PURPOSE: To prevent the runaway due to designation of an invalid address, by setting a CPU to the program processing stop state when the current address is larger (or smaller) than the upper (or lower) limit address of an instruction storage memory.

CONSTITUTION: The maximum (minimum) address of the upper (lower) limit of a prescribed area in an instruction register 2 where instructions are actually stored is stored in a register 11, and the maximum (minimum) address stored in this register 11 is compared with the current address, which a CPU1 designates with address terminals A0WA16, in a comparing circuit 12. If the current address is larger (smaller) than the maximum (minimum) address as the result of comparison, a control signal is transmirted to the CPU1 to reset the CPU1. When the CPU is reset, the CPU1 outputs specific address information. Thus, the runaway of a microcomputer is prevented.