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What is an IRQ?

An IRQ is an interrupt request from a device.

Currently they can come in over a pin, or over a packet.

Several devices may be connected to the same pin thus

sharing an IRQ.

An IRQ number is a kernel identifier used to talk about a hardware

interrupt source.  Typically this is an index into the global irq_desc

array, but except for what linux/interrupt.h implements the details

are architecture specific.

An IRQ number is an enumeration of the possible interrupt sources on a

machine.  Typically what is enumerated is the number of input pins on

all of the interrupt controller in the system.  In the case of ISA

what is enumerated are the 16 input pins on the two i8259 interrupt

controllers.

Architectures can assign additional meaning to the IRQ numbers, and

are encouraged to in the case  where there is any manual configuration

of the hardware involved.  The ISA IRQs are a classic example of

assigning this kind of additional meaning.