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Computer Architecture and Processing Systems

The fetch-execute cycle

  • 3 things to know
  • AS Unit 2

Overview

Fetch-execute cycle and execute stage of the cycle.

What you need to know

Answer each one before you reveal it.

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Key term

What is the fetch-execute cycle?

Key term

Which register supplies the address at the start of a fetch?

Key term

Describe what happens once the address is in the memory address register.

Questions from this subtopic

Each one with the model answer underneath.

  1. 1

    What is the fetch-execute cycle?

    Model answer

    The fetch-execute cycle is the repeating sequence in which a processor collects an instruction from memory, works out what it means and then carries it out.

  2. 2

    Which register supplies the address at the start of a fetch?

    Model answer

    The program counter supplies the address of the next instruction, and that address is copied into the memory address register.

  3. 3

    Describe what happens once the address is in the memory address register.

    Model answer

    The address travels along the address bus to main memory, and the contents of that location are returned along the data bus into the memory data register.

  4. 4

    At what point in the cycle is the program counter increased?

    Model answer

    The program counter is increased as soon as its address has been copied out, so the processor already knows where the following instruction lies while the current one is still being handled.

  5. 5

    Where is the fetched instruction placed before it is decoded?

    Model answer

    The fetched instruction is copied from the memory data register into the current instruction register.

  6. 6

    What happens during the decode stage?

    Model answer

    The control unit separates the instruction into its opcode and its operand and works out which circuits must be activated to carry the instruction out.

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