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10-16

MOTOROLA

 

This interrupt controller design also supports the concept of a software IACK. A software
IACK is a useful concept that allows an interrupt service routine to determine if there are
other pending interrupts so that the overhead associated with interrupt exception processing
(including machine state save/restore functions) can be minimized. In general, the software
IACK is performed near the end of an interrupt service routine, and if there are additional
active interrupt sources, the current interrupt service routine (ISR) passes control to the
appropriate service routine, but without taking another interrupt exception.

When the interrupt controller receives a software IACK read, it returns the vector number
associated with the highest level, highest priority unmasked interrupt source for that
interrupt controller. The IACKLPR register is also loaded as the software IACK is
performed. If there are no active sources, the interrupt controller returns an all-zero vector
as the operand. For this situation, the IACKLPR register is also cleared.

In addition to the software IACK registers within each interrupt controller, there are global
software IACK registers. A read from the global SWIACK will return the vector number
for the highest level and priority unmasked interrupt source from all interrupt controllers.
A read from one of the LnIACK registers will return the vector for the highest priority
unmasked interrupt within a level for all interrupt controllers.

The interrupt controllers have a fixed priority, where INTC0 has the highest priority, and
INTC1 has the lowest priority. If both interrupt controllers have active interrupts at the
same level and priority, then the INTC0 interrupt will be serviced first. If INTC1 has an
active interrupt that has a higher level or priority than the highest INTC0 interrupt, then the
INTC1 interrupt will be serviced first.

7

6

4

3

0

Field

VECTOR

Reset

0000_0000

R/W

R

Address

See Table 10-2 and Table 10-3 for register offsets

Figure 10-10. Software and Level n IACK Registers (SWIACKR, L1IACK–L7IACK) 

7–0

VECTOR Vector number. A read from the SWIACK register returns the vector number associated with the 

highest level, highest priority unmasked interrupt source. A read from one of the LnACK registers 
returns the highest priority unmasked interrupt source within the level.