PC Hardware in a Nutshell, 3rd Edition

Interrupt Request Line (IRQ)

When a component or peripheral, such as a network adapter or sound card, needs to get the CPU’s attention, it does so by generating a signal on an Interrupt Request Line (IRQ). Table 1-2 lists IRQs and the devices that typically use them.

Table 1-2. /16/32-bit ISA/PCI standard IRQ assignments

IRQ	Bus type	Typically used by
00	none	Non-maskable Interrupt (NMI); system timer
01	none	Keyboard port
02	none	Programmable Interrupt Controller (PIC); cascade to IRQ 09
03	8/16-bit	Communications Port 2 (COM2:)
04	8/16-bit	Communications Port 1 (COM1:)
05	8/16-bit	Sound card; printer port (LPT2:)
06	8/16-bit	Floppy disk controller
07	8/16-bit	Printer port (LPT1:)
08	none	System CMOS/real-time clock
09	8/16-bit	Redirected from IRQ 02; network interface
10	16-bit	Network interface; USB host controller
11	16-bit	Video adapter; SCSI host adapter
12	16-bit	PS/2 mouse port
13	none	Numeric data processor (math coprocessor)
14	16-bit	Primary IDE interface
15	16-bit	Secondary IDE interface

An 8-bit ISA slot contains physical IRQ lines only for IRQ 02 through IRQ 07 because IRQ 00 and IRQ 01 are reserved for system functions. A 16-bit ISA slot contains physical IRQ lines for IRQs 03 through 07, 09 through 12, 14, and 15. IRQ 09 is mapped to IRQ 02, allowing 8-bit ISA cards to recognize IRQ 09 as IRQ 02. IRQs 00, 01, 02, 08, and 13 are not present in any slot, and so cannot be assigned to devices.

If the processor receives ...