1. In a control system for electronically controlling the operation of an internal combustion engine said control system including sensors for detecting operating conditions of said engine, an arithmetic unit for arithmetically determining control quantities for controlling said engine through digital processing of output signals produced from said sensors, memory means for storing therein programs and data for performing said arithmetic operations, and control means for controlling said engine in response to the results of said arithmetic operation, said programs being classified as a plurality of tasks on the basis of control functions thereof, each classified task having a start period respectively set in accordance with its influence on the engine and priority level respectively; a method of controlling the operation of said control system comprising executing said programs by performing the steps of: A a first step of setting a start request flag into the memory means in response to the expiration of a start period, B a second step of searching for said start request flag successively from tasks of higher levels, C a third step of reading out a start address of the particular task on the basis of the search results of the second step, D a fourth step of executing the particular task, setting a run flag by use of a start address and resetting the start request flag, and E a fifth step for resetting said run flag on the basis of an end of execution of said particular task, and F a sixth step for repating the first to fifth steps, whereby control of the operation of the system is effected by execution of said tasks in the course of performing said first through sixth steps.