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Resource Mapping

The PIIX4E PCI-to-ISA bridge has four programmable interrupt request (PIRQ) input signals. Any PCI interrupt source (either onboard or from a PCI add-in board) connects to one of these PIRQ signals. Because there are only four signals, some PCI interrupt sources are mechanically tied together on the motherboard and therefore share the same interrupt. Table 37 lists the PIRQ signals and shows how the signals are connected to the onboard PCI interrupt sources.
Table 37. PCI Interrupt Routing Map
First PCI Expansion Slot: J4C1 INTA INTB INTC INTD Second PCI Expansion Slot: J4B1 INTB INTC INTD INTA Third PCI Expansion Slot: J4A3 INTC INTD INTA INTB INTD INTC AGP Video PCI Audio Power Management INTA

PIIX4E PIRQ Signal PIRQA PIRQB PIRQC PIRQD

USB INTA

For example, assume an add-in board has one interrupt (group INTA) and is plugged into the second PCI slot. In this slot, an interrupt source from group INTA connects to the PIRQB signal. This add-in board will not share an interrupt. Now, however, plug an add-in board that has one interrupt (group INTA) into the first PCI slot. Plug a second add-in board that has two interrupts (groups INTA and INTB) into the second PCI slot. INTA in the first slot is connected to signal PIRQA. INTA in the second slot is connected to signal PIRQB, and INTB is connected to signal PIRQC, which is already connected to the onboard audio. The second interrupt source on the add-in board in the second PCI slot will share an interrupt with the onboard audio.



NOTE
The PIIX4E can connect each PIRQ line internally to one of the IRQ signals (3, 4, 5, 7, 9, 11, 14, 15). Typically, a device that does not share a PIRQ line will have a unique interrupt. However, in certain interrupt-constrained situations, it is possible for two or more of the PIRQ lines to be connected to the same IRQ signal.

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