
Hawk PCI Host Bridge & Multi-Processor Interrupt Controller
MVME5100 Single Board Computer Programmer’s Reference (6806800H17B)
123
2.4.16.6 Current Task Priority Level
Each processor has a separate Current Task Priority Level register. The system software uses this
register to indicate the relative priority of the task running on the corresponding processor. The
interrupt controller will not deliver an interrupt to a processor unless it has a priority level which
is greater than the current task priority level of that processor. This value is also used in
determining the destination for interrupts which are delivered using the distributed deliver
mode.
2.4.17 Architectural Notes
The hardware and software overhead required to update the task priority register
synchronously with instruction execution may far outweigh the anticipated benefits of the task
priority register. To minimize this overhead, the interrupt controller architecture should allow
the task priority register to be updated asynchronously with respect to instruction execution.
Lower priority interrupts may continue to occur for an indeterminate number of cycles after the
processor has updated the task priority register. If this is not acceptable, the interrupt
controller architecture should recommend that, if the task priority register is not implemented
with the processor, the task priority register should only be updated when the processor enters
or exits an idle state.
Only when the task priority register is integrated within the processor, such that it can be
accessed as quickly as the MSRee bit, for example, should the architecture require the task
priority register be updated synchronously with instruction execution.
2.4.18 Effects of Interrupt Serialization
All external interrupt sources that are level sensitive must be negated at least N PCI clocks prior
to doing an EOI cycle for that interrupt source, where N is equal to the number of PCI clocks
necessary to scan in the external interrupts. In the example shown, 16 external interrupts are
scanned in, N = 16. Serializing the external interrupts causes a delay between the time that the
external interrupt source changes level and when MPIC logic actually sees the change. Spurious
interrupts can result if an EOI cycle occurs before the interrupt source is seen to be negated by
MPIC logic.
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