Specifications

Interrupt Controller (V40)
Automatic Finish Interrupt
When programmed for automatic FI, the ICU automatically executes
a nonspecific FI during the interrupt acknowledge cycle. The
advantage of the finish interrupt format is that you need no longer
issue a command to the ICU that a service routine is completed. Like
the nonspecific FI, the automatic FI must be used only in operating
modes in which the service routine of the highest priority request is
the first one completed. Two instances in which the automatic FI
command should not be used are if priorities are changed during an
interrupt service routine and if the exceptional nesting mode is used.
When using the automatic FI format, you must disable interrupt
requests during the execution of an interrupt service routine. This is
because the IIS bit is cleared immediately after the interrupt
acknowledge cycle or, in other words, before the service routine is
started. If interrupts are enabled during the service routine, there is no
indication that an interrupt is currently being serviced, and a second
request can be generated by the same interrupt if level-triggered
interrupts are used. This can continue until the level is removed,
resulting in an undefined operation.
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