Default (Fixed) Priority - Intel 80C188EC User Manual

8.3.3.1

Default (Fixed) Priority

After initialization, the 8259A module sets the priorities of the interrupt levels to the default con-
dition, in which IR7 is the lowest priority and IR0 is the highest (Figure 8-6). For systems using
fixed priority, the interrupt source with the highest priority is connected to IR0, the interrupt
source with the second-highest priority is connected to IR1, and so on. The lowest-priority device
is connected to IR7.
8.3.3.2 Changing the Default Priority: Specific Rotation
In some systems, it may be necessary to alter the default priority during program execution. Any
one of the IR lines can be reprogrammed to be the lowest-priority interrupt source. The priorities
of the remaining IR lines are then redefined in a circular fashion. For example, if IR5 is pro-
grammed to be the lowest-priority interrupt source, then IR6 becomes the highest-priority source
(see Figure 8-7). One could think of the priority pointer rotating through the IR sources. This
method of redefining the priority is called specific rotation.
The priorities of the IR lines cannot be set independently.
Highest
Priority
IR0 IR1 IR2 IR3
Decreasing relative priority
Figure 8-6. Default Priority
Highest
Priority
IR6 IR7 IR0 IR1
Decreasing relative priority
Figure 8-7. Specific Rotation
INTERRUPT CONTROL UNIT
Lowest
Priority
IR4 IR5 IR6 IR7
Lowest
Priority
IR2 IR3 IR4 IR5
A1242-0A
A1243-0A
8-11