2.6.2 Configuring an expansion card

Some expansion cards need an IRQ to operate. Generally, an IRQ must be exclusively assigned to one function at a time. In a standard design configuration, 16 IRQs are available but most are already in use. Normally, 6 IRQs are free for expansion cards. If the motherboard has PCI audio onboard, an additional IRQ will be used. If your motherboard also has MIDI enabled, another IRQ will be used, leaving 4 IRQs free. Sometimes IRQs are “shared” by more than one function; in this case, IRQ assignments are swapped automatically or adjusted through the BIOS firmware.

IMPORTANT! When using PCI cards on shared slots, ensure that the drivers support “Share IRQ” or that the cards do not need IRQ assignments. Otherwise, conflicts will arise between the three PCI groups, making the system unstable and the card inoperative.

Standard Interrupt Assignments
IRQPriorityStandard Function

0

1

System Timer

1

2

Keyboard Controller

2

N/A

Programmable Interrupt

3*

11

Communications Port (COM2)

4*

12

Communications Port (COM1)

5*

13

Sound Card (sometimes LPT2)

6

14

Floppy Disk Controller

7*

15

Printer Port (LPT1)

8

3

System CMOS/Real Time Clock

9*

4

ACPI Mode when used

10*

5

IRQ Holder for PCI Steering

11*

6

IRQ Holder for PCI Steering

12*

7

PS/2 Compatible Mouse Port

13

8

Numeric Data Processor

14*

9

Primary IDE Channel

15*

10

Secondary IDE Channel

*These IRQs are usually available for ISA or PCI devices.

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Chapter 2: Hardware information