KS3 Small Basic: Lesson 5 – Subroutines, Calculators & Coding Challenges (Dyslexia-Friendly)

A fully resourced, dyslexia-friendly KS3 Computer Science lesson introducing Subroutines (Sub…EndSub), branching with IF/ELSE, and creating a fully functional calculator program.

Includes PowerPoint, worksheet, worked examples, and challenge tasks—using the FREE online Small Basic compiler to ensure zero installation issues.

Designed with dyslexia-friendly formatting: pastel colour schemes, readable font style, reduced text density, and high-contrast code samples.

Students learn:

• How subroutines work and why programmers use them
• How Sub and EndSub create reusable blocks of code
• How to call subroutines in the correct order of execution
• How IF/ELSE statements allow the program to choose different outcomes
• How to build structured programs like calculators and mini-games

Practical coding tasks:

• Analyse example code to understand line order and execution flow
• Complete a partially written calculator by creating AddNumbers(), SubtractNumbers(), and MultiplyNumbers()
• Use IF/ELSE to decide which subroutine to run
• Integrate user input to customise calculations
• Extend the program with divide functionality and improved structure

Challenge activities (highly motivating for KS3):

• Add subroutines that fetch Number1 and Number2 directly from the user
• Create a command-driven calculator where the user selects add/subtract/multiply/divide
• Wrap the entire calculator inside a WHILE loop so it repeats until the user types EXIT
• Build a maths game: user chooses a skill, random numbers are generated, and the game continues until the correct answer is given

Assessment and evidence:

• Multiple screenshot prompts using the Windows Snipping Tool
• Written explanation boxes to ensure understanding of subroutines and branching
• Debugging and code-building tasks to demonstrate mastery

Lesson objectives:

• Create and call subroutines confidently
• Use IF/ELSE to control program flow
• Develop a working calculator using structured programming
• Apply logic, random numbers, and loops to create simple games

Ideal for KS3 Computing, coding clubs, enrichment, or as preparation for Python functions and selection.

This lesson is classroom-ready, accessible, and designed to make students feel like real programmers.