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2.7 PCI Interrupt Routing Map
This section describes interrupt sharing and how the interrupt signals are connected between the
PCI bus connectors and onboard PCI devices. The PCI specification specifies how interrupts can
be shared between devices attached to the PCI bus. In most cases, the small amount of latency
added by interrupt sharing does not affect the operation or throughput of the devices. In some
special cases where maximum performance is needed from a device, a PCI device should not share
an interrupt with other PCI devices. Use the following information to avoid sharing an interrupt
with a PCI add-in card.
PCI devices are categorized as follows to specify their interrupt grouping:
• INTA: By default, all add-in cards that require only one interrupt are in this category. For
almost all cards that require more than one interrupt, the first interrupt on the card is also
classified as INTA.
• INTB: Generally, the second interrupt on add-in cards that require two or more interrupts is
classified as INTB. (This is not an absolute requirement.)
• INTC and INTD: Generally, a third interrupt on add-in cards is classified as INTC and a
fourth interrupt is classified as INTD.
The ICH5 has eight Programmable Interrupt Request (PIRQ) input signals. All PCI interrupt
sources either onboard or from a PCI add-in card connect to one of these PIRQ signals. Some PCI
interrupt sources are electrically tied together on the Desktop Boards D865PCD and therefore
share the same interrupt. Table 15 shows an example of how the PIRQ signals are routed.
For example, using Table 15 as a reference, assume an add-in card using INTA is plugged into PCI
bus connector 3. In PCI bus connector 3, INTA is connected to PIRQB, which is already
connected to the ICH5 audio controller. The add-in card in PCI bus connector 3 now shares an
interrupt with the onboard interrupt source.