Units are needed to observe a protocol so which the interrupt-ask for (IRQ) lines is usually shared. The PCI bus contains 4 interrupt traces, INTA# via INTD#, all of which are offered to each machine. Around 8 PCI units share the identical IRQ line (INTINA# via INTINH#) in APIC-enabled x86 https://nathanlabsadvisory.com/data-protection-officer-service-virtual-dpo/