1. A method of operating a processor-controlled system, in which output signals are produced as a result of the processor carrying out a plurality of tasks the functions of which affect the operation of said system, and wherein there are provided a plurality of task programs through the execution of which said tasks are performed, each task program having a prescribed level of priority assigned thereto in accordance with its functional effect on the operation of said system, comprising the steps of: (a) generating successive interrupt signals to which said processor responds in executing said task programs; (b) in response to a respective one of said interrupt signals, issuing execution request signals for task programs currently requiring service; (c) upon completion of step (b), initiating the execution of the task program or programs, the task program level of which is highest among those programs for which execution request signals have been generated in step (b); and (d) upon completion of the execution of a task program or programs in step (c), determining which task program or programs is to be next executed and thereupon proceeding with its execution.