Lesson 2: The Computer Bus
This lesson discusses the set of hardware lines, or conductors, by which data is transferred internally in the components of a computer system.
After this lesson, you will be able to:
Estimated lesson time: 5 minutes
Understand the concept of an electronic bus.
For efficient use of system resources, most communications within a computer need to occur at a much quicker rate than processing signals one at a time would allow. Therefore, the computer moves information through a bus. Several types of buses are used within a computer, and they are discussed more fully in later tutorials. For now, let's simply look at what a bus is and how it works.
A bus is a group of electrical conductors (usually wires) running parallel to one another that can carry a charge from point a to point b. These conductors can be copper traces on a circuit board or wires in a cable. Usually, they are found in multiples of eight (8, 16, 32, 64, and so on). Early computers used eight conductors for the main system bus, thereby allowing the transmission of eight bits, or one byte, of information at a time. Figure 2.2 illustrates an 8-bit and a 16-bit bus.
Figure 2.2 Computer bus
The physical configuration of a bus isn't as important as its function. A bus provides a common path along which to transmit information in the form of code. It allows any device to receive or send information to any other device on the same bus. This is not unlike the telegraph system in which a single wire was strung from one end of the country to the other. Any town that tapped into the wire could exchange information with any other town also connected to the wire.
Another familiar example of a bus system is the electrical wiring in a home or office. The 110-volt AC outlets are wired with three wires-hot, neutral, and ground-that run in parallel from one outlet to another. Each time a device is plugged in, it is connected to the bus (in parallel). (See Figure 2.3.)
Figure 2.3 Connecting to a bus
Remember: in a computer, a bus is a set of parallel wires or lines to which the CPU, the memory, and all input/output devices are connected. Everything in a computer is connected to a bus. The actual number of wires, or lines, in a bus can vary from one computer to another or even from one part of a computer to another. The bus contains one line for each bit needed to give the address of a device or a location in memory. It also contains one line for each bit of data being transmitted from device to device.
A manufacturer might also use additional lines for power or other communication within the computer. When we speak of buses within a computer (data bus, expansion bus, or address bus), we are speaking of a specific numbers of wires, dedicated to a specific purpose-connecting parts of the computer to each other for the exchange of data between components.