491CHAPTER 21 INTERRUPT AND TEST FUNCTIONS21.4 Interrupt Servicing Operations21.4.1 Non-maskable interrupt acknowledge operationA non-maskable interrupt request is received without condition even when in the interrupt request receptionprohibited 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 program statusword (PSW) and then program counter (PC), the IE and ISP flags are reset to 0, and the vector table contents areloaded into PC and branched.A new non-maskable interrupt request generated during execution of a non-maskable interrupt service programis received after the execution of the non-maskable interrupt service program that is currently processing is completed(after the RETI command is executed) and 1 command of the main routine is executed. If a new non-maskable interruptrequest is 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.The flowchart from the time a non-maskable interrupt request is generated until it is received is shown in Figure21-10, the non-maskable interrupt request acknowledge timing is shown in Figure 21-11 and reception operationsin cases where multiple non-maskable interrupt requests are generated are shown in Figure 21-12.
