The arithmetic unit in a data processing system with a microprocessor and an additional arithmetic unit carries out special arithmetic operations, preferably integrated in a single semiconductor chip, is controlled by the microprocessor via a number of registers. Several sets of such registers are provided. The registers of one set are selected via a selection circuit. As a result, a set of registers which is not required during execution of a calculation by the arithmetic unit can be filled with new data by the microprocessor and, after completion of the calculation in the arithmetic unit, switching over to a newly filled set of registers takes place so that the arithmetic unit can continue with a new set of operands without having to observe a waiting period.