Units Are needed to observe a protocol so that the interrupt-ask for (IRQ) traces is often shared. The PCI bus includes four interrupt lines, INTA# through INTD#, all of which are offered to each unit. Approximately 8 PCI equipment share the identical IRQ line (INTINA# via INTINH#) in APIC-enabled x86 https://nathanlabsadvisory.com/cybersecurity-fintechs/