NEC switch - Manual (Page 171)

CHAPTER 11 INTERRUPT FUNCTIONS
User’s Manual U12978EJ3V0UD
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11.4 Interrupt Servicing Operation
11.4.1 Non-maskable interrupt acknowledgment operation
The non-maskable interrupt is unconditionally acknowledged even when interrupts are disabled. It is not subject
to interrupt priority control and takes precedence over all other interrupts.
When the non-maskable interrupt request is acknowledged, the PSW and PC are saved to the stack in that order,
the IE flag is reset to 0, the contents of the vector table are loaded to the PC, and then program execution branches.
Caution During non-maskable interrupt service program execution, do not input another non-maskable
interrupt request; if it is input, the service program will be interrupted and the new interrupt
request will be acknowledged.