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The PCI bus uses its own internal interrupt system for dealing with requests from the cards on the bus. It's usually not seen except in the bios where it is configurable. When the interrupts are needed, they are mapped through IRQ 9-12. And of course as you remember, IRQ 9-12 are mapped through IRQ 2.
The PCI slots in most systems can be mapped to at most four regular IRQs. In systems that have more than four PCI slots, or that have four slots and a USB controller (which uses PCI), two or more of the PCI devices share an IRQ. PCI Steering is used in Win 95 OSR2 and above. This enables the IRQ used for PCI devices to be controlled by the operating system to avoid resource problems.
Now we come to PCI Bus Mastering. Bus mastering is the capability of devices on the PCI bus (other than the system chipset, of course) to take control of the bus and perform transfers directly.
PCI supports full device bus mastering, and provides bus arbitration facilities through the system chipset. PCI's design allows bus mastering of multiple devices on the bus simultaneously, with the arbitration circuitry working to ensure that no device on the bus locks out any other device. At the same time though, it allows any given device to use the full bus throughput if no other device needs to transfer anything.
From PCGuide
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