My NIC won’t work, what do I do? (PCI IRQ Conflicts)

Unlike ISA and EISA cards, PCI cards do not select their own IRQ line. Instead they can only select one of four ’interrupt pins’, which are mapped by the motherboard chipset to IRQ lines. That is, only the motherboard BIOS knows how the INT to IRQ mapping is wired, and thus only the BIOS can change the IRQ mapping for the card. The IRQ mapping is usually configured at boot time by the PCI BIOS setup, with the the exact assignment algorithm being BIOS-specific. In What Order Are PCI Cards detected? There is no fixed rule, but use the following guidelines:

• The primary PCI bus is Bus #0. (Most machines have only one PCI bus, so this usually presents no problem.)
• PCI device numbers are usually assigned in slot order.
• The left-most PCI slot facing the back panel (usually the one closest to the CPU) is often the lowest numbered.
• On-motherboard devices are assigned the highest device numbers on Bus #0. (This is so that a plugged-in adapter card overrides a built-in one.)
• Adapter cards with on-board PCI bridges, e.g. the 4 port AHA6944TX, are assigned Bus#1 and up, and thus these devices are the last detected.