PURPOSE: To prevent the occurrence of bugs that never occurred in the existing memories and also to restore the memory that was once changed into an original unchanged state by allotting additionally the memory to be changed to the end of an existing memory.

CONSTITUTION: It is supposed that the address parts 300-400 of a memory 2 are changed. A memory 3 is newly obtained when a new memory part 9 prepared to the changed part of the memory 2 is added to the memory 2 together with a history record part 8. Under such conditions, the added addresses 1400-1499, new 100 lines, the date, and the time of a new memory part 7 are recorded to an additional memory control part 4 of a memory 1. If a bug is included in the part 9 during the operation of the memory 3, this operating state is returned to the state of the memory 2. At the same time, a desired address of the part 9 to be separated from the part 4 of the memory 1 is calculated. Then, the part 8 is separated from the part 9 with the processing of an additional memory control part 5 of the memory 1.