Devices are required to comply with a protocol so the interrupt-request (IRQ) strains can be shared. The PCI bus involves four interrupt lines, INTA# by INTD#, all of which can be found to each unit. Around 8 PCI equipment share the same IRQ line (INTINA# via INTINH#) in APIC-enabled x86 https://nathanlabsadvisory.com/vapt-services/