Computers as Components, 2nd Edition

106 CHAPTER 3 CPUs

Device 1

Device 2 Device 3

CPU

L3 L2 L1

FIGURE 3.4

Using polling to share an interrupt over several devices.

As shown in Figure 3.4, you can use a small amount of logic external to the CPU

to generate an interrupt whenever any of the devices you want to group together

request service. The CPU will call the interrupt handler associated with this priority;

that handler does not know which of the devices actually requested the interrupt.

The handler uses software polling to check the status of each device: In this example,

it would read the status registers of 1, 2, and 3 to see which of them is ready and

requesting service.

Example 3.7