System bus
A system bus is a single computer bus that connects the major components of a computer system. The technique was developed to reduce costs and improve modularity.
It combines the functions of a data bus to carry information, an address bus
Address bus
An address bus is a computer bus that is used to specify a physical address. When a processor or DMA-enabled device needs to read or write to a memory location, it specifies that memory location on the address bus...

 to determine where it should be sent, and a control bus
Control bus
A control bus is a computer bus, used by CPUs for communicating with other devices within the computer. While the address bus carries the information on which device the CPU is communicating with and the data bus carries the actual data being processed, the control bus carries commands from the...

 to determine its operation.
Although popular in the 1970s and 1980s, modern computers use a variety of separate buses adapted to more specific needs.


Many early electronic computers were based on the First Draft of a Report on the EDVAC
First Draft of a Report on the EDVAC
The First Draft of a Report on the EDVAC was an incomplete 101-page document written by John von Neumann and distributed on June 30, 1945 by Herman Goldstine, security officer on the classified ENIAC project...

report published in 1945. In what became known as the Von Neumann architecture
Von Neumann architecture
The term Von Neumann architecture, aka the Von Neumann model, derives from a computer architecture proposal by the mathematician and early computer scientist John von Neumann and others, dated June 30, 1945, entitled First Draft of a Report on the EDVAC...

, a central control unit and arithmetic logic unit
Arithmetic logic unit
In computing, an arithmetic logic unit is a digital circuit that performs arithmetic and logical operations.The ALU is a fundamental building block of the central processing unit of a computer, and even the simplest microprocessors contain one for purposes such as maintaining timers...

 (ALU, which he called the central arithmetic part) were combined with computer memory
Computer memory
In computing, memory refers to the physical devices used to store programs or data on a temporary or permanent basis for use in a computer or other digital electronic device. The term primary memory is used for the information in physical systems which are fast In computing, memory refers to the...

 and input and output
In computing, input/output, or I/O, refers to the communication between an information processing system , and the outside world, possibly a human, or another information processing system. Inputs are the signals or data received by the system, and outputs are the signals or data sent from it...

 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
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...


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 backplane
A backplane is a group of connectors connected in parallel with each other, so that each pin of each connector is linked to the same relative pin of all the other connectors forming a computer bus. It is used as a backbone to connect several printed circuit boards together to make up a complete...

s were used to hold printed circuit board
Printed circuit board
A printed circuit board, or PCB, is used to mechanically support and electrically connect electronic components using conductive pathways, tracks or signal traces etched from copper sheets laminated onto a non-conductive substrate. It is also referred to as printed wiring board or etched wiring...

s in these early machines.
The name "bus" was already used for "bus bars
In electrical power distribution, a bus bar is a strip of copper or aluminium that conducts electricity within a switchboard, distribution board, substation or other electrical apparatus....

" that carried electrical power to the various parts of electric machines, including early mechanical calculators.
The advent of integrated circuit
Integrated circuit
An integrated circuit or monolithic integrated circuit is an electronic circuit manufactured by the patterned diffusion of trace elements into the surface of a thin substrate of semiconductor material...

s 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.


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
Digital Equipment Corporation
Digital Equipment Corporation was a major American company in the computer industry and a leading vendor of computer systems, software and peripherals from the 1960s to the 1990s...

 (DEC) further reduced cost for mass-produced minicomputer
A minicomputer is a class of multi-user computers that lies in the middle range of the computing spectrum, in between the largest multi-user systems and the smallest single-user systems...

s, and memory-mapped I/O
Memory-mapped I/O
Memory-mapped I/O and port I/O are two complementary methods of performing input/output between the CPU and peripheral devices in a computer...

 into the memory bus, so that the devices appeared to be memory locations. This was implemented in the Unibus
The Unibus was the earliest of several computer bus technologies used with PDP-11 and early VAX systems manufactured by the Digital Equipment Corporation of Maynard, Massachusetts.-History:...

 of the PDP-11
The PDP-11 was a series of 16-bit minicomputers sold by Digital Equipment Corporation from 1970 into the 1990s, one of a succession of products in the PDP series. The PDP-11 replaced the PDP-8 in many real-time applications, although both product lines lived in parallel for more than 10 years...

 around 1969, eliminating the need for a separate I/O bus.
Even computers such as the PDP-8
The 12-bit PDP-8 was the first successful commercial minicomputer, produced by Digital Equipment Corporation in the 1960s. DEC introduced it on 22 March 1965, and sold more than 50,000 systems, the most of any computer up to that date. It was the first widely sold computer in the DEC PDP series of...

 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.

Many early microcomputers (with a CPU generally on a single integrated circuit
Integrated circuit
An integrated circuit or monolithic integrated circuit is an electronic circuit manufactured by the patterned diffusion of trace elements into the surface of a thin substrate of semiconductor material...

) were built with a single system bus, starting with the S-100 bus
S-100 bus
The S-100 bus or Altair bus, IEEE696-1983 , was an early computer bus designed in 1974 as a part of the Altair 8800, generally considered today to be the first personal computer...

 in the Altair 8800
Altair 8800
The MITS Altair 8800 was a microcomputer design from 1975 based on the Intel 8080 CPU and sold by mail order through advertisements in Popular Electronics, Radio-Electronics and other hobbyist magazines. The designers hoped to sell only a few hundred build-it-yourself kits to hobbyists, and were...

 computer system in about 1975.
The IBM Personal Computer, commonly known as the IBM PC, is the original version and progenitor of the IBM PC compatible hardware platform. It is IBM model number 5150, and was introduced on August 12, 1981...

 used the Industry Standard Architecture
Industry Standard Architecture
Industry Standard Architecture is a computer bus standard for IBM PC compatible computers introduced with the IBM Personal Computer to support its Intel 8088 microprocessor's 8-bit external data bus and extended to 16 bits for the IBM Personal Computer/AT's Intel 80286 processor...

 (ISA) bus as its system bus in 1981. The passive backplanes of early models were replaced with the standard of putting the CPU on a motherboard
In personal computers, a motherboard is the central printed circuit board in many modern computers and holds many of the crucial components of the system, providing connectors for other peripherals. The motherboard is sometimes alternatively known as the mainboard, system board, or, on Apple...

, with only optional expansion boards in system bus slots.
The Multibus
Multibus is a computer bus standard used in industrial systems. It was developed by Intel Corporation and was adopted as the IEEE 796 bus.The Multibus specification was important because it was a robust, well-thought out industry standard with a relatively large form factor so complex devices could...

 became a standard of the Institute of Electrical and Electronics Engineers
Institute of Electrical and Electronics Engineers
The Institute of Electrical and Electronics Engineers is a non-profit professional association headquartered in New York City that is dedicated to advancing technological innovation and excellence...

 as IEEE standard 796 in 1983.
Sun Microsystems
Sun Microsystems
Sun Microsystems, Inc. was a company that sold :computers, computer components, :computer software, and :information technology services. Sun was founded on February 24, 1982...

 developed the SBus
SBus is a computer bus system that was used in most SPARC-based computers from Sun Microsystems and others during the 1990s...

 in 1989 to support smaller expansion cards.
The easiest way to implement symmetric multiprocessing
Symmetric multiprocessing
In computing, symmetric multiprocessing involves a multiprocessor computer hardware architecture where two or more identical processors are connected to a single shared main memory and are controlled by a single OS instance. Most common multiprocessor systems today use an SMP architecture...

 was to plug in more that one CPU into the shared system bus, which was used through the 1980s.
However, the shared bus quickly became the bottleneck and more sophisticated connection techniques were explored.

As microprocessors evolved into using faster local buses and slower peripheral ones, Intel adopted the dual independent bus
Dual independent bus
Dual Independent Bus is a processor architecture that includes two buses: one to the main system memory and another to the level 2 cache...

 terminology, using the external front-side bus, and the internal back-side bus between one or more CPUs and the CPU cache
CPU cache
A CPU cache is a cache used by the central processing unit of a computer to reduce the average time to access memory. The cache is a smaller, faster memory which stores copies of the data from the most frequently used main memory locations...

s. This was introduced in the Pentium Pro
Pentium Pro
The Pentium Pro is a sixth-generation x86 microprocessor developed and manufactured by Intel introduced in November 1, 1995 . It introduced the P6 microarchitecture and was originally intended to replace the original Pentium in a full range of applications...

 and Pentium II
Pentium II
The Pentium II brand refers to Intel's sixth-generation microarchitecture and x86-compatible microprocessors introduced on May 7, 1997. Containing 7.5 million transistors, the Pentium II featured an improved version of the first P6-generation core of the Pentium Pro, which contained 5.5 million...

 products in the mid to late 1990s.
Modern personal and server computers use higher-performance interconnection technologies such as HyperTransport
HyperTransport , formerly known as Lightning Data Transport , is a technology for interconnection of computer processors. It is a bidirectional serial/parallel high-bandwidth, low-latency point-to-point link that was introduced on April 2, 2001...

 and Intel QuickPath Interconnect, while the system bus architecture continued to be used on simpler embedded microprocessors.
The systems bus can even be internal to a single integrated circuit, producing a system-on-a-chip
A system on a chip or system on chip is an integrated circuit that integrates all components of a computer or other electronic system into a single chip. It may contain digital, analog, mixed-signal, and often radio-frequency functions—all on a single chip substrate...

. Examples include AMBA
Advanced Microcontroller Bus Architecture
The Advanced Microcontroller Bus Architecture is used as the on-chip bus in system-on-a-chip designs. Since its inception, the scope of AMBA has gone far beyond microcontroller devices, and is now widely used on a range of ASIC and SoC parts including applications processors used in modern...

, CoreConnect
CoreConnect is a microprocessor bus-architecture from IBM for system-on-a-chip designs. Designed to ease the integration and reuse of processor, system, and peripheral cores within standard and custom SoC designs. As a standard SoC design point, it serves as the foundation of IBM or non-IBM devices...

, and Wishbone
Wishbone (computer bus)
The Wishbone Bus is an open source hardware computer bus intended to let the parts of an integrated circuit communicate with each other. The aim is to allow the connection of differing cores to each other inside of a chip...

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