When IRQ steering is enabled the BIOS still assigns IRQ's to PCI devices, and even though
Windows has the ability to change these settings, it generally does not.
PCI bus IRQ steering gives the operating system the flexibility to reprogram PCI IRQ's when it
rebalances Plug and Play resources. Without PCI bus IRQ steering, Windows can only rebalance
Plug and Play ISA IRQ's to resolve resource conflicts.
How OSR2 Uses PCI Bus IRQ Steering
PCI bus IRQ steering gives OSR2 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 IRQ's 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 will do the following:
Disable the PCI device.
Reprogram a free IRQ to a PCI IRQ, for example IRQ 11.
Assign an IRQ holder to IRQ 11.
Move the PCI device to IRQ 11.
Reprogram IRQ 10 to be an ISA IRQ.
Remove the IRQ holder for IRQ 10.
IRQ Holder
An IRQ Holder for PCI Steering may be displayed when you view the System Devices branch of
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 IRQ's that are
programmed for PCI-mode, 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 the IRQ Holder for PCI Steering you want to view, and then click the Resources tab.
How to Determine if Your Computer Is Using IRQ Steering
To determine if your computer is using 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. You should see one of the following
settings:
IRQ Steering Enabled
IRQ Steering Disabled
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