System Bus in Computers: Definition & Concept

Lesson Transcript
Instructor: Paul Zandbergen

Paul is a GIS professor at Vancouver Island U, has a PhD from U of British Columbia, and has taught stats and programming for 15 years.

A system bus in a computer is the way information travels from the central processing unit to computer memory to devices. Explore the definition and function of this system. Updated: 09/09/2021

Computer System & System Bus Definitions

Before we describe what a computer system bus is, let's describe what a computer system is. In very simple terms, a computer system is made up of three things:

  1. First, there is the CPU (or central processing unit). It's like a railway's grand central station where decisions are made, and just about everything that wants to go anywhere must get routed through it at some point.
  2. Second, computers have lots of destinations for the data to go (and stay) so it can be used later, which we call its addressable memory. These are kind of like houses with physical addresses, where the pieces of data live when they're not being processed by the CPU.
  3. Third, there are devices that do stuff with data instead of just storing it (called input and output devices) so we can see or hear what's going on or interact with the data in some way.

Okay, now that we have a basic idea of a computer system, here's what a system bus is. A system bus is kind of like all the train tracks that connect the CPU (the grand central station) with the computer memory (the houses of addressable data) and with the neat devices that let us interact (like the mouse, keyboard, screen, and audio system).

In early computer systems, all the data traveled across actual cables, which got pretty messy. Large bundles of wires were organized using bus bars, which is where the term 'bus' comes from. Nowadays, most of those wires are typically gold-plated traces (or tiny flat wires) on the computer's motherboard, a large flat surface that all the electronic stuff gets mounted to. The most critical connection of any computer system is the system bus.

Cables are still used in present-day computer systems, in particular, to connect external devices. The most common type of connection is a Universal Serial Bus (or USB), which you can probably recognize. Many peripheral devices, such as printers and scanners, use a USB connection.

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How the System Bus Works

The system bus is a little bit more complicated than a single train track, but not too much. Think of it as three rails per track, kind of like mass transit trains use. That's because each track has to carry three different things: data, address, and control.

  1. The data are the actual digital pieces of information that need to get somewhere or do something.
  2. The address information describes where the data is located and where it needs to go during a particular operation.
  3. The control part is like the instructions because data doesn't know what to do with itself (think 'Lego Movie'), so this manages the flow of address and data information. That includes which direction for the transfer of information and exactly how data needs to be routed through the computer system.

Because of these three different types of information, the system bus actually consists of three buses.

Three buses of system bus

Think of the system bus like an internal mass transportation system inside your computer, bussing data from place to place.

Here's an example. Say you want to listen to a song which is on your disk drive. That song is stored as data on that device. Somehow, that data has to get to a device in your computer that knows how to turn it into sound and then play that on your speakers. So, we specify the address of the MP3 file on the disk and the address of the sound card using the address bus. We also use the control bus to tell the song on the disk how to talk to the sound card. Now, all the traffic can get underway on the data bus, allowing the song to play until it's over. All of this is managed by the grand central station in the computer (the brains or CPU), but the system bus makes the actual transfers happen.

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