9-7
INTERRUPT CONTROL UNIT
Figure 9-2. Methods for Changing the Default Interrupt Structure
9.2.2.2 Determining Priority
There are three modes that determine relative priorities, i.e., whether a le vel higher, lower, orequal to another level has higher or lower interrupt priority.Fully nested In the fully nested mode, higher level IR signals have higher interrupt priority. In this mode, when an 82C59A receives multiple interrupt requests, it passes the highest level request to the core (or to the master if the 82C59A is a slave). The core stops processing the lower level request, processes the higher level request, then returns to finish the lower level request.Special fully nested The special fully nested mode allows higher or equal level IR signals to have higher interrupt priority. In this mode, if the core is processing an interrupt, a higher or equal level interrupt request is passed through to the core. Also, since all interrupts from the slave are directed into a single IR line (IR2) on the master (the master does not know the priorities of the slave interrupts it receives), this mode enables a higher-level interrupt on the slave to interrupt the
IR0
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A2303-02
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