Control Unit

The Control Unit (CU) is one of the main components inside the CPU. It carries out a job rather than storing anything, so it is a component and not a register. Its role is to direct the rest of the CPU: it works out what each instruction means, then makes the other components carry that instruction out at the right moment.

Decoding and controlling

Decoding is how the CU makes sense of an instruction. Instructions arrive as patterns of binary, and the CU recognises what each pattern means and which action is needed next.

Once it knows what is needed, the CU sends control signals along control lines. A signal might tell memory to read or to write, tell the ALU which operation to carry out, or move data from one place to another. Saying only that the CU "sends control signals" tells you very little on its own; what matters is what each signal causes a component to do. The CU also controls the flow of data inside the CPU and along the paths linking the CPU to memory and to input and output devices, so values arrive in the right place at the right time.

Managing the cycle

The CU manages the fetch-decode-execute cycle, keeping fetch, decode and execute in that order for every instruction and starting the next fetch once the current instruction has been carried out. Decode, the middle stage, happens here rather than in the ALU.

What it does not do

The CU performs no calculations, because the ALU does those, and it stores nothing, because registers hold the data and instructions being worked on. Picture a conductor in front of an orchestra: the conductor makes no sound but decides who plays and when.