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.

IRQ Number	Typical Use	Description
IRQ 0	System timer	This interrupt is reserved for the internal system timer. It is never available to peripherals or other devices.
IRQ 1	Keyboard	This interrupt is reserved for the keyboard controller. Even on devices without a keyboard, this interrupt is exclusively for keyboard input.
IRQ 2	Cascade interrupt for IRQs 8-15	This interrupt cascades the second interrupt controller to the first.
IRQ 3	Second serial port (COM2)	The interrupt for the second serial port and often the default interrupt for the fourth serial port (COM4).
IRQ 4	First 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 5	Sound card	This interrupt is the first choice that most sound cards make when looking for an IRQ setting.
IRQ 6	Floppy disk controller	This interrupt is reserved for the floppy disk controller.
IRQ 7	First parallel port	This 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 8	Real-time clock	This interrupt is reserved for the system's real-time clock timer and can not be used for any other purpose.
IRQ 9	Open interrupt	This interrupt is typically left open on devices for the use of peripherals.
IRQ 10	Open interrupt	This interrupt is typically left open on devices for the use of peripherals.
IRQ 11	Open interrupt	This interrupt is typically left open on devices for the use of peripherals.
IRQ 12	PS/2 mouse	This 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 13	Floating point unit/coprocessor	This 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 14	Primary IDE channel	This 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 15	Secondary IDE channel	This interrupt is reserved for use by the secondary IDE controller.