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Apple Wiki

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.


The term is almost certainly derived from the electrical engineering term "busbar" - 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".[1]


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 CSMA/CD and bus mastering.[2]

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 clock rate 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 ISA, EISA, Micro Channel, VL-bus and PCI. Other peripheral buses are TURBOchannel, VMEbus, MULTIBUS and STD bus.[2]

Internal buses adopted by Apple[]


  1. Omnibus bars by John Norris. 1998-12-16.
  2. 2.0 2.1 Bus at the Free On-Line Dictionary Of Computing. 2010-07-10.
  3. Apple Macintosh before System 7 by Eric Rasmussen. Archived 2012-01-09.
  4. 1995: Clones, the Worst Macs, Pippin, PCI Slots, and CPU Daughter Cards by Daniel Knight, Low End Mac. 1995-12-31.
  5. Apple Power Mac G4 specs, EveryMac. Accessed 2021-08-26.
  6. Apple Power Mac G5 specs, EveryMac. Accessed 2021-08-26.
  7. NVMe -- the future of the SSD Mac by Jonny Evans, Computerworld. 2015-04-27.

See also[]

External links[]

  • Bus at Wikipedia
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