linux/Documentation/IRQ.txt
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   1===============
   2What is an IRQ?
   3===============
   4
   5An IRQ is an interrupt request from a device.
   6Currently they can come in over a pin, or over a packet.
   7Several devices may be connected to the same pin thus
   8sharing an IRQ.
   9
  10An IRQ number is a kernel identifier used to talk about a hardware
  11interrupt source.  Typically this is an index into the global irq_desc
  12array, but except for what linux/interrupt.h implements the details
  13are architecture specific.
  14
  15An IRQ number is an enumeration of the possible interrupt sources on a
  16machine.  Typically what is enumerated is the number of input pins on
  17all of the interrupt controller in the system.  In the case of ISA
  18what is enumerated are the 16 input pins on the two i8259 interrupt
  19controllers.
  20
  21Architectures can assign additional meaning to the IRQ numbers, and
  22are encouraged to in the case  where there is any manual configuration
  23of the hardware involved.  The ISA IRQs are a classic example of
  24assigning this kind of additional meaning.
  25