Base Address Register

this term can be used at a couple different points (including mappings from physical addresses to physical hardware in the memory network), but a PCI BAR is a register in the configuration space that tells the card what PCI host addresses map to internal memory regions in the card. one BAR per region.

the PCI BARs are usually configured by the driver after allocating some address space from the kernel.

DRAM BARs in the switching network are generally configured by something running at the BIOS level based on probes of memory controllers and I2C reads from the DIMMS to find out capacity.