Abstract: In a data processing system having a plurality of tightly coupled data processing units connected by an asynchronous system bus, apparatus and an associated method are described for maintaining the coherency of data groups stored in instruction cache units and execution cache units. The apparatus includes a monitor unit as part of the bus interface unit, and a bus interface unit coupling each associated data processing unit to the system bus. The monitor unit receives signals, applied to the system bus, identifying data groups transferred between the memory unit and the data processing units, including those data groups originating from the bus interface unit of which the monitor unit is a component. The bus interface unit includes directories duplicating the contents of the instruction cache unit directory and the execution cache unit directory.