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IRQ Number: 5
16-Bit Priority: 13
Bus Line: 8/16-bit
Typical Default Use: Sound card (but varies widely).
Other Common Uses: LPT2 (second parallel port), COM3 (third serial port), COM4 (fourth serial port), modems, network cards, tape accelerator cards, hard disk controller on old PC/XT.
Description: This is probably the single "busiest" IRQ in the whole system. On the original PC/XT system this IRQ was used to control the (massive 10 MB) hard disk drive. When the AT was introduced, hard disk control was moved to IRQ14 to free up IRQ5 for 8-bit devices. As a result, IRQ5 is in most systems the only free interrupt below IRQ9 and is therefore the first choice for use by devices that would otherwise conflict with IRQ3, IRQ4, IRQ6 or IRQ7. IRQ5 is the default interrupt for the second parallel port in systems that use two printers for example. It is also the first choice that most sound cards make when looking for an IRQ setting. IRQ5 is also a popular choice as an alternate line for systems that need to use a third COM port, or a modem in addition to two COM ports.
Conflicts: Conflicts on IRQ5 are very common because of the large variety of devices that have it as an option. Since virtually every PC today uses a sound card, and they all like to grab IRQ5, it is almost always taken before you even start looking at more esoteric peripherals. If a second parallel port (LPT2) is being used to allow access to two printers or a printer and a parallel-port drive, then IRQ5 will usually be taken right away. If for some very strange reason you have three parallel ports, watch for a conflict here or with IRQ7, since 5 and 7 are the only two normally used as defaults for parallel ports. Sound cards that default to IRQ5 are generally best left there, to avoid problems with poorly written older software that just assumed the sound card would always be left at IRQ5. To whatever extent possible, move devices that can use higher-valued IRQs away from IRQ5. For example, you can't move COM3 to IRQ11, but you usually can move a network card to it. See the conflict resolution area of the Troubleshooting Expert for more ideas.