A processing device embodied in an integrated circuit may be divided into first and second functional units. A mount for the integrated circuit may be assigned to the first functional unit, which may define the external electrical connections of the processing device. Processing may take place in a second functional unit of the processing device, whose essential connections may normally be accessible from the outside via the external connections of the first functional unit. The processor device having first and second functional units in a mount may be similar to a hybrid circuit but may serve a different purpose. The first functional unit, which may also comprise more than one monolithic integrated circuit, may define the external connections and may make available suitable matching circuits for the second functional unit. In this manner, the processing device, which may be contained in the second functional unit and may also comprise more than one monolithic integrated circuit, may be considered to be “masked” as viewed from the outside. In addition to the matching circuits, the first functional unit may contain subcircuits performing more general functions, such as oscillators, supply voltage regulator circuits, buffer circuits, protective circuits, etc.