NEC uPD78P078 - Manual (Page 513)

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CHAPTER 22 INTERRUPT FUNCTIONS
22.4 Interrupt Servicing Operations
22.4.1 Non-maskable interrupt request acknowledge operation
A non-maskable interrupt request is unconditionally acknowledged even if in an interrupt request 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 acknowledged interrupt is saved in the stacks, PSW
and PC, in that order, the IE and ISP flags are reset to 0, and the vector table contents are loaded into PC and
branched.
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. If a new non-maskable interrupt request
is generated twice or more during non-maskable interrupt service program execution, only one non-maskable
interrupt request is acknowledged after termination of the non-maskable interrupt service program execution.
Figure 22-10 shows the flowchart from non-maskable interrupt request generation to acknowledge. Figure 22-
11 shows the non-maskable interrupt request acknowledge timing. Figure 22-12 shows the acknowledge operation
if multiple non-maskable interrupt requests are generated.