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{{Short description|
{{About|buses in computer hardware|buses in software|Software bus}}
{{Use American English|date=March 2016}}
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[[File:PCIExpress.jpg|250px|thumb|Four [[PCI Express]] bus card slots (from top to second from bottom: ×4, ×16, ×1 and ×16), compared to a 32-bit [[conventional PCI]] bus card slot (very bottom)]]
In [[computer architecture]], a '''bus''' (historically also called a '''data highway'''<ref name="Hollingdale_1958">{{cite conference |conference=Applications of Computers, University of Nottingham 15–19 September 1958 |title=Session 14. Data Processing |author-first=Stuart H. |author-last=Hollingdale |date=1958-09-19 |url=https://www.chilton-computing.org.uk/acl/literature/othermanuals/nottingham/p014.htm}}</ref> or '''databus''') is a communication system that transfers [[Data (computing)|data]] between components inside a [[computer]] or between computers.<ref>{{cite book |url=https://books.google.com/books?id=YVi8HVN-APwC&q=computer+buss+-sam&pg=PA27 |title=What Every Engineer Should Know about Data Communications |first=Carl |last=Clifton |publisher=CRC Press |date=September 19, 1986 |page=27 |isbn=9780824775667 |quote=The internal computer bus is a parallel transmission scheme; within the computer.... |url-status=live |archive-url=https://web.archive.org/web/20180117151300/https://books.google.com/books?id=YVi8HVN-APwC&lpg=PA27&dq=computer%20buss%20-sam&pg=PA27#v=onepage&q=computer%20buss%20-sam&f=false |archive-date=January 17, 2018}}</ref> It encompasses both [[Computer hardware|hardware]] (e.g., wires, [[optical fiber]]) and [[software]], including [[communication protocol]]s.<ref>{{cite web |url=https://www.pcmag.com/encyclopedia/term/39054/bus |title=bus Definition from PC Magazine Encyclopedia |publisher=pcmag.com |date=2014-05-29 |access-date=2014-06-21 |url-status=live |archive-url=https://web.archive.org/web/20150207204630/http://www.pcmag.com/encyclopedia/term/39054/bus |archive-date=2015-02-07}}</ref> At its core, a bus is a shared physical pathway, typically composed of wires, traces on a circuit board, or [[busbar]]s, that allows multiple devices to communicate. To prevent conflicts and ensure orderly data exchange, buses rely on a [[communication protocol]] to manage which device can transmit data at a given time.
Buses are categorized based on their role, such as [[system bus]]es (also known as internal buses, internal data buses, or memory buses) connecting the [[Central processing unit|CPU]] and [[Computer memory|memory]]. [[Expansion bus]]es, also called [[peripheral bus]]es, extend the system to connect additional devices, including [[peripheral]]s. Examples of widely used buses include [[PCI Express]] (PCIe) for high-speed internal connections and [[Universal Serial Bus]] (USB) for connecting external devices.
Modern buses utilize both [[parallel communication|parallel]] and [[serial communication]], employing advanced encoding methods to maximize speed and efficiency. Features such as [[direct memory access]] (DMA) further enhance performance by allowing data transfers directly between devices and memory without requiring CPU intervention.
==Address bus==
{{Unreferenced section|date=June 2023}}
An ''address bus'' is a bus that is used to specify a [[physical address]]. When a [[central processing unit|processor]] or [[direct memory access|DMA]]-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 ''2<sup>32</sup>'' (4,294,967,296) memory locations. If each memory ___location holds one byte, the addressable memory space is about {{val|4
=== Address multiplexing ===
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Accessing an individual byte frequently requires reading or writing the full bus width (a [[Word (data type)|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]] which lacks the two least significant bits, limiting this bus to [[Data structure alignment|aligned]] 32-bit transfers.
Historically, there were also some examples of computers
==Memory bus==
{{Unreferenced section|date=June 2023}}
The ''memory bus'' is the bus
==Implementation details==
Buses can be [[parallel bus]]es, which carry [[
The transition from parallel to serial buses was allowed by [[Moore's law]] which allowed for the incorporation of [[SerDes|serializer/deserializers]] in integrated circuits which are used in computers.<ref>{{cite book | url=https://books.google.com/books?id=aUCgNOpyUbgC&dq=parallel++serial++serdes+moore%27s+law&pg=PA275 | isbn=978-1-4020-7496-7 | title=The Boundary — Scan Handbook | date=30 June 2003 | publisher=Springer }}</ref>
[[computer network|Network]] connections such as [[Ethernet]] are not generally regarded as buses, although the difference is largely conceptual rather than practical. An attribute generally used to characterize a bus is that power is provided by the bus for the connected hardware. This emphasizes the [[busbar]] origins of bus architecture as supplying switched or distributed power. This excludes, as buses, schemes such as serial [[RS-232]], parallel [[Centronics]], [[IEEE 1284]] interfaces and Ethernet, since these devices also needed separate power supplies. [[Universal Serial Bus]] devices may use the bus supplied power, but often use a separate power source. This distinction is exemplified by a [[Plain old telephone service|telephone]] system with a connected [[modem]], where the [[RJ11]] connection and associated modulated signalling scheme is not considered a bus, and is analogous to an [[Ethernet]] connection. A phone line connection scheme is not considered to be a bus with respect to signals, but the [[telephone exchange|Central Office]] uses buses with [[cross-bar switch]]es for connections between phones.
However, this distinction{{mdashb}}that power is provided by the bus{{mdashb}}is not the case in many [[avionics|avionic systems]], where data connections such as [[ARINC 429]], [[ARINC 629]], [[MIL-STD-1553B]] (STANAG 3838), and EFABus ([[STANAG 3910]]) are commonly referred to as
The frequency or the speed of a bus is measured in Hz such as MHz and determines how many clock cycles there are per second; there can be one or more data transfers per clock cycle. If there is a single transfer
=== Bus multiplexing ===
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One common multiplexing scheme, [[#Address multiplexing|address multiplexing]], has already been mentioned.
Another multiplexing scheme re-uses the address bus pins as the data bus pins,<ref name="typewriter" /> an approach used by [[conventional PCI]] and the [[8086]].
The various
==History==
Over time, several groups of people worked on various computer bus standards, including the IEEE Bus Architecture Standards Committee (BASC), the IEEE
===First generation===
Early [[computer]] buses were bundles of wire that attached [[computer memory]] and peripherals. Anecdotally termed the
One of the first complications was the use of [[interrupt]]s. Early computer programs performed [[I/O]] by [[Busy waiting|waiting in a loop]] for the peripheral to become ready. This was a waste of time for programs that had other tasks to do. Also, if the program attempted to perform those other tasks, it might take too long for the program to check again, resulting in loss of data. Engineers thus arranged for the peripherals to interrupt the CPU. The interrupts had to be prioritized, because the CPU can only execute code for one peripheral at a time, and some devices are more time-critical than others.
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Early [[microcomputer]] bus systems were essentially a passive [[backplane]] connected directly or through buffer amplifiers to the pins of the [[CPU]]. Memory and other devices would be added to the bus using the same address and data pins as the CPU itself used, connected in parallel. Communication was controlled by the CPU, which read and wrote data from the devices as if they are blocks of memory, using the same instructions, all timed by a central clock controlling the speed of the CPU. Still, devices [[interrupt]]ed the CPU by signaling on separate CPU pins.
For instance, a [[disk drive]] controller would signal the CPU that new data was ready to be read, at which point the CPU would move the data by reading the
In some instances, most notably in the [[IBM PC]], although similar physical architecture can be employed, instructions to access peripherals (<code>in</code> and <code>out</code>) and memory (<code>mov</code> and others) have not been made uniform at all, and still generate distinct CPU signals, that could be used to implement a separate I/O bus.
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===Second generation===
{{Unreferenced section|date=June 2023}}
However, these newer systems shared one quality with their earlier cousins, in that everyone on the bus had to talk at the same speed. While the CPU was now isolated and could increase speed, CPUs and memory continued to increase in speed much faster than the buses they talked to. The result was that the bus speeds were now much slower than what a modern system needed, and the machines were left starved for data. A particularly common example of this problem was that [[video card]]s quickly outran even the newer bus systems like [[PCI Local Bus|PCI]], and computers began to include [[Accelerated Graphics Port|AGP]] just to drive the video card. By 2004 AGP was outgrown again by high-end video cards and other peripherals and has been replaced by the new [[PCI Express]] bus.
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{{See also|Bus network}}
Buses such as [[Wishbone (computer bus)|Wishbone]] have been developed by the [[open source hardware]] movement in an attempt to further remove legal and patent constraints from computer design.
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* [[UNI/O]]
* [[SMBus]]
* [[Advanced eXtensible Interface]]
* [[M-PHY]]
{{Div col end}}
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===Serial===
Many [[field bus]]es are serial data buses (not to be confused with the parallel
* [[CAN bus]] ("Controller Area Network")
* [[Modbus]]
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* [[Bus mastering]]
* [[Communication endpoint]]
* [[Computer port (hardware)]]
* [[Control bus]]
* [[Crossbar switch]]
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==References==
{{Reflist|refs=
}}
==External links==
* [http://pinouts.ru/pin_Slots.shtml Computer hardware buses and slots pinouts with brief descriptions]
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