Jump Relative (JR)

‍Introduction

In the last lesson, you learned how to jump to an absolute address using JP. This time, you’ll explore a different instruction: Jump Relative (JR).

JR is especially useful when writing loops and makes your programs far easier to read when combined with labels

What Is JR?

JR tells the CPU to jump a short distance forward or backward in memory relative to where it is now.

Key points:

• Uses 2 bytes (smaller than JP, which takes 3 bytes)
• Jumps up to 127 bytes forward or 128 bytes backward
• Ideal for tight loops or nearby branches
• Works seamlessly with labels — no need to calculate addresses by hand
• Module3 - Jump Relative

If you’re repeating something small, JR is usually your best choice.

Infinite Loop Using JR

       ORG 256
       LD A, 83         ; Start with ASCII 'S'

loop:
       ADD A, 1         ; Keep increasing A
       JR loop          ; Jump back to the 'loop' label

What This Does

• Loads ASCII 'S' (83) into register A
• Increments A by 1 each time
• Jumps back to the label loop forever

This results in an infinite loop. You’ll need to reset the emulator to stop it

‍Why Use Labels?

Previously, you might have written:

JP 262

That works, but if your program changes, the addresses change too, and you must recalculate them manually.

Using labels like this:

loop:
    JR loop

means the assembler automatically calculates the jump distance. This makes your code more flexible, readable, and error-proof. Thank goodness!

‍Important Notes About JR

• JR only works within ±128 bytes of the current instruction.
• For longer jumps, you’ll need JP.
• JR also has conditional forms, e.g.:
  
  • JR Z, label → jump if Zero flag is set
  • JR NZ, label → jump if Zero flag is not set
    These will be covered in a future lesson

Exercises

Exercise 3.1 — Start Small, Loop Forever

Write a program that:

1. Loads A with 5
2. Adds a value
3. Loops back using JR
4. Uses a label to make the loop readable
5. Module3 - Jump Relative

Summary

• JR is a compact jump for short distances.
• Best for loops and nearby instructions.
• Labels make your code clearer and easier to maintain.
• With JR and labels, you can build infinite loops or repeating sequences with ease

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