Interrupt Processing: Non Maskable Interrupt

Non maskable interrupt cannot be disabled.
If there is an interrupt request, the CPU will perform the interrupt processing unconditionally.
The non maskable interrupt is used for emergency processing, for example, data backup processing such as power outage processing.

There is a watchdog timer as the non maskable interrupt.
The watchdog timer checks whether the microcontroller is operating normally or not.
When an abnormal operation is detected, it generates the watchdog timer interrupt to notify the error to the CPU or resets the CPU. 

Reset
The microcontroller can be reset generally when the power is turned on.
Reset returns the microcontroller configurations to the initial state.
In reset, the program will run from the beginning.