Units are necessary to comply with a protocol so that the interrupt-ask for (IRQ) lines is usually shared. The PCI bus features 4 interrupt lines, INTA# by way of INTD#, all of which are available to every system. Around 8 PCI equipment share the same IRQ line (INTINA# through INTINH#) https://nathanlabsadvisory.com/open-source-scanning-oss/