Gadgets are needed to observe a protocol so which the interrupt-ask for (IRQ) lines can be shared. The PCI bus involves 4 interrupt strains, INTA# by INTD#, all of which are available to each gadget. Up to eight PCI units share the same IRQ line (INTINA# by means of INTINH#) https://nathanlabsadvisory.com/nist-800-171/