System bus

A system bus is a single computer bus that connects the major components of a computer system, combining the functions of a data bus to carry information, an address bus to determine where it should be sent or read from, and a control bus to determine its operation. The technique was developed to reduce costs and improve modularity, and although popular in the 1970s and 1980s, more modern computers use a variety of separate buses adapted to more specific needs. The system level bus (as distinct from a CPU's internal datapath busses) connects the CPU to memory and I/O devices. Typically a system level bus is designed for use as a backplane. Many of the computers were based on the First Draft of a Report on the EDVAC report published in 1945. In what became known as the Von Neumann architecture, a central control unit and arithmetic logic unit (ALU, which he called the central arithmetic part) were combined with computer memory and input and output functions to form a stored program computer. The Report presented a general organization and theoretical model of the computer, however, not the implementation of that model. Soon designs integrated the control unit and ALU into what became known as the central processing unit (CPU). Computers in the 1950s and 1960s were generally constructed in an ad-hoc fashion. For example, the CPU, memory, and input/output units were each one or more cabinets connected by cables. Engineers used the common techniques of standardized bundles of wires and extended the concept as backplanes were used to hold printed circuit boards in these early machines. The name "bus" was already used for "bus bars" that carried electrical power to the various parts of electric machines, including early mechanical calculators. The advent of integrated circuits vastly reduced the size of each computer unit, and buses became more standardized. Standard modules could be interconnected in more uniform ways and were easier to develop and maintain.