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0001 What is an IRQ?
0002 
0003 An IRQ is an interrupt request from a device.
0004 Currently they can come in over a pin, or over a packet.
0005 Several devices may be connected to the same pin thus
0006 sharing an IRQ.
0007 
0008 An IRQ number is a kernel identifier used to talk about a hardware
0009 interrupt source.  Typically this is an index into the global irq_desc
0010 array, but except for what linux/interrupt.h implements the details
0011 are architecture specific.
0012 
0013 An IRQ number is an enumeration of the possible interrupt sources on a
0014 machine.  Typically what is enumerated is the number of input pins on
0015 all of the interrupt controller in the system.  In the case of ISA
0016 what is enumerated are the 16 input pins on the two i8259 interrupt
0017 controllers.
0018 
0019 Architectures can assign additional meaning to the IRQ numbers, and
0020 are encouraged to in the case  where there is any manual configuration
0021 of the hardware involved.  The ISA IRQs are a classic example of
0022 assigning this kind of additional meaning.