The Windows 2000/XP operating system component responsible for PnP is Advanced Configuration and Power Interface (ACPI). ACPI allows devices to be controlled by operating system to perform power management. ACPI may put a device in a power saving state such as Standby, Suspend or Off. ACPI also allows dynamic handling of events like addition or removal of a USB device.

During boot process, BIOS builds a table to record which IRQs have been used by legacy ISA devices and then assigns unused ones to PCI bus controller. The PCI bus controller, which is part of motherboard chip set, manages PCI bus and expansion slots.

Each PCI device communicates its resource requirements to ACPI. Based on resource requests that each device makes, ACPI assigns resources such as IRQs, I/O addresses, and memory locations, and DMA channels. ACPI can reconfigure resource assignments when needed, for example, when a device is added to system.

The PCI bus overcomes limitation in available IRQ's with a feature called "PCI steering". PCI steering makes it possible for several devices to use same IRQ. PCI steering allocates an IRQ called "IRQ Holder" for PCI steering. The PCI controller steers that IRQ to its own interrupt systems on PCI bus. These interrupts are called A, B, C, D, and so on to avoid confusion with numbered system IRQs. One interrupt is assigned to each PCI expansion slot.

To see which IRQ has been assigned to IRQ Holder for PCI steering, open Control Panel "System" utility. In "System Properties" dialog box, select "Hardware" tab and click on "Device Manager..." button. In Device Manager's "View" menu select "Resources by type", then open "Interrupt request IRQ" branch.

pcirq.bmp

- Note that Windows 2000 uses IRQ 9 for PCI Steering, so never set a device to IRQ 9.

PnP resource allocation removes need to use jumpers of DIP switches to set IRQ and other resources for devices. But PnP involves a complex interaction between computers BIOS, chipset, PCI bus, operating system, and PnP devices.

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