IRQ Steering Explained

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Windows 95 OEM Service Release 2 (OSR2) and Windows 98 (all versions) provide support for PCI (Peripheral Component Interconnect) Bus IRQ steering. Through the use of  IRQ steering, Windows can dynamically assign or “steer” PCI bus IRQs to PCI devices.

While it is possible for multiple PCI devices to share a PCI IRQ, ISA (Industry Standard Architecture) devices cannot share an ISA IRQ. In addition, an IRQ cannot be programmed for both modes (PCI and ISA) at the same time.

Enabling (or verifying) PCI Bus IRQ steering in Windows 9x

In Windows 95 OSR2, PCI bus IRQ steering is disabled by default!

If you are using Windows 95 OSR2 (or Windows 98), and want to enable PCI bus IRQ steering, follow these steps:

1. Click Start, Settings, and then select Control Panel
2. In Control Panel, double-click the System icon
3. Now select the Device Manager tab
4. Now click the System Devices branch
5. Next, double-click PCI Bus, and then click the IRQ Steering tab
6. Select Use IRQ Steering
7. When you are prompted to restart your computer, click Yes

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. If your motherboard’s BIOS is dated prior to December 1995, in all probability it does not support PCI IRQ steering. In order to determine whether your BIOS can be updated, you should contact any of the following: The manufacturer of your computer, the manufacturer of your computer’s motherboard, or the company that wrote the BIOS software for the motherboard.
• The Use IRQ Steering check box is not selected. Attempt to check the box, after which you should be promoted by Windows to restart your computer.
• 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. You can attempt to clear the BIOS (CMOS) and then force a new routing table to be reported to Windows.
• The Get IRQ table from Protected Mode PCIBIOS 2.1 call check box is not selected. This box must be selected in order for Windows to obtain the required information from the BIOS. See Note 1 below.

Note 1: 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 2: If you have a VIA/ETEQ chipset on your motherboard, then you need to download the IRQ Steering update from the Via Web Site.

Disabling PCI Bus IRQ Steering in Windows

If you find that there are IRQ conflicts between PCI devices, you may need to disable PCI Bus IRQ steering in order to determine where the conflicts occur.

To disable PCI bus IRQ steering, follow these steps:

1. Click Start, Settings and then select Control Panel
2. From Control Panel, double-click the System icon
3. Now select the Device Manager tab
4. Next, click the System Devices branch “+” to expand it
5. Now double-click PCI Bus, and then select the IRQ Steering tab
6. Click the Use IRQ Steering check box to clear it, click OK twice
7. When you are prompted to restart your computer, click Yes

Note: In order to perform extensive troubleshooting, you may have to also disable PCI bus IRQ steering in your computer’s BIOS, provided that your motherboard’s BIOS permits you to do so. You should consult your computer or motherboard manuals to confirm this, 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, then resolve the conflict or problem.
• Get IRQ table using MS Specification table: When this check box is selected, the Microsoft 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 1: By default, the Get IRQ table from Protected Mode PCIBIOS 2.1 call check box is not selected. You should only check this check box if a PCI device is not working properly and some troubleshooting is required.

Note 2: 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. You should consult your motherboard’s manual in order to change this, or seek the advice of the BIOS developer about modifying the BIOS.

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