Devices are necessary to stick to a protocol so the interrupt-request (IRQ) traces can be shared. The PCI bus consists of 4 interrupt traces, INTA# through INTD#, all of which can be found to every system. Approximately eight PCI devices share the exact same IRQ line (INTINA# by means of https://nathanlabsadvisory.com/ncema-7000-2015-certification/