Each hardware device controller capable of issuing interrupt requests usually has a single
output line designed as the Interrupt ReQuest (IRQ) line. All existing IRQ lines are connected
to the input line of a hardware circuit called the Programmable Interrupt Controller, which
performs the following actions.
1. Monitors the IRQ lines, checking for raised signals. If two or more IRQ lines are raised, selects
the one having the lower pin number.
2. If a raised signal occurs on an IRQ line:
a. Converts the raised signal received into a corresponding vector.
b. Stores the vector in an Interrupt Controller I/O port, thus allowing the CPU to read it via the
data bus.
c. Sends a raised signal to the processor INTR pin --- that is, issues an interrupt.
d. Waits until the CPU acknowledges the interrupt signal by writing into one of the Programmable
Interrupt Controller (PIC) I/O ports; when this occurs, clears the INTR line.
3. Go back to step 1.
ULK --- Chap 4: IRQs and Interrupts (Note)
原文：http://www.cnblogs.com/miaoyong/p/4997261.html