bus
One of the sets of conductors (wires, PCB tracks or connections in an {integrated circuit}) connecting
the various {functional units} in a computer. There are
busses 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, i.e. the number of parallel
connectors, and its {clock rate} determine its 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 32-bit busses both internally
and externally. 100 or 133 {megahertz} bus clock rates are
common. The bus clock is typically slower than the processor
clock.
Some processors have internal busses which are wider than
their external busses (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 the {PC}, including
{ISA}, {EISA}, {Micro Channel}, {VL-bus} and {PCI}. Other
peripheral busses are NuBus, TURBOchannel, VMEbus, MULTIBUS and
STD bus.
Some {networks} are implemented as a {bus} at the {physical
layer}, e.g. {Ethernet} - a one-bit bus operating at 10 (or
later 100) {megabits per second}.
The term is almost certainly derived from the electrical
engineering term "bus bar" - a substantial, rigid power supply
conductor to which several connections are made. This was
once written "'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".
{More on derivation
(http://foldoc.doc.ic.ac.uk/pub/omnibus.html)}.
(2000-03-20)