ByteTrail

Level 0 · Module 1

CPU instruction cycle

Trace how a processor fetches, decodes, executes, and records an instruction.

Difficulty
beginner
Time
9 min read
Reading progress0% read

Not completed

By the end, you can

  • List the core stages of an instruction cycle.
  • Trace a simple arithmetic instruction through those stages.
  • Explain why the cycle repeats many times per second.

Summary

At a high level, a CPU repeatedly gets an instruction, works out what it means, carries it out, and stores any result. Then it chooses the next instruction and repeats.

Introduction

Programs can look like one large, continuous action: “open a photo” or “add a score.” A processor handles them as a precisely ordered stream of tiny steps. It repeats this cycle billions of times each second.

Explanation

The CPU starts with a program counter, a small value that points to the next instruction. During fetch, it reads that instruction from memory. During decode, it works out the instruction’s action and data. During execute, it performs the action, such as adding two values. During write back, it records a result in a register or memory when there is one to record. Finally, it chooses where to fetch next. Usually it moves forward, but a jump instruction can choose a different address.

Real-world example

Updating a game score

The game code loads the current score, adds the points earned, stores the new score, and continues. Each source-level statement may become several instruction cycles.

One instruction, four stages

Interactive exploration

Fetch, decode, execute

Step 1 of 4

  1. fetch
  2. decode
  3. execute
  4. write back

PC → 104

Fetch. Read the next instruction from memory.

The program counter is the CPU’s bookmark: it identifies where to fetch next. Usually it moves forward after an instruction. A jump or conditional branch can change it, which lets programs make choices and repeat work.

Code example

instruction-cycle.txttext
1. fetch   instruction at programCounter
2. decode  operation and operands
3. execute add R1 and R2
4. write   result to R1

Common mistakes

  • Treating fetch and decode as the same task.
  • Assuming every instruction writes a result; a comparison or jump may primarily change control flow.
  • Thinking the CPU reads a full application into itself at once.

Quiz

Knowledge check

A CPU has fetched an instruction. Before it can add two values, which stage works out that the instruction means “add”?

Try it

Trace an increment

For a hypothetical ADD counter, 1 instruction, write one short sentence for fetch, decode, execute, and write back.

Key takeaways

  • Fetch, decode, execute, and write back form a useful CPU-cycle model.
  • The program counter helps select the next instruction.
  • Complex programs emerge from enormous numbers of small repeated cycles.
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