NEC PD754144 - Manual (Page 199)

CHAPTER 7 INTERRUPT AND TEST FUNCTIONS
199 User’s Manual U10676EJ3V0UM
(2) Nesting by changing interrupt status flags
Nesting can be implemented if the interrupt status flags are changed by program. In other words, nesting is
enabled when IST1 and IST0 are cleared to “0, 0” by an interrupt servicing program, and status 0 is set.
This method is used to nest two or more interrupts, or to implement nesting level 3 or higher.
Before changing IST1 and IST0, disable interrupts by using the DI instruction.
Figure 7-9. Interrupt Nesting by Changing Interrupt Status Flag
Interrupt disabled
IPS set
Interrupt enabled
Interrupt with low
or high priority occurs
Interrupt
disabled
IST changed
Interrupt enabled
Interrupt with
low or high
priority occurs
Status 1
Status 0
Status 0
Status 1
Normal processing
(status 0)
Nesting of one interrupt Nesting of two interrupts