As mentioned above, PCI INTx interrupts are implemented using side-band signals. Each PCI slot has four interrupt signals: INTA#, INTB#, INTC#, and INTD#. Each PCI function may use one of the interrupt signals associated with its slot. The function indicates which signal, if any, that it uses in one of the standard config space registers.
PCI interrupt lines are level-triggered and assert when the signal is pulled low. This allows multiple interrupt signals to share a single physical line.