513CHAPTER 22 INTERRUPT FUNCTIONS22.4 Interrupt Servicing Operations22.4.1 Non-maskable interrupt request acknowledge operationA non-maskable interrupt request is unconditionally acknowledged even if in an interrupt request acknowledgedisable 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, PSWand PC, in that order, the IE and ISP flags are reset to 0, and the vector table contents are loaded into PC andbranched.A new non-maskable interrupt request generated during execution of a non-maskable interrupt servicing programis acknowledged after the current execution of the non-maskable interrupt servicing program is terminated (followingRETI instruction execution) and one main routine instruction is executed. If a new non-maskable interrupt requestis generated twice or more during non-maskable interrupt service program execution, only one non-maskableinterrupt 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 operationif multiple non-maskable interrupt requests are generated.
