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Case Study: Visualizing the Fetch-Decode-Execute Cycle

Denny Denny
3 min read

Educator's Toolkit

📅 January 2026 • ⏱️ 15 min read • 🎓 For Educators

The fetch-decode-execute cycle is one of the most fundamental concepts in computer science, yet it's often difficult for students to visualize. This case study shows how interactive simulation brings this abstract concept to life, improving student comprehension and engagement.

The Teaching Challenge

When teaching CPU architecture, instructors face a common problem: the fetch-decode-execute cycle happens in nanoseconds on real hardware, making it invisible to human observation. Static diagrams fail to convey the dynamic flow of data through the processor.

Students often memorize the three phases without truly understanding:

  • How does the CPU know which instruction to fetch next?
  • What exactly happens during decoding?
  • How do different instructions result in different actions?
Sequential Instruction Executor circuit

DigiSim.io's Sequential Instruction Executor: a working CPU simulation where students can watch every stage of instruction processing.

The Three Phases Visualized

1 FETCH: Getting the Instruction

The CPU reads the next instruction from memory. In the simulation:

  • The Program Counter (PC) highlights, showing the current address
  • Data flows from memory to the Instruction Register (IR)
  • The PC increments to point to the next instruction

Students SEE the binary instruction bits move from memory into the processor.

2 DECODE: Understanding the Instruction

The control unit interprets the instruction bits. In the simulation:

  • The opcode field (first few bits) determines the operation
  • The control unit generates specific control signals
  • Register addresses are extracted and sent to the register file

Students watch the instruction "unpack" into meaningful control signals.

3 EXECUTE: Performing the Operation

The actual operation is carried out. In the simulation:

  • Operands flow from registers to the ALU
  • The ALU performs the specified operation (ADD, SUB, AND...)
  • Results flow back to the destination register or memory

Students observe data transformations in real-time.

Fetch-Decode-Execute Cycle lesson

SimCast lesson showing each phase of the cycle with highlighted data paths and explanatory annotations.

Example Instruction Walkthrough

Let's trace the instruction ADD R1, R2, R3 (R3 = R1 + R2):

FETCH: PC = 0x04 → Memory outputs 0x2113 → IR = 0x2113 → PC = 0x05 DECODE: IR[15:12] = 0010 → ADD operation IR[11:8] = 0001 → Read R1 (source A) IR[7:4] = 0001 → Read R1 (source B) [corrected: 0010 = R2] IR[3:0] = 0011 → Write R3 (destination) EXECUTE: R1 = 5, R2 = 3 → ALU ADD → Result = 8 → R3 = 8 Status flags updated (Z=0, C=0, N=0)

Key Insight: The Power of Slow Motion

Real CPUs execute billions of cycles per second. By slowing down to one step at a time, students can follow the logical flow and understand cause-and-effect relationships. DigiSim.io's step mode lets students advance one clock cycle at a time, or watch animations at adjustable speeds.

Learning Outcomes Improved

Educators using interactive CPU simulations report significant improvements:

  • Conceptual understanding: Students explain the cycle more accurately on assessments
  • Debugging skills: Students can trace through programs to find errors
  • Engagement: Visual feedback maintains attention during lectures
  • Retention: Hands-on exploration creates lasting memory

Classroom Implementation

Live Demonstration (15 minutes)

  1. Open the Sequential Instruction Executor template
  2. Load a simple program (e.g., load constant, add, store)
  3. Step through each cycle, narrating the data flow
  4. Ask students to predict what happens next before each step

Hands-On Lab Exercise

  1. Students open the simulation individually
  2. Provide a mystery program (just binary values)
  3. Students decode each instruction manually
  4. They verify by stepping through the simulation
  5. Students write their own simple program
Simple CPU lesson

A complete Simple CPU lesson where students can modify the program and observe execution.

Advanced Topics Enabled

Once students master the basic cycle, the same simulation platform enables teaching:

  • Pipelining: What happens when fetch, decode, and execute overlap?
  • Branching: How does a conditional jump change the PC?
  • Interrupts: What happens when the cycle is interrupted?
  • Memory hierarchy: What if the fetch has to wait for cache?

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