PURPOSE: To improve the performance of an EREG instruction without using a working register by controlling the selective restoration of specific contents from an old register saved in a stack area of a main storage.

CONSTITUTION: The instruction code and register number of an EREG instruction are stored in an instruction buffer 3. Information for instructing register writing is stored in registers 8, 9. The leading register number stored in the buffer 3 is transmitted to a register number comparator 29 through registers 10 to 13 and the final register number is also transmitted to the comparator 29 through registers 14 to 17. Since register numbers correspording to selected general registers can be identified, a control signal for controlling whether writing operation in a general register corresponding to a certain applied register number is to be permitted or suppressed can be outputted.