Asus P/I-AP55T - Manual (Page 15)

Feature Guide
System IRQs
Later in the manual you'll see something called an "IRQ" men­
tioned 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 compo­
nents of the system. Devices that need an IRQ line to operate some­
times 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 soimd 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 ГОЕ channel uses IRQ14 and the
Secondary channel uses IRQ15. For the PCI bus there is an additional
consideration. On the PCI bus, you must cissign an IRQ to the PCI slot
you will install an IRQ-using card in. There are two methods of gen­
erating 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.
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