General Description of IRQ Sharing in Windows 2000

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Article ID: 252420 - View products that this article applies to.
This article was previously published under Q252420
For a Microsoft Windows XP version of this article, see 314068.
NOTE: This article is for informational use only. It does not contain any troubleshooting information. If you are searching for troubleshooting information that is not mentioned in this article, search the Microsoft Knowledge Base again by using keywords that are listed in the following Microsoft Knowledge Base article:
242450 How to Query the Microsoft Knowledge Base Using Keywords
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SUMMARY

In Windows, peripheral component interconnect (PCI) devices can share interrupts (IRQs) by design. Per the Plug and Play capability that is defined by the PCI specification, adapters are configured by the computer's BIOS, and are then examined by the operating system and changed if necessary. It is normal behavior for PCI devices to have IRQs shared among them, especially for Advanced Configuration and Power Interface (APCI) computers with Windows ACPI support enabled.

MORE INFORMATION

In Windows, some or all of the devices on your ACPI motherboard may be listed when you view the resources in Device Manager 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). You cannot change the IRQ setting because the setting is unavailable. This occurs because Windows takes advantage of the ACPI features of the motherboard, including advanced PCI sharing. IRQ 9 is used by the PCI bus for IRQ steering. This feature lets you add more devices without generating IRQ conflicts.

Note that Windows 2000 does not have the ability to 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 invalid IRQ setting or I/O range for the bus that a device is on, Windows 2000 cannot rebalance the resource it assigned to that bus to compensate. Windows 2000 does not have this ability because of the more complex hardware schemas it is designed to support. Windows 98 does not have to support IOAPICs, multiple root PCI buses, multiple-processor systems, and so on. Rebalancing becomes risky when you are dealing with these hardware schemas, and will not be implemented in Windows 2000 except for very specific scenarios. However, PCI devices are required to be able to share IRQs. The ability to share IRQs should not prevent any hardware from working in general.

The Plug and Play operating system settings in the computer's BIOS should not affect how Windows handles the hardware in general. However, Microsoft recommends that you set this setting to "No" or "Disabled" in the computer's BIOS. For information about viewing or modifying your computer's BIOS settings, consult your computer's documentation or manufacturer.

Manually assigning IRQs to PCI slots in the System BIOS as a troubleshooting method may work on some non-ACPI systems when using a standard PC HAL, but these settings are ignored by Plug and Play in Windows if ACPI support is enabled. If you need to manually assign IRQ addresses through the BIOS to a device on an ACPI motherboard, you will need to re-install Windows to force the installation to use a Standard PC HAL. For additional information, click the article number below to view the article in the Microsoft Knowledge Base:
216251 How to Specify a Specific or Third-Party HAL During Windows 2000 Setup

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Article ID: 252420 - Last Review: March 2, 2007 - Revision: 3.2
APPLIES TO
  • Microsoft Windows 2000 Server
  • Microsoft Windows 2000 Advanced Server
  • Microsoft Windows 2000 Professional Edition
Keywords: 
kbhardware kbinfo kbplugplay KB252420

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