IRQ

Interrupts are in some ways the most "famous" system resources, since almost everyone who's used a computer has heard of them--even if they don't know what they are. An interrupt is basically what it sounds like, a message from one part of the computer to another (normally to the system processor) that tells it that it needs to stop what it is doing, and do something else instead. An IRQ is an interrupt request, and is the name for the actual signal that is used when a peripheral requests an interrupt of the processor.

Interrupts play a key role in how the processor performs input/output processing, and interfaces with every peripheral in the computer, from the keyboard and mouse to the hard disk and modem.

IRA Most common use Description
0 System Timer This is the reserved interrupt for the internal system timer. It is used exclusively for internal operations and is never available to peripherals or user devices.
1 Keyboard controller / Keyboard This is the reserved interrupt for the keyboard controller. It is used exclusively for keyboard input. Even on systems without a keyboard, IRQ1 is not available for use by other devices. Note that the keyboard controller also controls the PS/2 style mouse if the system has one, but the mouse uses a separate line, IRQ12.
2 Cascade for IRA's 8 to 15 Not generally used. Can be used by modems, very old (EGA) video cards, as an alternative IRQ for COM3 (third serial port) or COM4 (fourth serial port). Rerouted to IRQ9 and appears to software as IRQ9.
3 COM2/4 This interrupt is normally used by the second serial port, COM2. It is also the default interrupt for the fourth serial port, COM4, and a popular option for modems, sound cards and other devices. Modems often come pre-configured to use COM2 on IRQ3.
4 COM1/3 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.
5 Sound card(varies widely) 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.
6 Floppy Disk Controller This interrupt is reserved for use by the floppy disk controller. Technically, it is available for use by other devices, and some devices will allow you to select IRQ6. Most however do not, realizing that virtually every PC uses at least one floppy disk drive. The most common devices that will let you use IRQ6 are probably tape drive accelerator cards. This is probably because these cards are used for tape drives that run off the floppy interface, and many of them can be set to drive floppy disks themselves.
7 LPT1 This IRQ is used on most systems to drive the first parallel port, normally for the use of a printer. These days of course many other devices use parallel ports, including external drives. If you are not using a printer or other device then IRQ7 can be used in a similar way to IRQ5: as an alternate for any of the devices that would normally be fighting over IRQ3 or IRQ4.
8 Real-time clock This is the reserved interrupt for the real-time clock timer. This timer is used by software programs to manage events that must be calibrated to real-world time; this is done by setting "alarms", which trigger this interrupt at a specified time. For example, if you are using an electronic datebook and have it set to pop up screen messages or beep the PC when it is time for a meeting, the software will set a timer to count down to the appropriate time. When the timer finishes its countdown, an interrupt will be generated on IRQ8.
9 NIC's, Sound cards, rerouted IRQ 2 devices This is usually an open IRQ on most systems, and is a popular choice for use by peripherals, especially network cards. On most PCs it can be used freely since it has no default setting.
10 NIC's, sound cards, Video Cards, PCI devices This is usually open and one of the easiest IRQs to use since it is generally not contested by many devices. While the secondary IDE controller can sometimes be set to use IRQ10, it almost always uses IRQ15 instead.
11 NIC's, sound cards, tertiary IDE channel, Video cards This line is usually open and relatively easy to use since it is generally not contested by many devices. If you are using three IDE channels (the third typically being on a sound card), IRQ11 is typically the one that the tertiary controller will try to use. Also, some PCI video cards will try to use IRQ11.
12 PS/2 mouse On machines that use a PS/2 mouse, this is the IRQ reserved for its use. Using a PS/2 mouse frees up the COM1 serial port and the interrupt it uses (IRQ4) for other devices. Normally this is a good trade since free IRQs with numbers below 8 are harder to find than ones above 8. If a PS/2 mouse is not used, IRQ12 is a good choice for use by other devices such as network cards.
13 Math Coprocessor This is the reserved interrupt for the integrated floating point unit (on 80486 or later machines) or the math coprocessor (on 80386 or earlier machines that use one). It is used exclusively for internal signaling and is never available for use by peripherals.
14 Primary IDE channel On most PCs, this IRQ is reserved for use by the primary IDE controller, which provides access to the first two IDE/ATA devices (usually hard disk drives and/or CD-ROM drives). On machines that do not use IDE devices at all, this IRQ can be used for another purpose (such as a SCSI host adapter to provide SCSI drives). In order to do this, you will normally have to disable the IDE channel using either the appropriate BIOS setting (for integrated IDE support on newer boards) or jumpers on the controller board (for older machines that use an IDE controller card).
15 Secondary IDE channel On most newer PCs, this IRQ is reserved for use by the secondary IDE controller, which provides access to the third and fourth IDE/ATA devices (usually hard disk drives and/or CD-ROM drives). If you are not using IDE, or are using only two devices and want to put them on the primary channel to free up this IRQ, that can be done easily as long as you remember to disable the secondary IDE channel using either the appropriate BIOS setting (for integrated IDE support on newer boards) or jumpers on the controller board (for older machines that use an IDE controller card).
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