What is an interrupt?
   An interrupt is a hardware signal that tells the CPU to 
   temporarily stop what it is doing and go do something else.  
   Without interrupts, the CPU would have to constantly check for 
   external events; with interrupts, the CPU can work on 
   something else and still respond to an event as soon as it 
   occurs. 

   CPUs typically have an instruction to disable interrupts for 
   use when a section of code has to run without being disturbed 
   by external events.  Because of this, most CPUs also have a 
   special interrupt called a Non-Maskable Interrupt (NMI), which 
   is responded to even when all other interrupts are disabled.  
   The NMI is used to signal calamities such as memory failure or 
   imminent power loss.

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