Each piece of hardware in a PC has an Interrupt Request (IRQ) and an Address. These are usually used by the CPU to identify each item and communicate with it when needed. Should two components be given thesame IRQ and thesame address, then a hardware conflict will occur. An error Message May be displayed and in some cases the computer may malfunction. The following Table shows a typical allocation of IRQs but this may vary in some detail from one computer to another

IRQ-------- Used By

0 -------------- System Timer
1 -------------- Keyboard
2 -------------- Interrupt Controller
3 -------------- COM2 and COM4 (external modem usually on COM2)
4 -------------- COM1 and COM3 (old models have mouse on COM1)
5 -------------- LPT2 (may be sound card)
6 -------------- Floppy Disk
7 -------------- LPT1
8 -------------- Real-Time Clock
9 -------------- May be used for NIC (Network Interface Card)
10 ------------ Usually CD drive controller
11 ------------ Display Adapter
12 ------------ May be used for NIC or tape drive
13 ------------ Numeric data processor
14 ------------ Fixed (hard) disk
15 ------------ BIOS

When installing add-on cards, you may be asked to set the IRQ and the address for that card. Alternatively, if you have Plug and Play (PnP) enabled (PCI motherboards), the computer will select and allocate the IRQ and the address automatically. If you motherboard does not have PnP, you can obtain software that can be installed to emulate PnP.

Most computers have only one parallel port (LPT1) and although provision is made for LPT2 on IRQ5, in its absence other cards may use IRQ5. This is typically used when a sound card is installed. In other situations it may be used by a network interface card (NIC).

Addresses are usually quoted in hexadecimal and can be confusing. Typically COM1 has an address of 3F8h. The h indicates hexadecimal and usually left off. When a computer is completing its Power-On-Self-Test (POST), it usually displays a table of hardware items just before handing over to the software tool. This table shows COM port and the parallel port address and the 3F8h is for COM1 and 2F8h is for COM2. The parallel port LPT1 is given the address 378h. If you have an internal modem set to COM4, it may have the address 2E8h. This will depend on the jumper settings or the PnP configuration.

The serial ports COM2/COM4 share IRQs but have different addresses, so avoiding hardware conflicts.