Microsoft KB Archive/182628

= How to Disable PCI Bus IRQ Steering in Windows =

Article ID: 182628

Article Last Modified on 1/22/2007

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APPLIES TO


 * Microsoft Windows 95
 * Microsoft Windows 95
 * Microsoft Windows 95
 * Microsoft Windows 98 Standard Edition
 * Microsoft Windows 98 Second Edition
 * Microsoft Windows Millennium Edition

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This article was previously published under Q182628



If this article does not describe your hardware-related issue, please see the following Microsoft Web site to view more articles about hardware:

http://support.microsoft.com/default.aspx/w98?sid=460



SUMMARY
To disable PCI bus IRQ steering, follow these steps:
 * 1) Click Start, point to Settings, click Control Panel, and then double-click System.
 * 2) Click the Device Manager tab.
 * 3) Double-click the System Devices branch.
 * 4) Double-click PCI Bus, and then click the IRQ Steering tab.
 * 5) Click to clear the Use IRQ Steering check box, click OK, and then click OK again.
 * 6) When you are prompted to restart your computer, click Yes.

If you click No instead of Yes when you are prompted to restart your computer, the changes are not applied.

NOTE: You may also have to disable PCI bus IRQ steering in your computer's basic input/output system (BIOS). For information about how to do so, contact you BIOS manufacturer.

The following settings determine which routing tables Windows uses when programming IRQ steering:
 * Get IRQ table using ACPI BIOS:

When this check box is selected, the ACPI BIOS IRQ routing table is the first table Windows tries to use to program IRQ steering. If a PCI device is not working properly, click this check box to clear it.
 * Get IRQ table using MS Specification table:

When this check box is selected, the MS Specification routing table is the second table Windows tries to use to program IRQ steering.
 * Get IRQ table from Protected Mode PCIBIOS 2.1 call:

When this check box is selected, the Protected Mode PCIBIOS 2.1 routing table is the third table Windows tries to use to program IRQ steering.
 * Get IRQ table from Real Mode PCIBIOS 2.1 call:

When this check box is selected, the Real Mode PCIBIOS 2.1 routing table is the fourth table Windows tries to use to program IRQ steering.

NOTE: By default, the Get IRQ table from Protected Mode PCIBIOS 2.1 call check box is not selected. You should select this check box only if a PCI device is not working properly.

Note that "IRQ Holder For PCI Steering" may appear under IRQ Routing Status even though PCI bus IRQ steering is disabled. This can occur if the IRQ settings are being read by your computer's BIOS. For information about modifying the BIOS, contact your BIOS manufacturer.

For additional information about PCI bus IRQ steering, click the article number below to view the article in the Microsoft Knowledge Base:

182604 Description of PCI Bus IRQ Steering



MORE INFORMATION
The PCI local bus has become the industry-standard bus and is used in most Pentium-based computers. When a computer with a PCI local bus starts, the system BIOS dynamically configures all the PCI adapter resource settings and requirements because the PCI bus and PCI devices use agreed-upon mechanisms for identifying themselves and declaring their resource settings and requirements.

PCI devices can share the same IRQs because the system BIOS builds a table that is called the PCI IRQ routing table. This table includes entries for each PCI device that is given an ISA IRQ that is mapped to a particular PCI INT number that is associated with the specific PCI slot in which the device is installed. These are combined to create a link value. The link value is used when communicating with the device, and although PCI devices may use the same IRQ, they all have individual link values.

After the system BIOS assigns resource settings and builds the PCI IRQ routing table, Windows loads and extracts the PCI and ISA Plug and Play-compliant device resource information from the system BIOS and reads the information from the PCI IRQ routing table. Windows can reassign the ISA IRQs that are mapped to a particular PCI INT number. Windows can also dynamically reassign the IRQs when a Plug and Play event occurs, such as docking a laptop with its docking station.

Sometimes when is is reassigning IRQs in this way, Windows may hang, reboot, or have a device not work when it tries to reassign the IRQs that are set up by the BIOS. You can prevent Windows from dynamically allocating ISA interrupts by turning off IRQ steering. This prevents Windows from dynamically allocating interrupts, and relies on your system BIOS to do so.

Original Equipment Manufacturers (OEMs) may find it necessary to disable IRQ steering even on new PCs to prevent hardware resource conflicts on devices they have installed. This does not cause any loss of functionality for the devices, it simply ensures that the devices will remain enabled to use a specific IRQ configuration that the OEM has found to work properly when testing the system.

Additional query words: HWSYSPCI

Keywords: kbhowto kbenv KB182628

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