1. <electronics> {push-button}.

	2. <operating system> A graphical representation of an
	electrical {push-button} appearing as part of a {graphical
	user interface}.  Moving the {mouse pointer} over the
	graphical button and pressing one of the physical mouse
	buttons starts some software action such as closing a window
	or deleting a file.

	See also {radio button}.

	(1997-07-07)

<architecture> 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://www.foldoc.org/pub/omnibus.html)}.

	(2000-03-20)