IRQ

12/7/96

IRQs are hardware interrupt lines associated with the ISA bus. IRQs 0-7 were implemented on the original IBM PC. IRQs 8-15 (decimal numbers) were added on the PC-AT. IRQs should not be confused with INTs which are interrupt numbers seen by the software. IRQs 0-7 are tied to INTs 8 through F(hexadecimal numbers). IRQs 8-15 (decimal) are tied to INTs 70 through 77(hex). The PC architecture uses cascaded Interrupt controllers so that the IRQs added in the AT also cause an interrupt to appear on the IRQ2 line. For this reason, IRQ2 is often indicated as IRQ2/9.

Conventional IRQ assignments are:

IRQ No IRQ Type

IRQ0 54.9 mSec Motherboard Clock Interrupt

IRQ1 Keyboard Interrupt

IRQ2 Cascade for IRQs 8-15 on ATs

IRQ3 COM2 Serial Port

Also COM4, but much software will not support

Frequently default assignment for Network Interface Cards

IRQ4 COM1 Serial Port

Also COM3, but much software will not support

IRQ5 Second Printer, but DOS/Windows 3 do not use and OS/2,Novell use is optional

Also Sound Cards

Hard Drive Controller Interrupt on XT

IRQ6 Floppy Disk Attention (End of Operation)

IRQ7 Primary Printer, but DOS/Windows 3 do not use and OS/2,Novell use is optional

Also Sound Cards

Also triggered by some Interrupt Controller errors.

IRQ8 Real Time Clock 1KHz Interrupt
IRQ9 Network Interface Cards (but many default to IRQ3)

Microsoft Mouse (labeled IRQ2)

Vertical retrace on older video cards

IRQ10 No fixed Assignment

IRQ11 No fixed assignment. Used by some VGA cards

IRQ12 PS/2 Mouse

IRQ13 External math Coprocessor

IRQ14 IDE Disk/CDROM Primary Channel

IRQ15 IDE Disk/CDROM Secondary Channel

IRQ No	IRQ Type
IRQ0	54.9 mSec Motherboard Clock Interrupt
IRQ1	Keyboard Interrupt
IRQ2	Cascade for IRQs 8-15 on ATs
IRQ3	COM2 Serial Port
	Also COM4, but much software will not support
	Frequently default assignment for Network Interface Cards
IRQ4	COM1 Serial Port
	Also COM3, but much software will not support
IRQ5	Second Printer, but DOS/Windows 3 do not use and OS/2,Novell use is optional
	Also Sound Cards
	Hard Drive Controller Interrupt on XT
IRQ6	Floppy Disk Attention (End of Operation)
IRQ7	Primary Printer, but DOS/Windows 3 do not use and OS/2,Novell use is optional
	Also Sound Cards
	Also triggered by some Interrupt Controller errors.
IRQ8	Real Time Clock 1KHz Interrupt
IRQ9	Network Interface Cards (but many default to IRQ3)
	Microsoft Mouse (labeled IRQ2)
	Vertical retrace on older video cards
IRQ10	No fixed Assignment
IRQ11	No fixed assignment. Used by some VGA cards
IRQ12	PS/2 Mouse
IRQ13	External math Coprocessor
IRQ14	IDE Disk/CDROM Primary Channel
IRQ15	IDE Disk/CDROM Secondary Channel

In the PC architecture, IRQs are used to signal changes in hardware state that require software attention.

Although IRQs are, in concept, easily shared by properly written software, "properly written software" is virtually non-existent except for PCI/USB devices. In practice, it is almost always necessary to allocate each interrupting device to a different IRQ. Most PC hardware options require at least one IRQ and some require several. It is not uncommon to run out of IRQs.

Some IRQs are hard wired. The remainder are set by physical jumpers or by special software that configures the device temporarily or permanently.