<P> 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). </P> <P> 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 . </P> <P> To provide even more modularity with reduced cost, memory and I / O buses (and the required control and power buses) were sometimes combined into a single unified system bus . Modularity and cost became important as computers became small enough to fit in a single cabinet (and customers expected similar price reductions). Digital Equipment Corporation (DEC) further reduced cost for mass - produced minicomputers, and memory - mapped I / O into the memory bus, so that the devices appeared to be memory locations . This was implemented in the Unibus of the PDP - 11 around 1969, eliminating the need for a separate I / O bus . Even computers such as the PDP - 8 without memory - mapped I / O were soon implemented with a system bus, which allowed modules to be plugged into any slot . Some authors called this a new streamlined "model" of computer architecture . </P> <P> Many early microcomputers (with a CPU generally on a single integrated circuit) were built with a single system bus, starting with the S - 100 bus in the Altair 8800 computer system in about 1975 . The IBM PC used the Industry Standard Architecture (ISA) bus as its system bus in 1981 . The passive backplanes of early models were replaced with the standard of putting the CPU and RAM on a motherboard, with only optional daughterboards or expansion cards in system bus slots . </P>

System bus consists of a number of conducting wires