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IRQ Number: 4
16-Bit Priority: 12
Bus Line: 8/16-bit
Typical Default Use: COM1 (first serial port).
Other Common Uses: COM3 (third serial port), modems, sound cards, network cards, tape accelerator cards.
Description: This interrupt is normally used by the first serial port, COM1. On PCs that do not use a PS/2-style mouse, this port (and thus this interrupt) are almost always used by the serial mouse. IRQ4 is also the default interrupt for the third serial port, COM3, and a popular option for modems, sound cards and other devices. Modems sometimes come pre-configured to use COM3 on IRQ4.
Conflicts: Conflicts on IRQ4 are relatively common, although not as common as on IRQ3. On systems that do not use a serial mouse, problems are less common, because COM1 isn't automatically busy whenever the mouse is in use. The two biggest problem areas are modems that attempt to use COM3/IRQ4 and clash with COM1, and systems that attempt to use both COM1 and COM3 simultaneously on this same interrupt line. In most cases the problem can be avoided by changing the conflicting device to a different interrupt (IRQ2 and IRQ5 usually being the best choices). If a PS/2 mouse is being used, you can disable the built-in COM1 port in the BIOS setup, which will allow a modem to stay at COM3/IRQ4 without causing any problems. However, this is not really recommended. More general solutions to these issues can be found in the conflict resolution area of the Troubleshooting Expert.