Execute: the instruction is executed (carried out). This may include…

The idea

The execute stage is the third stage of the fetch–decode–execute cycle. Once an instruction has been fetched from main memory and decoded by the control unit, the processor carries it out. Execution is not one single action: it depends on what the instruction asks for. An arithmetic instruction such as adding two values held in registers is carried out inside the CPU's ALU. A load instruction, however, requires the processor to read a value from main memory into a register, and a store instruction requires it to write a register value back to main memory. So execution may involve reading from or writing to main memory, but it does not have to. The key idea is that execute means the decoded instruction is actually performed, producing the result or data movement it specifies.

What earns the marks

• The execute stage follows fetch and decode in the fetch–decode–execute cycle.
• Execution means the decoded instruction is actually carried out, not merely retrieved or interpreted.
• What happens during execution depends on the instruction type, for example arithmetic in the ALU or a data transfer.
• Execution may include reading from main memory, for example a load instruction bringing data into a register.
• Execution may include writing to main memory, for example a store instruction saving a register value.
• Execution does not always involve main memory; some instructions operate only on registers or the ALU.

Common mistakes and how to put them right

• Thinking execute always means accessing main memory.

  Put it right by noting that some instructions are carried out entirely within the CPU, such as register arithmetic.

• Confusing execute with decode.

  Put it right: Decode works out what the instruction means, whereas execute performs it.

• Assuming execute is the first stage.

  Put it right: It is the third stage, after fetch and decode.

• Believing the whole cycle runs only once.

  Put it right: The cycle repeats continuously for each instruction.

Exam tips

• Link each stage to its job: fetch brings the instruction, decode interprets it, execute performs it.
• When a question mentions main memory in the execute stage, think of load and store instructions as concrete examples.
• Use precise vocabulary such as register, ALU, control unit and main memory rather than vague words like 'process'.