Q:

The interrupt timeline for a single process doing output

I'm studying the book 'Operating System Concepts' 9th edition.
In the first chapter, part 1.2.1 computer system operation, I can't understand the figure 1.3:

Can any one make a quick interpretation on this for me? especially about the peaks of this graph?

A:

The I/O device (controller) is busy transferring data from the device buffer to the device. It goes from idle to transferring. This is the peak for I/O device. It goes back to idle when the transfer is done, until the next request. 
The  CPU curve shows a peak when the transfer is done because the CPU is notified by the device (through an interrupt).

