To address a PCI device, it must be enabled by being mapped into the system's I/O port address space or memory-mapped address space. The system's firmware, device drivers or the operating system program the "Base Address Registers" (commonly called BARs) to inform the device of its address mapping by writing configuration commands to the PCI controller. Because all PCI devices are in an "inactive" state upon system reset, they will have no addresses assigned to them by which the operating system or device drivers can communicate with them. Either the BIOS or the operating system geographically addresses the PCI slots (for example, the first PCI slot, the second PCI slot, or the third PCI slot, etc., on the motherboard) through the PCI controller using the per slot IDSEL (Initialization Device Select) signals.