PURPOSE: To prevent a runaway in the course of execution of a program and to prevent an abnormal loop operation, by adding a simple circuit to a program counter.

CONSTITUTION: An AND is taken with respect to a micro-order R generated by a return instruction RTN, and a signal which is generated when a stack pointer SP is holding the lowest stack address, and in that case, a specified value IN is inputted to a program counter PC from an IN input terminal. As a result, by the value IN in this case, the program counter PC can get out of a loop. The specified value IN inputted from an additional gate G10 can take various values, for instance, when it is set to the start address of a program to restart and the program is executed again.