Abstract: A channel for a data processing system is provided with a time of day clock that is synchronized with the time of day clock of the associated central processor. Both the central processor and the channel processor record times of particular events, and the channel uses these times to calculate two times called Function Pending and Function Active. Both times begin when the central processor executes an instruction to begin an I/O operation. Function Pending ends when the channel has made successful initial selection. This time shows delays by the channel processor in scheduling the channel control unit, and device resources for I/O operations. Function Active ends at Channel End. A new instruction, Set Channel Monitor, enables or disables these measurements. An information block for each subchannel defines one of several measurement modes for a subchannel or disables the subchannel from measurement.