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[ The PC Guide | Systems and Components Reference Guide | Motherboard and System Devices | System Buses | System Bus Types | Peripheral Component Interconnect (PCI) Local Bus ]

PCI Internal Interrupts

The PCI bus uses its own internal interrupt system for dealing with requests from the cards on the bus. These interrupts are often called "#A", "#B", "#C" and "#D" to avoid confusion with the normal numbered system IRQs, though they are sometimes called "#1" through "#4" as well. These interrupt levels are not generally seen by the user except in the BIOS setup screen for PCI, where they can be used to control how PCI cards operate.

These interrupts, if needed by cards in the slots, are mapped to regular interrupts, normally IRQ9 through IRQ12. 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.

If you are using Windows 95 OEM SR2, you may see additional entries for your PCI devices under the Device Manager. Each device may have an additional entry entitled "IRQ Holder for PCI Steering". PCI steering is in fact a feature that is part of the Plug and Play portions of the system, and enables the IRQ used for PCI devices to be controlled by the operating system to avoid resource problems. Having this listed in addition to another device under the IRQ list does not mean you have a resource conflict.

Next: PCI Bus Mastering

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