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Posted on 1998-09-24
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holder for PCI steering
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Article last modified on 05-11-1998
 

 

 

======================================================================
********************************************************************
     BETA INFORMATION  BETA INFORMATION  BETA INFORMATION  BETA
 
     This article discusses a Beta release of a Microsoft
     product. The information in this article is provided as-is
     and is subject to change without notice.
 
     No formal product support is available from Microsoft for
     this Beta product. For information about obtaining support
     for a Beta release, please see the documentation included
     with the Beta product files, or check the Web location
     from which you downloaded the release.
 
     BETA INFORMATION  BETA INFORMATION  BETA INFORMATION  BETA
********************************************************************
 
---------------------------------------------------------------------
The information in this article applies to:
 
 - Microsoft Windows 95 OEM Service Release versions 2, 2.1
 - Microsoft Windows 98
---------------------------------------------------------------------
 
SUMMARY
=======
 
This article describes Peripheral Component Interconnect (PCI) bus
interrupt request (IRQ) steering.
 
MORE INFORMATION
================
 
Windows 95 OEM Service Release 2 (OSR2) and Windows 98 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 IRQs
----------------
 
Multiple 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 Devices
------------------------------------
 
In Windows 95 (retail release) and OSR1, the Basic Input/Output System
(BIOS) assigns IRQs to PCI devices. With OSR2 and Windows 98, 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 has the ability to change these settings,
it generally does not.
 
CardBus Cards and OSR2
----------------------
 
PCI 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 only rebalance Plug
and Play ISA IRQs to resolve resource conflicts.
 
How OSR2 and Windows 98 Use PCI Bus IRQ Steering
------------------------------------------------
 
PCI bus IRQ steering gives OSR2 and Windows 98 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:
 
 - Disables the PCI device.
 - Reprograms a free IRQ to a PCI IRQ, for example IRQ 11.
 - Assigns an IRQ holder to IRQ 11.
 - Moves the PCI device to IRQ 11.
 - Reprograms IRQ 10 to be an ISA IRQ.
 - Removes 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 IRQs
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
 
NOTE: PCI bus IRQ steering is disabled by default in OSR2. If you are
using OSR2, and IRQ steering is disabled, verify that the Use IRQ Steering
check box is selected on the IRQ Steering tab.
 
IRQ Steering may be displayed as disabled in Device Manager for any of the
following reasons:
 
 - 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 Use IRQ Steering check box is not selected.
 
 - The "Get IRQ table from Protected Mode PCIBIOS 2.1 call" check box is
   not selected.
 
 - Your computers BIOS may not support PCI bus IRQ steering. For more
   information, contact the manufacturer of your BIOS.
 
Additional query words: 95
======================================================================
Keywords          : osr2
Issue type        : kbinfo
=============================================================================
Copyright Microsoft Corporation 1998.


 

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