Computer Bus Systems

A bus is a shared set of electrical pathways that carries data, addresses and control signals between the components of a computer. Rather than wiring every part directly to every other part, a bus provides a common channel that many devices connect to, which simplifies the design. The width of a bus, meaning how many bits it can carry at once, and its clock speed together determine how much data it can move per second.

Buses are often divided by function. A data bus carries the information being transferred, an address bus identifies the memory location or device involved, and a control bus carries signals that coordinate the transfer. In a PC, the processor communicates with memory over a fast memory bus, while expansion buses connect add-in cards. The PCI bus has long been a general-purpose expansion bus, and the AGP bus was a variant dedicated to graphics cards needing a fast link to memory.

Buses can be parallel, sending many bits side by side over separate wires, or serial, sending bits one after another over fewer lines. Although parallel buses once dominated, modern high-speed connections are usually serial, because a fast serial link avoids the timing problems of keeping many parallel signals aligned. Examples include serial buses for peripherals and SATA for storage drives, while specialised memory interfaces such as Rambus addressed the need for high memory bandwidth. The trend has been toward fast serial point-to-point links replacing older shared parallel buses.

Frequently asked questions

What is a computer bus?

It is a shared set of pathways that carries data, addresses and control signals between the processor, memory and other components.

What is the difference between a parallel and a serial bus?

A parallel bus sends many bits at once over separate wires, while a serial bus sends bits one after another, which scales better to high speeds.

What was the AGP bus used for?

AGP was a bus dedicated to graphics cards, giving them a fast connection to main memory separate from the general-purpose PCI bus.