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IRQ Lines and the System Bus
The devices that use interrupts trigger them by signaling over lines provided on the ISA system bus. Most of the interrupts are provided to the system bus for use by devices; however, some of them are only used internally by the system, and therefore they are not given wires on the system bus. These are interrupts 0, 1, 2, 8 and 13, and are never available to expansion cards (remember, IRQ2 is now wired to IRQ9 on the motherboard).
As explained in this section on the ISA bus, the original bus was only 8 bits wide and had a single connector for expansion cards. The bus was expanded to 16 bits and a second connector slot added next to the first one; you can see this if you look at your motherboard, since all modern PCs use 16-bit slots.
The addition of this extra connector coincided with the addition of the second interrupt controller, and the lines for these extra IRQs were placed on this second slot. This means that in order to access any of these IRQs--10, 11, 12, 14 and 15--the card must have both connectors. While almost no motherboards today have 8-bit-only bus slots, there are still many expansion cards that only use one ISA connector. The most common example is an internal modem. These cards can only use IRQs 3, 4, 5, 6 and 7 (and 6 is almost always not available since it is used by the floppy disk controller). They can also use IRQ 9 indirectly if they have the ability to use IRQ2, since 9 is wired to where 2 used to be.
Note: All of this applies
to ISA and VESA local bus slots only. PCI slots handle interrupts differently, using their
own internal interrupt system. If a PCI
card needs to use a regular IRQ line the BIOS/chipset will normally "map" the
PCI interrupt to a regular system interrupt. This is normally done using IRQ9 to IRQ12.
Next: Interrupt Priority