What Is the Fetch–Decode–Execute Cycle?

The Fetch–Decode–Execute cycle (FDE cycle) is the process used by the CPU to execute instructions stored in memory.

Every program that runs on a computer follows this cycle repeatedly.

The cycle consists of three main stages:

1️⃣ Fetch
2️⃣ Decode
3️⃣ Execute

This process happens millions or billions of times per second in modern computers.

Step 1: Fetch

During the fetch stage, the CPU retrieves the next instruction from memory (RAM).

Key components involved:

• Program Counter (PC) – stores the address of the next instruction

• Memory Address Register (MAR) – stores the memory address to access

• Memory Data Register (MDR) – stores the data retrieved from memory

Process

1️⃣ The PC contains the address of the next instruction
2️⃣ The address is copied to the MAR
3️⃣ The instruction at that memory address is fetched from RAM
4️⃣ The instruction is placed in the MDR

The PC is then incremented to point to the next instruction.

Step 2: Decode

In the decode stage, the CPU interprets the instruction.

Key component involved:

• Control Unit (CU)

Process

1️⃣ The instruction in the MDR is copied to the Current Instruction Register (CIR)
2️⃣ The Control Unit decodes the instruction
3️⃣ The CPU determines what operation must be performed

The control unit then sends signals to the appropriate components.

Step 3: Execute

In the execute stage, the CPU performs the required action.

Key component involved:

• Arithmetic Logic Unit (ALU)

Possible actions include:

• Performing calculations

• Comparing values

• Moving data between registers

• Sending output to devices

The result may be stored in the Accumulator (ACC) or another register.

Summary of the FDE Cycle

StageDescriptionFetchInstruction is retrieved from memoryDecodeInstruction is interpreted by the control unitExecuteInstruction is carried out by the CPU

This cycle repeats continuously while the computer is running programs.

Why the Fetch–Decode–Execute Cycle Is Important

The FDE cycle explains how the CPU processes instructions step by step.

Understanding this cycle helps students learn:

• How programs run on computers

• How instructions are executed

• How CPU components work together

It is one of the most fundamental concepts in computer architecture.

Exam Tip (2210 / 0478)

Students are frequently asked to:

• Describe the fetch stage

• Describe the decode stage

• Describe the execute stage

• Explain the role of PC, MAR, MDR and CIR

A common exam question is:

Describe the fetch–decode–execute cycle.

Practice Question

Explain one role of the Program Counter (PC).

Answer

The Program Counter stores the memory address of the next instruction to be executed.

Study Computer Science with IVY Online

Preparing for Cambridge O Level Computer Science (2210) or IGCSE Computer Science (0478) requires clear understanding of concepts and exam-focused practice.

With the IVY Online App, students can access:

• Complete syllabus coverage

• Interactive concept explanations

• Past paper practice

• Exam-focused revision

📱 Download the IVY Online App and start preparing smarter for Computer Science exams.