NEC PD754144 - Manual (Page 198)

CHAPTER 7 INTERRUPT AND TEST FUNCTIONS
198
User’s Manual U10676EJ3V0UM
7.5 Nesting Control of Interrupts
The
µPD754244 can nest interrupts by the following two methods.
(1) Nesting with interrupt having high priority specified
This method is the standard nesting method of the µPD754244. One interrupt source is selected and nested.
An interrupt with a higher priority specified by the interrupt priority select register (IPS) can occur when the
status of the processing under execution is 0 or 1, and the other interrupts (interrupts with a lower priority)
can occur when the status is 0 (refer to Figure 7-8 and Table 7-3).
Therefore, if you use this method when you wish to nest only one interrupt, operations such as enabling and
disabling interrupts, that is, changing the interrupt status flag, while the interrupt is serviced do not need to
be performed, and the nesting level can be kept to 2.
Figure 7-8. Nesting of Interrupt with High Priority
Interrupt disabled
IPS set
Interrupt enabled
Interrupt with
low or high
priority occurs
Interrupt
with high
priority
occurs
Normal
processing
(status 0) Interrupt servicing
with low or high
priority (status 1) Interrupt servicing
with high priority
(status 2)