Abstract: A programmable controller for operating a machine to perform programmed functions allows the user to allocate the amount of processing time to be devoted to a machine control program and to user defined background tasks. The allocation is defined by an entry stored in one of several locations in a data table. Each occurrence of a timed interrupt steps the operating system through the data table. When the operating system accesses the location at which the entry is stored, the machine control program is assigned as lower execution priority than any background tasks to be executed. When the last storage location of the data table is accessed, the user control program is restored to a higher execution priority than the background tasks before initiating another stepping sequence through the data table.