IBM nec pd78p081(a2) User Manual

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 contents of program status word (PSW) and program

counter (PC), in that order, are saved to the stack, the IE flag and ISP flag are reset (0), and the contents of the vector

table are loaded in the 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.

The flowchart showing the flow from non-maskable interrupt request generation to acknowledgment is shown in

Figure 12-8, the non-maskable interrupt request acknowledge timing is shown in Figure 12-9, and acknowledge

operation in the case where multiple non-maskable interrupt requests are generated, is shown in Figure 12-10.