PURPOSE: To prevent an area storing other information from being destructed and to suppress the occurrence of the system trouble by comparing key informa tion on a top address with that on a final address and confirming that the memory area is a correct storage area.

CONSTITUTION: A comparator means 4 reads out the key information KH' on the top address A in the prescribed area in a memory 6 with the key information KT' on the final address (A+l-1), and compares KH' with a parameter KH in a key information specification means 3 and KT' with a parameter KT in said means 3. If an operator mistakenly specifies the top address, the dissidence of either one of a bits of key information or both of them is detected. The comparator means 4 outputs a signal showing the occurrence of the dissidence to an error reporting means 5. Receiving said signal, the error reporting means 5 displays a message that a microinstruction is executed on the display of a user input means 1. Thus the destruction of other areas in the memory can be prevented which is caused by an error in terms of specifying the address.