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Address bus
From Wikipedia, the free encyclopedia
An address bus is (part of) 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 = 64Ki memory locations, whereas a 32-bit address bus (common in today's PC processors) can address 232 = 4,294,967,296 = 4Gi locations.
In most microcomputers the addressable elements are 8-bit bytes (so a "Ki" in that case is equal to a "KiB", i.e. a kibibyte), while there are also many examples of computers with larger "chunks" of data as their minimum physically addressable elements, notably some mainframes and some microprocessors.
For many years, computer users and manufacturers used the SI prefixes for powers of 103 (kilo-,mega-,giga-) to refer instead to powers of 210. The IEC has now standardized on a binary prefix nomenclature to resolve this ambiguity, but the older usage is still common as of 2006.