Specifications

Interrupt Controller (V40)
Finish Interrupts
The ICU must be told when a service routine is completed so that the
in-service bit of the IIS register can be cleared. The three finish
interrupt formats that the ICU recognizes are the nonspecific finish
interrupt, specific finish interrupt, and automatic finish interrupt.
Nonspecific FI
The ICU accepts a nonspecific FI as an indication of interrupt service
completion of the highest priority request. The advantage of the finish
interrupt format is that you need not specify the level to be cleared.
The catch is that the nonspecific FI command 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
nonspecific FI command should not be used are if priorities are
changed during an interrupt service routine and if the exceptional
nesting mode is used.
Specific FI
The second finish interrupt format is the specific FI command. This
command includes the level of the request to be cleared. The specific
FI command must be used if the highest priority interrupt is not
always the first one completely serviced. Such is the case if the
interrupt priorities are changed during a service routine (called
specific rotation). If the nonspecific FI were used in this case, it is
possible that the wrong in-service bit might be reset by the ICU. The
specific FI can be used in all modes of ICU operation.
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