Chapter 13: System Resources^295
Each PCI expansion slot has four interrupts of its own, which are designated PIRQs
(PCI interrupt requests) A through D. The PCI expansion card determines which of the
four it will use, normally PIRQ A. Without IRQ steering, the system BIOS would assign
each slot to a different IRQ and potentially cause conflicts or a lack of resources for other
devices. IRQ steering is available on Windows versions beginning with Windows 95
OSR2 (OEM). For IRQ steering to work correctly, the BIOS, chipset, PCI cards, and the
software drivers must all support it.
However, if there are IRQ conflicts between PCI devices, you may need to disable PCI
bus IRQ steering to determine where the conflicts occur.
To check if IRQ steering is enabled on your system, follow these steps:
- Open the Windows Device Manager. Click the plus sign (+) to expand the
System Devices device type. - Highlight the selection for PCI Bus and click Properties.
- Select the IRQ Steering tab to display the window shown in Figure 13-10.
In order for IRQ steering to be activated, the Use IRQ Steering check box must be
checked. The other check boxes on this window tell IRQ steering where it should look for
its IRQ routing information:
ACPI (advanced configuration and power interface) BIOS This setting
indicates that this is the first IRQ routing table Windows should use to
program IRQ steering. ACPI is a power management specification that
provides hardware status information to the operating system.
MS specification table This setting indicates that the MS (Microsoft)
specification table is the second IRQ routing table that Windows should
use to program IRQ steering.
PCI BIOS 2.1 real mode This selection is not checked by default and should
be selected only if a PCI device is not working properly. When checked, it
specifies that this is the third IRQ routing table that Windows should use to
program IRQ steering.
PCI BIOS 2.1 protected mode This setting indicates that this routing table is
also to be used by Windows to program IRQ steering.If the BIOS is struggling with a PCI device, you may want to try a different combina-
tion of options, including selecting the PCI BIOS 2.1 real mode. But, in most cases, if the
default selections do not work, it is more likely that you need to update the BIOS. One
surefire way to tell that you may need a BIOS update is that IRQ steering is causing the
system to lock up or kernal32.dll error messages.
To deselect IRQ steering, merely click the Use IRQ Steering check box and reboot
the system.