1. A data processing system including one or more processing units, a plurality of storage modules and a plurality of peripheral equipments, and each storage module and each peripheral equipment being provided with an access unit and each processing unit being provided with an associated unique data communication path which is connected to an individual port on each access unit, each access unit including means for storing a unique identity code indicative of the system identity of the associated art, each access unit further including an identity address recognition circuit which activates the access unit upon receipt of an associated identity code, a processing unit access operation involving a peripheral equipment or storage module location being performed by the processing unit associated with a communication path of the processing unit, an address comprising at least two fields, the first address filed defining the system identity of the required storage module or peripheral equipment and the second address field defining a location within the required storage module or peripheral equipment, wherein the improvement comprises in each processing unit a bus interface unit connected to a unique data communication path of each processing unit and an internal register selection means and said bus interface unit includes identity storing means for storing an identity code indicative of the processing unit and an identity address recognition means arranged to compare the first field of an address extended to the data communication path with said identity code stored in said identity storing means and upon detection of said identity code of the processing unit in the first field of an address extended to the data communication path, said identity address recognition means being connected to the second field of the presented address to the internal register selection means so that a selected internal register can be used as the location for a processing unit access operation.