Products are required to observe a protocol so that 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 product. Around 8 PCI devices share the exact same IRQ line (INTINA# via https://nathanlabsadvisory.com/nist-800-cyber-security-frame-work/