Subroutienes‍

Introduction

As your programs grow in complexity, you’ll often want to reuse the same block of instructions, like printing a character or updating a score. Instead of copying and pasting code in multiple places, assembly lets you create subroutines: reusable chunks you can call whenever needed.

Subroutines keep your code clean, efficient, and easier to understand.

What Is a Subroutine?

A subroutine is a block of code that performs a specific task.

• You call it from somewhere else in your program.
• Once finished, it returns control to where it was called.

If you’ve used BASIC before, it’s just like GOSUB

Key Instructions: CALL and RET

CALL label

• Tells the CPU to jump to a subroutine.
• Saves the address of the next instruction on the stack.
• Execution continues at the label you specify.

Example:

CALL PRINT

This jumps to the subroutine labelled PRINT, then resumes where it left off after a RET

‍RET

• Short for return.
• Tells the CPU to jump back to where the last CALL came from.

Example:

RET

Every subroutine must end with RET, otherwise your program will crash or behave unpredictably

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Example Program: Print with a Subroutine

        ORG 256
        LD A, 200        ; Character to print
        LD C, 50         ; Number of times to print

NXT:    CALL PRINT       ; Call subroutine to print
        DEC C
        INC A
        JP NZ, NXT
        RET

PRINT:  RST 8
        DEFB 158
        RET

Walkthrough

• Sets A to 200 (starting character)
• Sets C as a loop counter
• Calls the PRINT subroutine
• PRINT prints the character in A using RST 8 and DEFB 158
• RET returns to the loop at NXT

Subroutines vs Loops: What’s the Difference?

They might look similar, both jump around in your code, but how they return is key.

Loop Example

LD A, 0

loop:
    ADD A, 1
    JP loop

This repeats forever. It never returns

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Subroutine Example

LD A, 42
CALL printVal

printVal:
    RST 8
    DEFB 158
    RET

Here, CALL jumps to printVal, which prints the character, then returns back to the main program

‍Conditional Subroutines

Just like jumps, you can also do conditional calls.

Example:

CALL Z, PSPC

This calls PSPC only if the Z (zero) flag is set

‍Why Use Subroutines?

• Avoid duplicating code
• Make logic modular
• Simplify changes — update once, apply everywhere
• Keep programs clean and readable
• Module6 - Subroutines

Summary

• Use CALL label to jump to a reusable block of code.
• Use RET to return from that block to the main program.
• Together, CALL/RET act like function calls.
• Subroutines keep code organised and reduce repetition.
• You can also use conditional subroutines (CALL Z,label)

Exercises

Exercise 6.1 — Loop Countdown

Write a loop that counts down from 10 to 0 using DEC and JR NZ.

Exercise 6.2 — Subroutine Math

Create a subroutine that adds 1 to register A and returns. Call it from the main program using CALL.

Exercise 6.3 — Rewriting Old Code

Take your previous “print the alphabet” program and:

• Move the character-printing logic into a subroutine
• Call it from the loop

Exercise 6.4 — Conditional Subroutine

Write a program that calls a subroutine only if the result of a previous instruction was zero, using CALL Z,label

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