PC Hardware A Beginner’s Guide

(^240) PC Hardware: A Beginner’s Guide

Interrupt Requests (IRQs)

Let’s say that it is your job to provide a service to a large number of people in an office

setting. In your office, there is a desk with a single bare light bulb on it. If any of the

people you are to take care of need your help, they flip a switch on their desk and it turns

on your light. Each time the light lights up, you have to drop whatever you were doing to

take care of whatever is needed for whomever lit it. As willing as you are to serve, the

problem is that you don’t know who turned on the light to request help. The solution to

this problem is to place numbered individual lights on your desk, one for each person

who can request your services. Now when a light lights up, you will be able to determine

just whom you should be assisting.

As strange as this situation may seem, it is essentially the way that the CPU interfaces

with the devices installed on the PC. Like the people in the office, each device is assigned

an IRQ (interrupt request) that they can turn on to signal to the CPU that some kind of service

is needed. The services needed might be to move data from RAM to a device, transfer

data from a device to RAM, or the like. Whatever the need, the device requests service

from the CPU by turning on its IRQ. The CPU interrupts whatever it is doing and services

the request.

Chapter 13 discusses IRQs in more detail, but one issue that is common to nearly all

expansion cards is IRQ conflicts. When two devices are assigned the same IRQ, the CPU

cannot know which device requested the service. IRQ conflicts occur for a number of

reasons, but the most common is proprietary installation software that preassigns the

system resources, including the IRQ, rather than assigning available resources. Although

there is no established standard for the assignment of IRQs, the list in Table 11-1 represents

the default assignments used by the majority of processors and BIOS manufacturers.

The available IRQs listed in Table 11-1 are available to be used by any expansion card

added to the system. IRQs 3 and 4 are shared among the COM (serial) ports because all

four ports are rarely installed on a PC and, if they are, they are rarely in use at the same

time. On the PC, two devices can be assigned to share an IRQ, but only in situations

where just one of the devices is active at a time.

IRQ Default Assignment

0 System timer

1 Keyboard

2 Video card

3 COM2, COM4

Table 11-1. Default IRQ Assignments on a PC