It will be assumed that the disk controller 50 generates a memory read request including an address coded to specify controller 200 with address bits BSAD19-21 set to binary ZEROS and address bit BSAD22 set to a binary ONE. Also, for the request, disk controller 50 operates to switch line BSDBPL to a binary ZERO, line BSDBWD to a binary ONE and line BSWRIT to a binary ZERO indicating that the memo