DATA PROCESSOR AND MICROCOMPUTER

In a microcomputer incorporating a microprocessor, the coprocessor having product-sum operation arithmetic units executes residue multiplications given by "A=A.multidot.B.multidot.R.sup.-1 mod N+kN", "A=A.sup.2 .multidot.R.sup.-1 mod N+kN" and "A=A.multidot.R.sup.-1 mod N 30 kN" and is provided with a multiplication function of executing a preprocessing "R.sup.2 mod N" at high speed. It is possible to perform a power-residue operation "X.sup.Y mod N" at high speed by the microprocessor, using these calculation functions of the coprocessor.