IO is always between device and device controller. CPU reads/write the data in Giga Hedge speed but disk are very slow , due to this disk looses some data during reading the data from CPU. So Disk controller has local buffers for solving this issue. First CPU place the data in to Disk buffers from that Disk reads. same thing for write also.
CPU is processing process p1, in process p1 there is an instruction 'read opd.txt file'. then CPU gives this request to Device controller (disk controller) then disk controller is reading the data from disk mean wile CPU should not Idle, CPU context switching to process p2 and process p1 is preempted . Once disk is finished reading the data from disk (and placed this data into disk controller), disk controller raises an interrupt to CPU "saying that ready is finished" at time P2 or P3 or P4 may running in CPU, then ISR executes and data is read from disk controller to memory.
This is called Hardware interrupt.
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