Branch instruction (B) in ARM Architecture

Introduction

The branch instruction (B) in ARM lets the processor jump to a different part of the program. This helps control the flow of instructions, like making decisions or repeating actions.

When you want to skip some instructions and jump to another part of the program.

To create loops by jumping back to an earlier instruction.

To jump to a function or subroutine to perform a task.

To implement conditional actions by combining with condition codes.

To exit from a section of code and continue elsewhere.

Core Concept

ARM Architecture

B label

The 'label' is the address or name of the instruction to jump to.

The branch instruction changes the program counter to the label's address.

Key Points

This jumps to the instruction labeled 'loop_start'.

ARM Architecture

B loop_start

This jumps to the instruction labeled 'end', often used to finish a program.

ARM Architecture

B end

Detailed Explanation

This program counts from 0 to 5 using a loop. The branch instruction 'BNE loop' jumps back to 'loop' if R0 is not equal to 5, repeating the addition until the count reaches 5.

ARM Architecture

start:
    MOV R0, #0      ; Set R0 to 0
loop:
    ADD R0, R0, #1 ; Increase R0 by 1
    CMP R0, #5     ; Compare R0 with 5
    BNE loop       ; If not equal, jump back to loop
end:
    MOV R1, R0     ; Move final value to R1

Important Notes

The branch instruction changes the flow immediately, so instructions after it are skipped unless jumped back to.

Branch instructions can be combined with conditions like BEQ (branch if equal) or BNE (branch if not equal) for decision making.

Summary

The branch instruction (B) lets the program jump to a different place.

It is useful for loops, decisions, and calling functions.

The label tells where to jump, changing the program flow.