Interrupt types
  Mask-able or plain interrupts. (IRQ) 
    Represents conditions that affect activity but not system integrity.
      can be ignored.
      can be delayed.
      can be processed.

    IBM 360 has a decimal overflow mask. If overflow occurs and mask off, 
      program crashes. If masked, program continues - programmer is expected
      to code a check where overflow might occur.
  
  Non-maskable. (NMI)
    must be processed (usually represent catastrophic condition).
      generated by software.
        invalid commands.
        illegal memory accesses.

      generated by hardware.
        hardware failure.
        real time critical data transfers.