PURPOSE: To utilize the newly prepared phase of an operating system (OS) without stopping or restarting the system while operating the system.

CONSTITUTION: A phase address translation program 2 checks which address on a system common area 6 is processed by an OS 1 and when it is recognized from the contents of a phase address translation table 3 that the address of the system common area 6 to be processed by the system 1 is coincident with the leading address of a former (loaded) phase 5 to be replaced, the OS executes the newly prepared phase while skipping to the address on the system common area where a new (loaded) phase 4' exists. After the processing is completed, this time, it is instructed to return to an address adding '1' to a terminating address on the system common area of the former phase recorded in the phase address translation table.