2-54 Computer Group Literature Center Web Site
Hawk PCI Host Bridge & Multi-Processor Interrupt Controller
2
Processor’s Current Task Priority
Each processor has a task priority register which is set by system soft ware
to indicate the relative importance of the task running on that processor.
The processor will not receive interrupts with a priority level equal to or
lower than its current task priority. Therefo re, setting the current task
priority to 15 prohibits the delivery of all interrupts to the associated
processor.
Nesting of Interrupt Events
A processor is guaranteed never to have an in service interrupt preempted
by an equal or lower priority source. An inte rr upt is considered to be in
service from the time its vector is returned during an interrupt
acknowledge cycle until an EOI (End of Interrupt) is received for that
interrupt. The EOI cycle indicates the end of processing for the highest
priority in service interrupt.
Spurious Vector Generation
Under certain circumstances the MPIC will not have a valid vector to
return to the processor during an interrupt acknowledge cycle. In these
cases the spurious vector from the spurious vector register will be returned.
The following cases would cause a spurious vector fetch:
❏INT is asserted in response to an externally sourced interrupt, which
is activated with level sensitive logic, and the asserted level is
negated before the interrupt is acknowledged.
❏INT is asserted for an interrupt source, which is masked using the
mask bit, in the Vector-Priority register be fo re the interrupt is
acknowledged.