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PC Hardware in a Nutshell, 3rd Edition by Barbara Fritchman Thompson, Robert Bruce Thompson

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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 ...

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