The Central Processing Unit (CPU) is the main component of a computer that executes instructions in order to run programs. It continuously carries out the fetch-execute cycle (also called the fetch-decode-execute cycle) to process instructions one at a time.
Key points
Definition:CPU: the central processing unit is the part of the computer that carries out program instructions by processing data and controlling other components using the fetch, decode and execute cycle.
Definition:Fetch-Execute Cycle: the repeated process where the CPU retrieves an instruction from memory, decodes it, executes it, and then moves on to the next instruction.
FETCH:The address of the next instruction is copied from the Program Counter (PC) to the Memory Address Register (MAR).
FETCH:The instruction is fetched from memory and copied into the Memory Data Register (MDR).
FETCH: The Program Counter is incremented to point to the next instruction.
DECODE:The instruction in the MDR is decoded by the Control Unit (CU).
EXECUTE: The decoded instruction is executed — this may involve the ALU performing a calculation.
Exam Tip:You must describe the cycle using the CORRECT REGISTER NAMES (PC, MAR, MDR). Vague answers like 'the instruction is fetched' without naming registers will not score full marks.
Common Mistake:Forgetting that the Program Counter increments DURING the fetch stage, not after execution.
Exam Tip:Use the word 'stores' when describing registers (e.g. 'the MAR stores the address'). Do NOT use action verbs like 'fetches' or 'retrieves' — registers STORE data, they don't perform actions.
Exam Example:Common 2-mark question: 'Identify two events that take place during the fetch-execute cycle.' Use specific events like 'the address is copied from the PC to the MAR' and 'the PC is incremented'.