An address bus is a type of computer bus found in most modern microprocessors and other semiconductor devices. It is used to communicate the location of data words within memory, and can also be used to select bits and bytes within a word. The bus is usually composed of multiple lines, each of which can carry one bit or address code from the processor. When communicating to memory, the bus contains both address and data lines.
Address buses have been around since the early days of computers, when they were used to connect cards on the ZipBUS and later, the Unibus. The precise timing and control of the bus was encoded in the timing signals, which ensured that the information went to the right device. These signals were replaced by more efficient signals as the technology developed.
The length of an address bus affects the amount of memory a processor can access. For example, a bus consisting of 8 address lines will allow a processor to access up to 256 bytes of memory. A bus with 16 address lines will allow access to up to 65,536 bytes of memory.
In recent years, newer microprocessors have incorporated technologies such as ECC (error-correcting codes) and spare bits into their address buses. This enables the processor to use the extra address bits to detect and possibly correct any memory address errors.
Also, some modern processors use a single-instruction multiple-data (SIMD) instruction set with multiple address lines, enabling them to access multiple memory addresses with a single instruction. The Intel Core 2 processor, for example, uses MID (multiple instruction decode) and ECC on the address bus to enable faster memory access.
The address bus is vital to the operation of computer systems. With it, a processor can access, select, and manipulate memory addresses in order to read, write, and execute instructions. Without an address bus, modern computers would not be able to operate.
