Devices are required to observe a protocol so the interrupt-request (IRQ) traces may be shared. The PCI bus involves 4 interrupt traces, INTA# by means of INTD#, all of which are offered to every product. Around 8 PCI products share the identical IRQ line (INTINA# as a result of INTINH#) https://nathanlabsadvisory.com/soc-1/