Gadgets are needed to follow 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 can be found to every gadget. Nearly eight PCI products share the identical IRQ line (INTINA# via INTINH#) in APIC-enabled https://nathanlabsadvisory.com/achieve-pci-dss-compliance-in-the-usa-secure-your-business-today/