Asus P I-AP55T P/I-AP55T User's manual - Page 15

Feature Guide System IRQs Later in the manual you'll see something called an "IRQ" mentioned several times. If you're not familiar with these, this is a short explanation of what they are and why you may need to know about them if you upgrade your system. An IRQ, or interrupt request, is the process whereby an input or output device tells the CPU to temporarily interrupt whatever it is doing and immediately process something from the source of the interrupt. When finished the CPU goes back to what it was already processing. This happens very quickly. There are 16 IRQs, IRQ 0 through IRQ 15, some of which are already in use by basic components of the system. Devices that need an IRQ line to operate sometimes must have the exclusive use of that line. Many expansion cards require the use of an IRQ line to operate, for example, network interface cards and sound cards. When you install a card that uses an IRQ, it will have a default IRQ setting that you might need to change if that IRQ is already in use and cannot be shared. There are different ways of setting an IRQ assignment, with jumpers being the most common. The ISA and PCI buses all use the same set of system IRQs. On this mainboard the PCI-Bus Primary IDE channel uses IRQ14 and the Secondary channel uses IRQ15. For the PCI bus there is art additional consideration. On the PCI bus, you must assign an IRQ to the PCI slot you will install an IRQ-using card in. There are two methods of generating an IRQ on the PCI bus, level-triggering (level-sensitive) and edge-triggering. Most PCI expansion cards use the level-triggered design. A few cards may use the edge-triggered design instead. The BIOS PCI Slot Configuration therefore provides the means to set the IRQ assignment for a PCI slot for either method. This is explained in detail in Chapter 3.