Address bus
An address bus is a computer bus, used by CPUs or DMA-capable units for communicating the physical addresses of computer memory elements/locations that the requesting unit wants to access (read/write).
The width of an address bus, along with the size of addressable memory elements, determines how much memory can be accessed. For example, a 16-bit wide address bus (commonly used in the 8-bit processors of the 1970s and early 1980s) reaches across 216 = 65,536 = 64 Ki memory locations, whereas a 32-bit address bus (common in PC processors as of 2004) can address 232 = 4,294,967,296 = 4 Gi locations.
In most microcomputers such addressable "locations" are 8-bit bytes. In such case the above examples translate to 64 kibibytes (KiB) and 4 gibibytes (GiB) respectively. Historically, there were also some examples of computers, which were able to address only areas of a larger size (words), such as 16, 32, 36 bits long.