The original design of Plug and Play was that the BIOS would configure only boot devices. However assigning IRQs in the OS turned out to be more complex than anticipated. In the design that has evolved, the BIOS assigns IRQs for all PCI devices. Starting with Win95 OSR2 the Operating System may, under some circumstances, reassign one or more devices
IRQ Holders are fictitious "devices" within Microsoft Windows that have no function other than to indicate that an IRQ is available for PCI Interrupt assignment. Devices with IRQ Holders assigned can not be used by ISA even though everyone but Windows may know that an ISA device resides there. Since a PCI device IRQ assignment may be moved during system setup, IRQ Holders are assigned to IRQs even when a device is also assigned to the IRQ. Users sometimes waste substantial time trying to 'resolve' the apparent, but nonexistent, device conflict between the real device(s) and the IRQ Holder.
Even on Windows versions that support PCI Steering by the OS, the capability can be turned off at the user's option. It will also be turned off if the BIOS does not deliver an ESCD table to the OS indicating the assignments it has made, or if the ESCD is defective.
Return To Index Copyright 1994-2008 by Donald Kenney.