Systems architecture: bus.

The idea

A bus is a set of parallel wires that carries data, addresses or control signals between components of a computer, such as the CPU, memory and input/output devices. Because one bus is shared, only one transfer happens at a time, which is why bus width and clock speed affect performance. The three main types are the data bus, which carries the actual data or instruction; the address bus, which carries the location being accessed; and the control bus, which carries signals such as read, write and clock pulses. A wider data bus moves more bits per transfer, and a faster clock means more transfers per second. In a multiple-choice question you may be asked which bus carries a particular signal or how bus width affects performance, so match each bus to its job and remember that the address bus is one-directional from the CPU while the data bus is bidirectional.

What earns the marks

• A bus is a set of wires carrying signals between computer components.
• The data bus carries data and instructions between the CPU and memory or devices.
• The address bus carries the memory location being read from or written to.
• The control bus carries control signals such as read, write and clock pulses.
• Bus width and clock speed affect how much data can be transferred and how quickly.

Common mistakes and how to put them right

• Treating the data bus and address bus as interchangeable.

  Put it right by stating that the data bus carries the contents while the address bus carries the location.

• Believing a bus carries several transfers at once.

  Put it right by explaining that a shared bus handles one transfer at a time.

• Ignoring the control bus.

  Put it right by naming the read, write and clock signals it carries.

Exam tips

• Match each option to one of the three bus types before deciding.
• Use the words 'data', 'address' and 'control' to check whether an option describes the right signal.
• If an option mentions performance, check whether it correctly links bus width or clock speed to transfer rate.