A Semi-inserted PCI-X card inside of a 32-bit PCI slot, illustrating the need for that rightmost notch and the additional space about the motherboard to stay backward compatible PCI provides individual memory and memory-mapped I/O port address spaces with the x86 processor relatives, 64 and 32 bits, respectively. Addresses in https://nathanlabsadvisory.com/3rd-party-security-audit-assessment/