At 07:43 06-05-98 -0700, Dick H. Fredericksen wrote:
>I printed out a "Resource Summary Report" which includes the following:
>   IRQ Usage Summary:
>   11 - IRQ Holder for PCI Steering
>   11 - Iomega Jaz Jet PCI SCSI Host Adapter
>Question: What is the "IRQ Holder for PCI Steering"?

The following should explain what "PCI Bus IRQ Steering" and "IRQ
Holders for PCI Steering" are.

First, recall that there is a fundamental difference between ISA
devices and PCI devices. More than one ISA device cannot share the
same IRQ. However, multiple PCI devices *can* share one IRQ. Also,
generally the computer's BIOS assigns the IRQs to devices in PCI
slots. Finally, an IRQ can be programmed as either an ISA type IRQ
(no sharing), or a PCI type IRQ (sharing allowed), but not both.

In Win95 and Win95a (OSR1), the IRQs assigned to PCI devices by the
motherboard BIOS could not be changed. With Win95b (OSR2), a feature
called "PCI bus IRQ steering" was added which enables Windows to
override the IRQ assigned by the BIOS to a PCI device.

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. But 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.

For example, suppose your computer's BIOS is unaware of non-Plug
and Play ISA cards and you are using Win95 or Win95a. If 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, windows can resolve this IRQ
resource conflict. To do so, windows first disables the PCI device
and reprograms a free IRQ to be a PCI IRQ, for example IRQ 11.
Then it assigns an "IRQ holder" to IRQ 11 and then moves the PCI
device to IRQ 11. Finally it reprograms IRQ 10 to be an ISA IRQ
and removes the IRQ holder for IRQ 10.

An IRQ Holder for PCI Steering may be displayed when you view the
System Devices branch of Device Manager. This 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 determine if your computer is using IRQ Steering, go to Device
Manager and double-click the System Devices branch. Double-click
PCI Bus, and then click the IRQ Steering tab. You should see one of
the following settings: IRQ Steering Enabled - or - IRQ Steering
Disabled. (PCI bus IRQ steering is disabled by default in OSR2.)

See "Description of PCI Bus IRQ Steering" at
<http://support.microsoft.com/support/kb/articles/Q182/6/04.asp>
and "How to Disable PCI Bus IRQ Steering in Windows" at
<http://support.microsoft.com/support/kb/articles/q182/6/28.asp>.

Regards,
Bill