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80C186EC/80C188EC Microprocessor User's Manual

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INTERRUPT CONTROL UNIT<br />

8.3.3.1 Default (Fixed) Priority<br />

After initialization, the 8259A module sets the priorities of the interrupt levels to the default condition,<br />

in which IR7 is the lowest priority and IR0 is the highest (Figure 8-6). For systems using<br />

fixed priority, the interrupt source with the highest priority is connected to IR0, the interrupt<br />

source with the second-highest priority is connected to IR1, and so on. The lowest-priority device<br />

is connected to IR7.<br />

Highest<br />

Priority<br />

Lowest<br />

Priority<br />

IR0<br />

IR1<br />

IR2<br />

IR3<br />

IR4<br />

IR5<br />

IR6<br />

IR7<br />

Decreasing relative priority<br />

A1242-0A<br />

Figure 8-6. Default Priority<br />

8.3.3.2 Changing the Default Priority: Specific Rotation<br />

In some systems, it may be necessary to alter the default priority during program execution. Any<br />

one of the IR lines can be reprogrammed to be the lowest-priority interrupt source. The priorities<br />

of the remaining IR lines are then redefined in a circular fashion. For example, if IR5 is programmed<br />

to be the lowest-priority interrupt source, then IR6 becomes the highest-priority source<br />

(see Figure 8-7). One could think of the priority pointer rotating through the IR sources. This<br />

method of redefining the priority is called specific rotation.<br />

The priorities of the IR lines cannot be set independently.<br />

Highest<br />

Priority<br />

Lowest<br />

Priority<br />

IR6<br />

IR7<br />

IR0<br />

IR1<br />

IR2<br />

IR3<br />

IR4<br />

IR5<br />

Decreasing relative priority<br />

A1243-0A<br />

Figure 8-7. Specific Rotation<br />

8-11

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