Definition: bus

A shared digital pathway between resources and devices. In a computer, there are two major types: the system bus and peripheral bus. The system bus, also known as the "frontside bus" or "local bus," is the internal path from the CPU to memory and is split into address bus and data bus subsets. Addresses are sent over the address lines to signal a memory location, and data are transferred over the data lines to that location.

System buses transfer data in parallel. In a 32-bit bus, data are sent over 32 wires simultaneously. A 64-bit bus uses 64 wires.

Peripheral Buses
The peripheral bus is the pathway to the peripheral devices such as a disk or printer. PCI and PCI Express are widely used peripheral buses. Devices connect to these parallel buses with cables to controller cards that plug into slots on the motherboard. Another common bus is USB, and devices are cabled to ports on the computer. USB is a serial bus, in which data travels over one wire.

Other peripheral buses have been used, including ISA, EISA, Micro Channel, VL-bus, NuBus, TURBOchannel, VMEbus, MULTIBUS and STD bus. The CAN bus and FlexRay bus are automotive buses.

Why Is It Called a Bus?
Inaccurate naming. Electronic buses are shared pathways, in which all devices receive the same signals. They do not function like a passenger bus that makes prescribed stops, one after the other. The data transfer technology similar to a passenger bus is a ring network (see Token Ring). See bus network, software bus, serial bus, PCI, PCI Express, USB and AGP.

System and Peripheral Buses

This illustration shows how chips, memory and peripherals in a PC interconnect via the system bus (top) and peripheral buses (AGP, PCI and PCI Express).