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Non-Maskable Interrupts (NMI)
All of the regular interrupts that we normally use and refer to by number are called maskable interrupts. The processor is able to mask, or temporarily ignore, any interrupt if it needs to, in order to finish something else that it is doing. In addition, however, the PC has a non-maskable interrupt (NMI) that can be used for serious conditions that demand the processor's immediate attention. The NMI cannot be ignored by the system unless it is shut off specifically.
When an NMI signal is received, the processor immediately drops whatever it was doing and attends to it. As you can imagine, this could cause havoc if used improperly. In fact, the NMI signal is normally used only for critical problem situations, such as serious hardware errors. The most common use of NMI is to signal a parity error from the memory subsystem. This error must be dealt with immediately to prevent possible data corruption.