Intel AN430TX Product Specification - Page 51

Motherboard Resources 2.6 PCI Interrupt Routing Map The PCI specification allows for sharing of interrupts between devices attached to the PCI bus. In most cases, the small amount of latency added by interrupt sharing does not affect the normal operation or throughput of the devices. However, in some special cases where maximum performance is needed from a device, you may want to ensure that it does not share an interrupt with other PCI devices. This section describes the interrupt sharing mechanism and how the interrupt signals are connected between the motherboard's PCI expansion slots and onboard PCI devices. Use this information to avoid sharing an interrupt for 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 PIIX4 PCI-to-ISA bridge has four Programmable Interrupt Request (PIRQ) input signals. Any PCI interrupt source (either onboard or from a PCI add-in card) 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 39 lists the PIRQ signals and shows how the signals are connected to the PCI expansion slots and to onboard PCI interrupt sources. Table 39. PCI Interrupt Routing Map PIIX4 PIRQ Signal PIRQA PIRQB PIRQC PIRQD First PCI Expansion Slot: J4E2 INTA INTB INTC INTD Second PCI Expansion Slot: J4E1 INTD INTA INTB INTC Third PCI Expansion Slot: J4D1 INTC INTD INTA INTB Fourth PCI Expansion Slot: J4C1 INTB INTC INTD INTA Onboard Video X USB X For example, assume that you plug an add-in card that has one interrupt (group INTA) into the fourth PCI slot. In this slot, an interrupt source from group INTA connects to the PIRQD signal, which is already connected to the onboard video and USB PCI sources. The add-in card shares an interrupt with these onboard interrupt sources. The PCI interrupts will dynamically configure an available interrupt on the interrupt controller contained within the PIIX4. 51