Editing Bus (computing) (section)

=== Bus multiplexing ===
{{main | Bus encoding#Other examples of bus encoding }}

The simplest [[system bus]] has completely separate input data lines, output data lines, and address lines.
To reduce cost, most microcomputers have a bidirectional data bus, re-using the same wires for input and output at different times.<ref name="typewriter" >
Don Lancaster.
[https://www.tinaja.com/ebooks/tvtcb.pdf "TV Typewriter Cookbook"]. ([[TV Typewriter]]).
Section "Bus Organization".
p. 82.
</ref>

Some processors use a dedicated wire for each bit of the address bus, data bus, and the control bus.
For example, the 64-pin [[STEbus]] is composed of 8 physical wires dedicated to the 8-bit data bus, 20 physical wires dedicated to the 20-bit address bus, 21 physical wires dedicated to the control bus, and 15 physical wires dedicated to various power buses.

Bus multiplexing requires fewer wires, which reduces costs in many early microprocessors and DRAM chips.
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 ''serial buses'' can be seen as the ultimate limit of multiplexing, sending each of the address bits and each of the data bits, one at a time, through a single pin (or a single differential pair).