PURPOSE: To pursue errors in a program logic, by installing registers which store, when a specific address is accessed, the program No. to which the access instruction belongs, the address of the instruction, the distinction between setting and resetting, etc.

CONSTITUTION: Programs 1 and 2 are executed in the order. A flag 3 is set by an instruction 4, reset by an instruction 5, and set by an instruction 6. The address of the flag 3 is stored in advance in a register 7, and, when an access is given to the address, the program No. to which the access instruction belongs, the address at which the access instruction exists, and the distinction between the setting and resetting, etc., are stored in another register 8. Since the content of the register 8 is changed whenever the address is accessed, the register 8 always holds the last accessed program No, address, etc., and therefore, the program No. and address which are not reset are easily known. Thus causes are easily pursued.