PURPOSE: To keep the continuity of processing, by limiting the instruction execution address with the set of switching, and performing self restoration to the failure of tentative address through the automatic set in correct region.

CONSTITUTION: In the computer system controlling with the microprogram, the microprogram memory 2, operation processing circuit 4, instruction execution address calculation microprogram pick up circuit 5 and instruction execution address limit circuit 8 are provided. Further, the program performing the instruction and execution address with a given microprogram is picked up, and the agreement signal 6 picked up and the signal 7 of the result of calculation of the instruction execution address are fed to the limit circuit 8, the instruction execution address is limited to the elective program area, and self restoration is made to the failure of tentative program address.