Asus P55SP4 - Manual (Page 16)

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, in the ISA bus design. Devices that need an IRQ line
to operate sometimes must have the use of that line exclusively.
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 carmot be
shared. There are different ways of setting an IRQ assignment, with
jumpers being the most common.
Both the ISA bus and the PCI bus use the same set of system IRQs.
For the PCI bus there is an 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 ex­
pansion cards use the level-triggered design. Some very few cards
may use the edge-triggered design instead. The mainboard design
therefore provides the means to set the IRQ assignment for a PCI slot
for either type of card, but performs this fimction automatically by
default. This is explained in detail in Chapters 2 & 3.
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