Control Unit

What is Control Unit?

The unit within the CPU that coordinates the CPU's operations: it decodes each instruction, sends out the control signals that make other components act, and manages the movement of data and instructions around the CPU and to and from memory, so directing the whole fetch-decode-execute cycle.

This definition is part of the Hardware chapter in Cambridge O Level Computer Science.

Concepts related to Control Unit

• control bus
• decode
• cir

Common mistakes with Control Unit

• 6. “The CIR stores the address of the next instruction.” Why wrong This is misconception 5 in reverse — the two registers have been swapped. Correct model The CIR holds the current instruction, so the control unit can decode it and then execute it. The address of the next instruction is in the PC. Exam-safe “The CIR stores the instruction currently being decoded and executed.” Check: which register does the control unit read in order to decode?The CIR.
• 8. “The ALU controls every CPU component.” Why wrong The ALU is passive: it acts on values put in front of it. Nothing in its name or its definition involves control. Correct model The control unit coordinates the CPU: it decodes instructions, sends control signals and manages the movement of data. The ALU performs arithmetic, logic and comparisons. Exam-safe “The control unit coordinates the operations of the CPU; the ALU performs calculations, logical operations and comparisons.” Check: which unit sends the memory-read signal during a fetch?The control unit, along the control bus.
• 9. “Execute always means performing arithmetic.” Why wrong Many instructions never touch the ALU — storing a value, loading a value, jumping to an address, sending data to a device. Correct model In execute, the control unit sends the signals needed to carry out whatever the instruction was, using the ALU only when a calculation, logical operation or comparison is required. Exam-safe “The control unit sends the control signals needed to carry out the instruction, using the ALU where a calculation, logical operation or comparison is required.” Check: is the ALU used when executing STO 302?No. Storing the accumulator's value at an address moves data; no calculation is performed.