One of the major improvements the PCI Local Bus had over other I/O architectures was its configuration mechanism. In addition to the normal memory-mapped and I/O port spaces, each device function on the bus has a "configuration space", which is 256 bytes long, addressable by knowing the eight-bit PCI bus, five-bit device, and three-bit function numbers for the device (commonly referred to as the "BDF" or "B/D/F", as abbreviated from "bus/device/function"). This allows up to 256 buses, each with up to 32 devices, each supporting eight functions. A single PCI expansion card can respond as a device and must implement at least function number zero. The first 64 bytes of configuration space are standardized; the remainder are available for vendor-defined purposes.