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Interrupts, Multiple Devices and Conflicts
In general, interrupts are single-device resources. Because of the way the system bus is designed, it is not feasible for more than one device to use an interrupt at one time, because this can confuse the processor and cause it to respond to the wrong device at the wrong time. If you attempt to use two devices with the same IRQ, an IRQ conflict will result. This is one of the types of resource conflicts.
It is possible to share an IRQ among more than one device, but only under limited conditions. In essence, if you have two devices that you seldom use, and that you never use simultaneously, you may be able to have them share an IRQ. However, this is not the preferred method since it is much more prone to problems than just giving each device its own interrupt line.
One of the most common problems regarding shared IRQs is the use of the third and fourth serial (COM) ports, COM3 and COM4. By default, COM3 uses the same interrupt as COM1 (IRQ4), and COM4 uses the same interrupt as COM2 (IRQ3). If you have a mouse on COM1 and set up your modem as COM3--a very common setup--guess what happens the first time you try to go online? :^) You can share COM ports on the same interrupt, but you have to be very careful not to use both devices at once; in general this arrangement is not preferred. See here for ideas on dealing with COM port difficulties.
Many modems will let you change the IRQ they use to IRQ5 or IRQ2, for example, to avoid this problem. Other common areas where interrupt conflicts occur are IRQ5, IRQ7 and IRQ12. The conflict resolution area of the Troubleshooting Expert can sometimes help with these situations.