PURPOSE: To simplify the constitution of a circuit which detects the end of an instruction by obtaining the difference between an address indicated by a control storage access instruction and an address register of the control storage through an arithmetic circuit.

CONSTITUTION: A CS (control storage) access instruction is read out of a control storage CS1 and then set to an instruction register 2. A CS address register 3 stores a CS address of the CS access instruction and receives +1 every time when the contents of the CS1 are read out to a buffer register 6. Then the address of the CS1 to which the next access is given is displayed. A difference detecting circuit 7 detects the difference of contents between a CS address A indicated by the CS access instruction stored in the register 2 and the address indicated by a CS address register 4. The access is through when a comparison/ detection circuit 8 detects that said difference of contents reaches the number of access words.