translation is compared with the present number in the counter. If they do not match a No Compare signal is provided.