PURPOSE: To prevent data from being erased on a screen and to easily and surely change the key data by showing the data controlled by keys on the screen in the order of process addresses and then controlling the change of those data at the physical storing positions of keys.

CONSTITUTION: The data are called with a process address '0101' corresponding to a process number feed BK2 of a sort key BK and used as a key. Thus the date are displayed on a screen in the order of keys of the process addresses and also the physical storing positions of keys are stored in a memory. Even when the process address of the element work B of the 2nd line is changed from '0101' to '0102', the physical storing positions of keys are never erased on the screen since they are memorized. Thus the input/output operations are carried out to these physical storing positions. Then the memorized physical storing positions of keys are cleared, when a calling instruction is produced for the next key information. Thus the physical storing position of the key called next is stored in the memory.