An address bus is a computer bus

Computer bus

In computer architecture, a bus is a subsystem that transfers data between components inside a computer, or between computers.Early computer buses were literally parallel electrical wires with multiple connections, but the term is now used for any physical arrangement that provides the same...

 (a series of lines connecting two or more devices) that is used to specify a physical address

Physical address

In computing, a physical address, also real address, or binary address, is the memory address that is represented in the form of a binary number on the address bus circuitry in order to enable the data bus to access a particular storage cell of main memory.In a computer with virtual memory, the...

. When a processor

Central processing unit

The central processing unit is the portion of a computer system that carries out the instructions of a computer program, to perform the basic arithmetical, logical, and input/output operations of the system. The CPU plays a role somewhat analogous to the brain in the computer. The term has been in...

 or DMA

Direct memory access

Direct memory access is a feature of modern computers that allows certain hardware subsystems within the computer to access system memory independently of the central processing unit ....

-enabled device needs to read or write to a memory location, it specifies that memory location on the address bus (the value to be read or written is sent on the data bus). The width of the address bus determines the amount of memory a system can address. For example, a system with a 32-bit address bus can address 2³² (4,294,967,296) memory locations. If each memory address holds one byte, the addressable memory space is 4 GiB.

Implementation

Early processors used a wire for each bit of the address width. For example, a 16-bit address bus had 16 physical wires making up the bus. As the buses became wider, this approach became expensive in terms of the number of chip pins and board traces. Beginning with the Mostek

Mostek

Mostek was an integrated circuit manufacturer, founded in 1969 by ex-employees of Texas Instruments. Initially their products were manufactured in Worcester, Massachusetts, however by 1974 most of its manufacturing was done in the Carrollton, Texas facility on Crosby Road...

 4096 DRAM, multiplexed addressing became common. In a multiplexed address scheme, the address is sent in two equal parts. This halves the number of address bus signals required to connect to the memory. For example a 32-bit address bus can be implemented by using 16 wires and sending the first half of the memory address, immediately followed by the second half.

Interesting examples

Accessing an individual byte frequently requires reading or writing the full bus width (a word) at once. In these instances the least significant bits of the address bus may not even be implemented - it is instead the responsibility of the controlling device to isolate the individual byte required from the complete word transmitted. This is the case, for instance, with the VESA Local Bus

VESA Local Bus

The VESA Local Bus was mostly used in personal computers. VESA Local Bus worked alongside the ISA bus; it acted as a high-speed conduit for memory-mapped I/O and DMA, while the ISA bus handled interrupts and port-mapped I/O.-Historical overview:In the early 1990s, the I/O bandwidth of...

 which lacks the two least significant bits, limiting this bus to aligned

Data structure alignment

Data structure alignment is the way data is arranged and accessed in computer memory. It consists of two separate but related issues: data alignment and data structure padding. When a modern computer reads from or writes to a memory address, it will do this in word sized chunks...

32-bit transfers.

Historically, there were also some examples of computers which were only able to address words.