So what exactly is NMI?  the Interrupt Request Level defines the hardware priority at which a processor operates.  These interrupt request levels allow processes to mask (or block) interrupt requests to have the processor perform a task.  Thus, NMI – the Non-Maskable Interrupt -  is basically “God Mode” for Interrupt Requests, and by extension the processor.  NMI requests cannot be blocked and are reserved for very high priority tasks.

Full Article