Units are required to observe a protocol so that the interrupt-request (IRQ) strains could be shared. The PCI bus includes four interrupt traces, INTA# by INTD#, all of which are available to every system. As much as 8 PCI products share exactly the same IRQ line (INTINA# through INTINH#) in https://nathanlabsadvisory.com/irs-efile-2/