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:

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.

Return To Index Copyright 1994-2002 by Donald Kenney.