conditions_and_branches

Conditions and Branches in ARM Assembly

Introduction

In ARM assembly, branching allows for altering the flow of execution based on conditional tests. These conditions are evaluated through flags set by previous instructions, typically comparison operations.

Branching Instructions

• B: Unconditional branch, jumps to the specified label.
• BEQ: Branch if equal, jumps if the last comparison was equal.
• BNE: Branch if not equal, jumps if the last comparison was not equal.
• BGT: Branch if greater than, jumps if the last comparison showed greater than.
• BLT: Branch if less than, jumps if the last comparison showed less than.
• BLE: Branch if less than or equal to, jumps if the last comparison showed less than or equal.
• BAL: Branch always, an unconditional jump (equivalent to B).

Example Code Explanation

.global _start
_start:
    MOV R0,#1
    MOV R1,#2
    
    CMP R0,R1 
    
    BGT greater
    BAL default
    
greater:
    MOV R2,#1

default:
    MOV R2,#2

Line-by-Line Breakdown:

1. MOV R0,#1: Load the immediate value 1 into register R0.
2. MOV R1,#2: Load the immediate value 2 into register R1.
3. CMP R0,R1: Compare the values in registers R0 and R1.
4. BGT greater: If R0 is greater than R1, branch to the label 'greater'.
5. BAL default: Unconditionally branch to the label 'default'.

Execution Flow:

• The CMP instruction sets condition flags based on the comparison of R0 and R1.
• If R0 is greater than R1, execution jumps to greater, setting R2 to 1.
• If not, it falls through to default due to the unconditional branch (BAL), setting R2 to 2.

In this example, since R0 contains 1 and R1 contains 2, the condition for BGT is not met. Therefore, execution proceeds to the 'default' label where R2 is set to 2.