Systems architecture: control unit

The idea

The control unit (CU) is the part of the CPU that directs the fetch–decode–execute cycle rather than performing arithmetic itself. It fetches each instruction from main memory, decodes it to work out what operation is required, then sends control signals along the control bus to the relevant components so the operation happens in the right order. For example, for an instruction to add two values, the CU coordinates the registers, the ALU and memory so data is moved and processed at the correct moment. It also uses the clock to time each step. A common exam question asks you to identify the CU's role or distinguish it from the ALU, so focus on 'directs and coordinates' rather than 'calculates'.

What earns the marks

• The control unit directs the flow of data and coordinates the fetch–decode–execute cycle.

Common mistakes and how to put them right

• Saying the control unit performs calculations.

  Put it right by stating that arithmetic and logic are done by the ALU, while the CU directs and coordinates.

• Confusing the control unit with main memory.

  Put it right by explaining that the CU fetches instructions from memory but is not where programs are stored.

• Describing the CU as storing data long-term.

  Put it right by noting registers hold small amounts of data temporarily and the CU coordinates their use rather than acting as storage.

Exam tips

• Learn the phrase 'directs and coordinates' as the core function of the CU.
• Be ready to contrast the CU with the ALU in a single sentence.
• Link the CU to the fetch–decode–execute cycle and to control signals when explaining its role.