- Compare the IRQ assignments of your PC to those in Table 13-1. They should
match for the most part. Any exceptions are likely because of Plug-and-Play
devices or adjustments that were made to avoid conflicts (more on that later).
If there are differences, don’t change your IRQ settings. Table 13-1 lists typical,
or what are called default, settings. They are by no means the only settings
that will work.
IRQ Connections
Interrupt requests are wires in the system bus on the motherboard. Each IRQ wire is con-
nected to every one of the expansion slots on the motherboard. Regardless of which ex-
pansion slot an I/O adapter is placed in, it has access to the IRQs of the PC. As illustrated
in Figure 13-3, each expansion slot can be assigned a particular IRQ line. The type of con-
troller or adapter and its preset values determines which IRQ line it is assigned. Each de-
vice can have only one IRQ assignment. The number of the assigned IRQ is what
identifies the device to the CPU when the device requests services.
When a device sends an IRQ signal over its bus line, the bus line number identifies the
device. When the CPU has completed the requested task, it sends a clearing signal over
the IRQ bus line and the device knows it may proceed.
An IRQ can be assigned to multiple devices, but you must do so carefully. If two ac-
tive devices are sharing a single IRQ, the CPU will have no way of knowing which device
sent the request. In fact, the CPU never knows if there is more than one device on an IRQ;
it only knows that whatever is attached to the other end of the IRQ line has sent a request
for services. If two devices are in contention for a single IRQ line, one device could be try-
ing to process data intended for the other. In addition, there is the physical danger of two
active devices sending bus signals (which means a certain number of volts on the line) at
the same time. This could short the bus, the motherboard, or the device controller.(^286) PC Hardware: A Beginner’s Guide
Figure 13-3. IRQ bus wires connected to the expansion ports