Units are required to follow a protocol so which the interrupt-request (IRQ) strains could be shared. The PCI bus involves 4 interrupt lines, INTA# as a result of INTD#, all of which can be found to every system. As much as 8 PCI devices share a similar IRQ line (INTINA# https://nathanlabsadvisory.com/iso-iec-19770-1-2017-it-asset-management/