PURPOSE: To ensure the interchangeability of software by using an operating system to simulate an instruction to be originally executed by a CPU and therefore omitting the correction caused by the change of an instruction set.

CONSTITUTION: An operating system 11 and a job program 12 are loaded into a main storage 10, and the program 12 is being executed. The program 12 includes an undefined instruction and an illegal instruction interruption is produced when the undefined instruction is executed. Thus the control is shifted to an illegal instruction interruption process 13 of the system 11. The action of the undefined instruction is simulated in the process 13. Thus the system 11 simulates the undefined instruction. As a result, the interchangeability of software is maintained without giving any correction to the program 12.