Compare and branch patterns in ARM Architecture

Introduction

Compare and branch patterns help the processor decide what to do next based on a condition. They let the program choose different paths, like making decisions in real life.

When you want to check if two numbers are equal and do something if they are.

When you need to repeat actions only if a condition is true.

When you want to skip some instructions if a condition is false.

When you want to handle different cases based on a value.

When you want to stop a loop when a condition is met.

Core Concept

ARM Architecture

CMP Rn, Operand2
B{cond} label

CMP compares two values by subtracting but does not store the result.

B{cond} branches (jumps) to a label if the condition is true.

Key Points

Compare R0 with zero. If equal, branch to zero_label.

ARM Architecture

CMP R0, #0
BEQ zero_label

Compare R1 and R2. If not equal, branch to not_equal_label.

ARM Architecture

CMP R1, R2
BNE not_equal_label

Compare R3 with 10. If R3 is greater, branch to greater_than_label.

ARM Architecture

CMP R3, #10
BGT greater_than_label

Detailed Explanation

This program compares two registers R0 and R1. If they are equal, it sets R2 to 1. Otherwise, it sets R2 to 0.

ARM Architecture

    MOV R0, #5
    MOV R1, #5
    CMP R0, R1
    BEQ equal_label
    BNE not_equal_label

equal_label:
    MOV R2, #1
    B end

not_equal_label:
    MOV R2, #0

end:

Important Notes

The CMP instruction updates special flags in the processor to remember the comparison result.

Branch instructions use these flags to decide if they should jump or continue.

Common conditions include EQ (equal), NE (not equal), GT (greater than), LT (less than), GE (greater or equal), and LE (less or equal).

Summary

Compare and branch patterns let the processor make decisions by checking conditions.

CMP compares values without changing them but sets flags.

Branch instructions jump to different code parts based on these flags.