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What Are IRQ Numbers?

Prior to plug and play devices, users had to set IRQ values of devices manually when adding the device, such as a modem or printer, to a system. The following list of IRQ numbers specifies what each of the 16 IRQ lines are used for.

Prior to plug-and-play devices, users had to set IRQ values of devices manually when adding the device, such as a modem or printer, to a system. The following list of IRQ numbers specifies what each of the 16 IRQ lines are used for.

RQ Number
Typical Use
Description
IRQ 0System timerThis interrupt is reserved for the internal system timer. It is never available to peripherals or other devices.
IRQ 1KeyboardThis interrupt is reserved for the keyboard controller. Even on devices without a keyboard, this interrupt is exclusively for keyboard input.
IRQ 2Cascade interrupt for IRQs 8-15This interrupt cascades the second interrupt controller to the first.
IRQ 3Second serial port (COM2)The interrupt for the second serial port and often the default interrupt for the fourth serial port (COM4).
IRQ 4First serial port (COM1)This interrupt is normally used for the first serial port. On devices that do not use a PS/2 mouse, this interrupt is almost always used by the serial mouse. This is also the default interrupt for the third serial port (COM3).
IRQ 5Sound cardThis interrupt is the first choice that most sound cards make when looking for an IRQ setting.
IRQ 6Floppy disk controllerThis interrupt is reserved for the floppy disk controller.
IRQ 7First parallel portThis interrupt is normally reserved for the use of the printer. If a printer is not being used, this interrupt can be used for other devices that use parallel ports.
IRQ 8Real-time clockThis interrupt is reserved for the system's real-time clock timer and can not be used for any other purpose.
IRQ 9Open interruptThis interrupt is typically left open on devices for the use of peripherals.
IRQ 10Open interruptThis interrupt is typically left open on devices for the use of peripherals.
IRQ 11Open interruptThis interrupt is typically left open on devices for the use of peripherals.
IRQ 12PS/2 mouseThis interrupt is reserved for the PS/2 mouse on machines that use one. If a PS/2 mouse is not used, the interrupt can be used for other peripherals, such as network card.
IRQ 13Floating point unit/coprocessorThis interrupt is reserved for the integrated floating point unit. It is never available to peripherals or other devices as it is used exclusively for internal signaling.
IRQ 14Primary IDE channelThis interrupt is reserved for use by the primary IDE controller. On systems that do not use IDE devices, the IRQ can be used for another purpose.
IRQ 15Secondary IDE channelThis interrupt is reserved for use by the secondary IDE controller.









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