PCI Interrupts: PCI Interrupt handling is inordinately complex. Numerous variations seem to exist in both the implementation and the way the variations are presented to the user in SETUP. The PCI specification calls for an unlimited number of PCI slots although loading considerations generally limit PC implementations to 4 slots. Each slot should have up to 4 interrupts identified as 1 through 4 or A through D associated with it. These Interrupt lines are sometimes referred to as INTs, not to be confused with the term INT used for CPU interrupts. These interrupts are mapped to IRQs -- the familiar interrupt lines also used by ISA devices -- by the BIOS during machine initialization.
The problem with this scheme is that it is difficult or impossible to implement without interrupt assignment conflicts. Although PCI is able to share interrupts, there are sometimes performance issues if too many devices attempt to share the same interrupt. Actual implementations may map one set of four interrupts to all PCI slots on a first-come first-served basis. Other implementations simply assign one or two IRQs to each PCI slot.
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Copyright 1994-2002 by Donald Kenney.