Description of IRQ Steering (HW164)

Windows 95 OEM Service Release 2 (OSR2) and Windows 98 provide support for PCI (Peripheral Component Interconnect) bus IRQ steering. By using IRQ steering, Windows can dynamically assign ("steer") PCI bus IRQs to PCI devices.

It is possible for multiple PCI devices to share a PCI IRQ, however ISA (Industry Standard Architecture) devices cannot share an ISA IRQ, and an IRQ cannot be programmed for both modes at once.

Note: PCI bus IRQ steering is disabled by default in OSR2.

If you are using OSR2, and want to enable PCI bus IRQ steering, follow there steps:

Select Control Panel > System
Select the Device Manager tab
Double-click the System Devices branch
Double-click PCI Bus, and then click the IRQ Steering tab
Select Use IRQ Steering
When you are prompted to restart your computer, click Yes

Select IRQ Steering

IRQ Steering may be displayed as disabled in Device Manager for any of the following reasons:

	Your computers BIOS may not support PCI bus IRQ steering. For more information, contact the manufacturer of your BIOS.
	The Use IRQ Steering check box is not selected
	The IRQ routing table that must be provided by the BIOS to the operating system may be missing or contain errors. The IRQ routing table provides information on how the motherboard is configured for PCI IRQ's
	The Get IRQ table from Protected Mode PCIBIOS 2.1 call check box is not selected

Note I: By default, the Get IRQ table from Protected Mode PCIBIOS 2.1 call check box is not selected. You should only click this check box to place a check mark if a PCI device is not working properly.

Note II: If you have a VIA/ETEQ chipset on your motherboard, then you need to download the IRQ Steering update from the Via Web Site.

How to Disable PCI Bus IRQ Steering in Windows

If there are IRQ conflicts between PCI devices, you may need to disable PCI bus IRQ steering to determine where the conflicts occur. To disable PCI bus IRQ steering, follow these steps:

Select Control Panel > System
Select the Device Manager tab
Double-click the System Devices branch
Double-click PCI Bus, and then select the IRQ Steering tab
Click the Use IRQ Steering check box to clear it, click OK (twice)
When you are prompted to restart your computer, click Yes

Note: You may also have to disable PCI bus IRQ steering in your computer's BIOS. For information about how to do so, consult you motherboard manual, or contact your 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 II: 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, consult you motherboard manual, or contact your BIOS manufacturer.

BY: MRA - 04/2002