Bus

A bus is a set of electrical conductors (wires, PCB tracks, or connections within an integrated circuit) connecting various "stations", which can be functional units in a computer or nodes on a network.

Origin
The term is almost certainly derived from the electrical engineering term "" - a substantial, rigid power supply conductor to which several connections are made. This was once written as 'bus bar as it was a contraction of "omnibus bar" - a connection bar "for all", by analogy with the passenger omnibus - a conveyance "for all".

Implementation
A bus is a broadcast channel, meaning that each station receives every other station's transmissions and all stations have equal access to the bus. As a result, various schemes were developed to address the problem of collisions: multiple stations trying to transmit at once, such as and.

There are buses both within the CPU and connecting it to external memory and peripheral devices. The data bus, address bus and control signals, despite their names, really constitute a single bus since each is useless without the others.

The width of the data bus is usually specified in bits and is the number of parallel connectors. This and the determine the bus's data rate (the number of bytes per second which it can carry). This is one of the factors limiting a computer's performance. Most current microprocessors have 64-bit buses both internally and externally, with bus clock rates measured in gigahertz (GHz). The bus clock is typically slower than the processor clock.

Some processors have internal buses which are wider than their external buses (usually twice the width) since the width of the internal bus affects the speed of all operations and has less effect on the overall system cost than the width of the external bus. Various bus designs have been used in PCs, including, , , and PCI. Other peripheral buses are, , and.

Internal buses adopted by Apple

 * NuBus, first included in the Macintosh II, which shipped in April 1987.
 * PCI, first included in the Power Macintosh 9500 in June 1995.
 * AGP, first included in the Power Mac G4 ("Sawtooth") in August 1999.
 * PCI-X, first included in the Power Mac G5 (high-end) in June 2003.
 * PCIe, first included in the Power Mac G5 in October 2005.
 * NVMe, first included in the 2015 MacBook, Air, and Pro with OS X 10.10.3.