The Fetch-Decode-Execute Cycle (Cambridge (CIE) O Level Computer Science)

Fetch-Decode-Execute Cycle (FDE)

What is the purpose of the CPU?

• The purpose of the Central Processing Unit (CPU) is to fetch, decode and execute instructions

• The CPU is the brain of the computer and its job is to take an input, process data and produce an output

What is the Fetch-Decode-Execute cycle?

• The Fetch-Decode-Execute Cycle is the cycle that the central processing unit (CPU) runs through billions of times per second to make a computer work

• A computer takes an input, processes the input and then delivers an output for the user

  • Input: Clicking a button on the gamepad 

  • Process: The CPU inside the console follows a set of instructions to carry out the task

  • Output: The player moving on screen

The Fetch-Decode-Execute cycle stages

Fetch stage

• During the fetch stage of the cycle, the program counter holds the address of the next instruction to be fetched from memory

• The address of the next instruction or data to be fetched is copied into the memory address register (MAR)

• The address of the instruction or data is then sent along the address bus and awaits a signal from the control bus

• The signal sent along the control bus is sent from the control unit (CU) to the main memory

• The data or instructions received from main memory is fetched to the memory data register (MDR) via the data bus

• A copy of the instruction or data is stored in the current instruction register (CIR)

• The program counter (PC) increments by 1 so it is pointing to the next instruction to be executed

Decode stage

• During the decode stage of the cycle, the CPU needs to work out what is required from the instruction 

• This is done as the instruction is split into two parts:

  • Opcode - what the instruction is

  • Operand - what to do it to

    • This could be either data or an address where the data is stored

Execute stage

• During the execute stage of the cycle, the CPU will carry out the instruction that was fetched

• Some examples that would take place at this stage are

  • Performing a calculation

  • Storing a result or data back in main memory (RAM)

  • Going to main memory to fetch data from a different location

The important things to remember are:

• An instruction or data is fetched from memory

• The instruction is decoded 

• The instruction is executed

• The cycle repeats billions of times per second