Gadgets are required to abide by a protocol so the interrupt-request (IRQ) lines could be shared. The PCI bus involves 4 interrupt traces, INTA# by way of INTD#, all of which are available to every system. As much as eight PCI products share the same IRQ line (INTINA# via INTINH#) https://nathanlabsadvisory.com/penetration-testing/