Decode: the instruction is decoded to work out what it is

The idea

Decode is the middle stage of the fetch–decode–execute cycle. Once the instruction has been fetched into the current instruction register, the control unit examines it to work out what operation it represents and what operands it needs. Decoding turns the binary instruction into signals the CPU can act on, so the control unit can then direct the right parts of the processor during execute. Decode does not carry out the operation and does not fetch the instruction; it interprets it. A useful check: if the description is about the control unit interpreting the instruction to determine what it is, that is decode. If it describes the arithmetic logic unit actually adding numbers, that is execute.

What earns the marks

• Decode is the second stage of the fetch–decode–execute cycle.
• The control unit decodes the instruction held in the current instruction register.
• Decoding works out what the instruction is and what operation it requires.
• Decoding produces the control signals used in the execute stage.
• Decode does not perform the operation itself; that happens in execute.
• Decode follows fetch and precedes execute.

Common mistakes and how to put them right

• Saying decode carries out the instruction.

  Put it right: Decode only interprets it; execution happens in the execute stage.

• Confusing decode with fetch.

  Put it right: Fetch brings the instruction in, while decode works out what it means.

• Thinking the ALU decodes instructions.

  Put it right: The control unit decodes, and the ALU performs arithmetic and logic operations.

Exam tips

• Name the control unit as the component that decodes the instruction.
• State that decoding determines what the instruction is, ready for execution.
• Keep the three stages in order: fetch, decode, execute.