Units are needed to observe a protocol so that the interrupt-ask for (IRQ) traces might be shared. The PCI bus contains 4 interrupt lines, INTA# by way of INTD#, all of which are offered to each gadget. As many as 8 PCI gadgets share the same IRQ line (INTINA# as https://nathanlabsadvisory.com/cism-certified-information-security-manager/