Fetch: the next instruction is fetched to the CPU from main memory

The idea

The fetch stage is the first part of the fetch–decode–execute cycle. The CPU needs the next instruction before it can do anything with it, and instructions are held in main memory (RAM) alongside data. The program counter (PC) holds the address of the next instruction; the CPU copies that address along the address bus to memory, and memory returns the instruction along the data bus into the current instruction register (CIR). The PC is then incremented so it points to the following instruction. So 'fetch' means moving the next instruction from main memory into the CPU, not carrying it out and not moving data to be stored. A useful check: if the question describes an address being sent and an instruction coming back, that is fetch.

What earns the marks

• Fetch is the first stage of the fetch–decode–execute cycle.
• The next instruction is copied from main memory (RAM) into the CPU.
• The program counter holds the address of the next instruction, which is sent to memory.
• The fetched instruction is placed in the current instruction register (CIR).
• The program counter is incremented to point to the following instruction.
• Fetch does not execute or decode the instruction; those are later stages.

Common mistakes and how to put them right

• Confusing fetch with execute.

  Put it right: Fetch only brings the instruction into the CPU; carrying it out is the execute stage.

• Saying the instruction is fetched from the hard disk or cache.

  Put it right: The next instruction is fetched from main memory.

• Thinking fetch stores a result back to memory.

  Put it right: Fetch reads the instruction in; writing results back is a later step.

Exam tips

• Link each stage to a register: PC supplies the address, CIR holds the fetched instruction.
• Use the phrase 'next instruction' to show you know fetch looks ahead, not at the current one.
• If asked to order stages, put fetch first, then decode, then execute.