The fetch-execute cycle and its registers

Overview

Three phases of the fetch-execute cycle and first action of the fetch phase.

What you need to know

Answer each one before you reveal it.

Questions from this subtopic

Each one with the model answer underneath.

1. 1

   State the three phases of the fetch-execute cycle.

   Model answer

   The three phases are fetch, decode and execute. The cycle then repeats for the next instruction.

2. 2

   What is the first action of the fetch phase?

   Model answer

   The address held in the program counter is copied into the memory address register. The address bus then carries that address to main memory.

3. 3

   Which register places the address on the address bus during a fetch?

   Model answer

   The memory address register places the address on the address bus. The control unit then issues a memory read signal.

4. 4

   Which register receives the instruction arriving from the data bus?

   Model answer

   The memory data register receives the instruction from the data bus. The instruction is then copied into the current instruction register.

5. 5

   When is the program counter incremented, and why then?

   Model answer

   The program counter is incremented during the fetch phase. The processor therefore already holds the address of the next instruction when execution finishes.

6. 6

   What happens during the decode phase?

   Model answer

   The control unit splits the instruction in the current instruction register into an opcode and an operand. The opcode identifies which operation is to be carried out.

Other ways to work this

The same content, four other ways.

What is inside Levo

Six ways to work the same material.