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CHAPTER 19 INTERRUPT FUNCTIONS
User’s Manual U15798EJ2V0UD
19.4 Interrupt Servicing Operations
19.4.1 Non-maskable interrupt request acknowledge operation
A non-maskable interrupt request is unconditionally acknowledged even if in an interrupt acknowledge disable
state. It does not undergo interrupt priority control and has highest priority over all other interrupts.
If a non-maskable interrupt request is acknowledged, the contents are saved into the stacks in the order of PSW,
then PC, the IE flag and ISP flag are reset (0), and the contents of the vector table are loaded into PC and branched.
At this time, the NMIS flag is set (1) to disable the acknowledgment of multiple interrupts.
A new non-maskable interrupt request generated during execution of a non-maskable interrupt servicing program
is acknowledged after the current execution of the non-maskable interrupt servicing program is terminated (following
RETI instruction execution) and one main routine instruction is executed. However, if a new non-maskable interrupt
request is generated twice or more during non-maskable interrupt servicing program execution, only one non-
maskable interrupt request is acknowledged after termination of the non-maskable interrupt servicing program.
Figures 19-7, 19-8, and 19-9 show the flowchart of non-maskable interrupt request generation through acknowledg-
ment, acknowledgment timing of non-maskable interrupt request, and acknowledgment operation at multiple non-
maskable interrupt request generation, respectively.
Remark
The NMIS flag is set (1) by acknowledging a non-maskable interrupt and cleared by the RET or RETI
instruction. When a non-maskable interrupt occurs, restore from the interrupt using the RETI instruction.
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