Interrupt Requests

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An IRQ is a wire incorporated into the motherboard's bus that is used by a device, such as a printer or a keyboard, as a mechanism to capture the attention of the CPU for a request of service.

The default IRQ assignment for a standard 101 keyboard is IRQ 1. When you enter data using the keyboard, you are requesting the CPU to stop what it is doing and take notice of your request to input data. Your request is sent to the CPU by the use of IRQ 1.

There are 16 IRQ assignments in a computer system. (Refer to Table 5.3 for typical system default IRQ settings.) Early computers had eight IRQs. As the need for more devices increased, another eight IRQs were added.

For the exam, remember that the system reserves IRQ 2 to connect the two sets of eight IRQs. In other words, IRQ 2 is ‘cascaded' to IRQ 9, which provides the usage of IRQs 9 through 15. Of the 15 IRQs available, 10 are used for I/O devices and 5 are reserved for system devices.

Table 5.3: Typical System Default IRQ Assignments

IRQ	Device Assigned
0	System timer
1	Standard 101/102 keyboard
2	Interrupt controller (cascaded to IRQ 9)
3	COM2 and COM4 (serial ports 2 and 4)
4	COM1 and COM3 (serial ports 1 and 3)
5	LPT2 (extra printer or sound card)
6	Floppy drive controller
7	LPT1 (parallel port)
8	Real-Time Clock (RTC)
9	Cascaded to IRQ 2
10	Available (advanced audio)
11	Available (SCSI or VGA card)
12	PS/2 mouse
13	Math coprocessor
14	Primary hard drive controller (IDE)
15	Secondary hard drive controller

An IRQ is connected to every port, slot, and device on the motherboard. The IRQ assignments are typically handled by the system BIOS settings on start-up. Some expansion cards and peripheral devices (usually legacy, non-plug-and-play devices) are configured manually through the use of jumpers on the motherboard or on the device. A Network Interface Card (NIC), for example, may allow you to change the memory address and IRQ settings with the use of plastic jumpers on the card. It is important to note that an IRQ can only be assigned to one active device at a time. Multiple devices can be assigned to the same IRQ. If two devices attempt to use the same IRQ at the same time, an IRQ conflict may occur. IRQ conflicts typically occur when new devices, such as sound cards, modems, and NICs, are added to a system with their manufacturer's default settings. For example, suppose a sound card is installed that is assigned to IRQ 5. Then you install a NIC that has a preassigned manufacturer's setting of IRQ 5. You reboot the system and notice that the new NIC is not recognized or won't function. Chances are that the sound card is currently using IRQ 5, and a conflict has occurred. You will have to manually assign the NIC to an open IRQ.

To check for any device conflicts on a system (assuming use of Windows 2000), click Start > Settings > Control Panel > System > Hardware > Device Manager. If you see a yellow diamond containing a black exclamation point, you have a device conflict.

IRQ 14 is reserved for the primary IDE or ATA controller (hard drive controller). Two devices can be attached to the primary IDE controller. The first device is the master, or primary hard drive. The second device attached to your primary IDE/ATA controller is the slave, or secondary device. This secondary device is typically another hard drive, CD-ROM, DVD-ROM, or DVD-R. If you wanted to add a third and fourth drive, you would need to use the secondary IDE controller and IRQ 15, which also allow two more devices to be attached. Refer to Figure 5.2 for a typical Windows 2000 display of IRQ assignments.

Figure 5.2: Windows 2000 display of IRQ settings.

The old A+ Hardware core exam asked simple IRQ questions such as, 'What is IRQ 2 used for?' The current A+ Hardware core test will likely have you resolve at least one basic IRQ conflict. The review questions at the end of this chapter will help you to sharpen your IRQ conflict-resolution skills.

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