A data processor comprises a processing unit which processes an instruction in pipeline stages, the number of which is switchable between n and m, m being a larger number than n. The data processor also comprises a switching unit for switching the number of the pipeline stages of the processing unit between n and m. The switching unit comprises an indicating unit for indicating whether the data processor is in a first operating condition or in a second operating condition, depending either on the frequency of the operation clock provided for the data processor or on the power source voltage supplied to the data processor, and a pipeline control unit for ordering a processing unit to operate in n stages under the first operation condition, and for ordering the processing unit to operate in m stages under the second operating condition.