Devices are needed to abide by a protocol so the interrupt-ask for (IRQ) traces could be shared. The PCI bus contains 4 interrupt lines, INTA# through INTD#, all of which are offered to each system. Around 8 PCI units share the identical IRQ line (INTINA# via INTINH#) in APIC-enabled x86 https://nathanlabsadvisory.com/testing-for-compliance/