PURPOSE: To perform high-speed, high-precision arithmetic through a small-scale circuit by providing a correcting circuit which makes the binary-coded decimal correction of the output signal of a binary arithmetic circuit and adds zero, and determining which of decimal correction and zero addition is selected with a microcode.

CONSTITUTION: When a microcode for setting a binary arithmetic mode is stored in a program memory 6 and executed, an instruction decoder 7 outputs a control signal for selecting the binary arithmetic mode to a selecting circuit 5 and the correcting circuit 4 adds zero to the output of the binary arithmetic circuit 3, so that the binary arithmetic results of temporary registers 1 and 2 are outputted to a data bus 8. This is effective for all or specific arithmetic instructions until a microcode for changing the setting to the decimal arithmetic code is executed newly.