Branching with Labels in Assembly Language

Branching with labels in assembly language involves using conditional and unconditional jumps to alter the flow of program execution based on specific conditions.

The steps involved in branching with labels in assembly language are as follows:

  1. Define a label at the point in the program where you want to jump to.
  2. Use a branching instruction to jump to the label.

The following assembly language code shows how to branch with labels:

; Define a label. my_label: ; ... ; Jump to the label `my_label`. jmp my_label

This code will jump to the label my_label, regardless of where it is in the program.

Types of branching instructions

There are a number of different branching instructions that can be used in assembly language. The most common branching instructions are:

  1. jmp: Jumps to the specified address.
  2. je: Jumps if equal.
  3. jne: Jumps if not equal.
  4. jl: Jumps if less than.
  5. jle: Jumps if less than or equal to.
  6. jg: Jumps if greater than.
  7. jge: Jumps if greater than or equal to.

Explanation with examples, using x86 assembly as a reference:

Define Labels

Place labels at specific points in your code where you want to be able to jump to.

section .text global _start _start: ; Your code here jmp my_label ; Unconditional jump to my_label next_instruction: ; Code after the jump

Unconditional Jump

Use the jmp instruction for unconditional jumps.

section .text global _start _start: ; Your code here jmp my_label ; Unconditional jump to my_label next_instruction: ; Code after the jump my_label: ; Code at my_label

Conditional Jump

Use conditional jump instructions like je (Jump if Equal) or jne (Jump if Not Equal) to perform conditional branching.

section .text global _start _start: ; Your code here cmp eax, ebx ; Compare eax and ebx je equal_label ; Jump if equal not_equal_label: ; Code if not equal jmp end_label equal_label: ; Code if equal end_label: ; Code after the conditional jump
Full Source

Here's an example that checks if a number is even or odd:

section .data num dw 6 section .text global _start _start: ; Your code here mov ax, [num] test ax, 1 ; Test the least significant bit jz even_label ; Jump if zero (even) jmp odd_label ; Jump if not zero (odd) even_label: ; Code for even number jmp end_label odd_label: ; Code for odd number jmp end_label end_label: ; Exit program mov eax, 60 ; System call number for exit xor edi, edi ; Exit code 0 syscall ; Invoke system call

In this example, the program checks the least significant bit of the number (test ax, 1) and jumps to either the even_label or odd_label based on the result. Adjust the code inside each label according to your specific requirements.

Conclusion

Branching with labels involves using conditional and unconditional jumps to direct the flow of program execution based on specific conditions. Labels serve as markers within the code, allowing for the creation of decision points and facilitating both structured and conditional programming in low-level languages.






Related Topics
  1. Conditional Execution and Branching in Assembly Language
  2. Loops in Assembly Language
  3. Functions in Assembly Language
  4. Procedures and Subroutines in Assembly Language
  1. Stack and its usage in assembly language