Figure: I/O-CPU Processing

1. In any C program, there are any of two things will happen- either I/O or CPU processing.
2. I/O - > CPU processing - > I/O - CPU processing - > I/O -> CPU processing and so on is called I/O- Processing – I/O cycle.
3. At the time of Input or Output, CPU is free.
4. As we know that speed of computer is very fast, so, it performs Output and CPU processing at very fast speed. (Although speed of Output depends on the speed of output device and speed of CPU processing depends on speed of processor used.)
5. But Input is not in the hand of computer because it is to be provided by user to the computer. The computer stops only when user provides input to the user.