A general description of IRQ sharing in Windows XP (314068)

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In Windows, peripheral component interconnect (PCI) devices can share IRQs. In accordance with the Plug and Play capability that is defined by the PCI specification, adapters are configured by the computer BIOS and are then examined by the operating system and changed if necessary. Typically, PCI devices have IRQs shared among them, especially on Advanced Configuration and Power Interface (ACPI) computers that have Windows ACPI support enabled.

In Windows XP, Device Manager may list some or all the devices on your ACPI motherboard as using the same IRQ (IRQ 9). (To view the list of resources, click either Resources by type or Resources by connection on the View menu). No option is available to change the IRQ setting. Windows takes advantage of the ACPI features of the motherboard, including advanced PCI sharing. The PCI bus uses IRQ 9 for IRQ steering. With this feature, you can add more devices without IRQ conflicts.

Windows XP cannot rebalance resources in the same way that Microsoft Windows 98 does. After PCI resources are set, they generally cannot be changed. If you change to an incorrect IRQ setting or I/O range for the bus that a device is on, Windows XP cannot compensate by rebalancing the resource that was assigned to that bus.

Windows XP does not have this ability because of the more complex hardware schemas that Windows XP is designed to support. Windows 98 does not have to support IOAPICs, multiple root PCI buses, multiple-processor systems, and other highly complex hardware schemas. When you are dealing with these hardware schemas, rebalancing becomes risky and therefore is not implemented in Windows XP except for very specific scenarios. However, PCI devices must be able to share IRQs. Generally, the ability to share IRQs does not prevent any hardware from working.

The Plug and Play operating system settings in the computer BIOS do not generally affect how Windows XP handles the hardware. However, Microsoft recommends that you set the Plug and Play operating system setting to No or Disabled in the computer BIOS. For information about viewing or modifying the computer BIOS settings, see the computer documentation, or contact the computer manufacturer.

Manually assigning IRQs to PCI slots in the system BIOS as a troubleshooting method may work on some non-ACPI systems that use a standard PC hardware abstraction layer (HAL), but these settings are ignored by Plug and Play in Windows if ACPI support is enabled. If you must manually assign IRQ addresses through the BIOS to a device on an ACPI motherboard, you must reinstall Windows to force the installation to use a Standard PC HAL.

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