The fetch-execute cycle is a program-driven model of computation. But computers are not totally program driven. To some extent they are hardware driven. CPUs are equipped with input signal pins which allow the fetch-execute cycle to be interrupted. The original purpose of these pins was to increase processor efficiency by reducing the amount of time spent in communication with I/O devices.

In chapter 8 we saw how a device to increase memory efficiency was adopted as a device for controlling memory access. Interrupts have undergone a similar transition. This chapter begins by describing how interrupts economize on processor time. Then, starting with section 9.6, we will see how interrupts are used to control processor access. ...

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