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Description of PCI Bus IRQ Steering
Article ID: 182604 - View products that this article applies to.
This article was previously published under Q182604
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This article describes Peripheral Component Interconnect (PCI) bus interrupt request (IRQ) steering.
Windows 95 OEM Service Release 2 (OSR2), Windows 98, Windows 98 Second Edition, and Windows Millennium Edition (Me) provide support for PCI bus IRQ steering. By using PCI bus IRQ steering, Windows can dynamically assign or "steer" PCI bus IRQs to PCI devices. Note that Microsoft Windows 95 (retail release) and Microsoft Windows 95 OEM Service Release 1 (OSR1) do not provide support for PCI bus IRQ steering.
ISA and PCI IRQsMultiple Industry Standard Architecture (ISA) devices cannot share an ISA IRQ; however, multiple PCI devices can share a PCI IRQ. On computers that use a PCI bus, the 16-standard IRQs can be programmed to either PCI or ISA mode. An IRQ cannot be programmed for both modes at once.
How IRQs Are Assigned to PCI DevicesIn Windows 95 (retail release) and OSR1, the basic input/output system (BIOS) assigns IRQs to PCI devices. With OSR2 and Windows 98, Windows 98 Second Edition, and Windows Me, if PCI bus IRQ steering is disabled in Windows the BIOS assigns IRQs to PCI devices, but if PCI bus IRQ steering is enabled, Windows assigns IRQs to PCI devices. When IRQ steering is enabled the BIOS still assigns IRQs to PCI devices and, even though Windows can change these settings, it generally does not.
CardBus Cards and OSR2PCI R3 Personal Computer Memory Card International Association (PCMCIA) cards (CardBus cards), look very similar to the more common ISA R2 PCMCIA cards. CardBus cards use 32-bit addressing and require a PCI-type shareable IRQ. R2 PCMCIA cards use 16-bit addressing with an ISA-type non- shareable IRQ. PCI bus IRQ steering gives the operating system the flexibility to reprogram PCI IRQs when it rebalances Plug and Play resources. Without PCI bus IRQ steering, Windows can rebalance only Plug and Play ISA IRQs to resolve resource conflicts.
How OSR2 and Windows 98, Windows 98 Second Edition, and Windows Me Use PCI Bus IRQ SteeringPCI bus IRQ steering gives Windows 98, Windows 98 Second Edition, and Windows Me the flexibility to reprogram PCI interrupts when rebalancing Plug and Play PCI and ISA resources around non-Plug and Play ISA devices. Windows 95 (retail release) and OSR1 cannot rebalance PCI and ISA IRQs for Plug and Play devices around non-Plug and Play ISA devices to solve resource conflicts.
For example, if your computer's BIOS is unaware of non-Plug and Play ISA cards, the operating system does not have PCI bus IRQ steering, and the BIOS has set a PCI device to IRQ 10, you may have a resource conflict when you add a non-Plug and Play ISA device that is configured for IRQ 10.
However, with PCI bus IRQ steering the operating system can resolve this IRQ resource conflict. To do so, the operating system:
IRQ HolderAn IRQ Holder for PCI Steering may be displayed when you view the System Devices branch in Device Manager. An IRQ Holder for PCI Steering indicates that an IRQ has been programmed to PCI mode and is unavailable for ISA devices, even if no PCI devices are currently using the IRQ. To view IRQs that are programmed for PCI-mode:
How to Determine If Your Computer Is Using IRQ SteeringTo determine if your computer is using IRQ Steering:
IRQ Steering may be displayed as disabled in Device Manager for any of the following reasons: