Control unit
Device selection progresses from the channel to each control unit in the order they are physically attached to their channel. At the end of the chain the selection process continues in reverse back towards the channel. If the selection returns to the channel then no control unit accepted the command and SIO returns Condition Code 3. Control units marked as High Priority check the outbound CUU to be within their range. If so, then the I/O was processed. If not, then the selection was passed to the next outbound CU. Control units marked as Low Priority check for inbound (returning) CUU to be within their range. If so, then the I/O is processed. If not, then the selection is passed to the next inbound CU (or the channel). The connection of three controls unit to a channel might be physically -A-B-C and, if all are marked as High then the priority would be ABC. If all are marked low then the priority would be CBA. If B was marked High and AC low then the order would be BCA. Extending this line of reasoning then the first of N controllers would be priority 1 (High) or 2N-1 (Low), the second priority 2 or 2N-2, the third priority 3 or 2N-3, etc. The last physically attached would always be priority N.