Writing Functions in Assembly

Functions in assembly language involve defining, calling, and returning from subroutines.

The following assembly language code defines a function called add_numbers:

add_numbers: add eax, ebx ret

This function adds the values of the registers EAX and EBX and stores the result in the register EAX.

The following assembly language code calls the add_numbers function:

mov eax, 10 mov ebx, 20 call add_numbers ; The value of the register EAX is now 30.

Here's a step-by-step explanation with examples, using x86 assembly as a reference:

Function Definition

Define a label to mark the beginning of the function.

section .text global _start my_function: ; Function code here ret ; Return instruction

Function Parameters

Pass parameters to functions using registers or the stack.

section .text global _start my_function: mov eax, [ebp+8] ; Access the first parameter on the stack ; Function code here ret

Function Call

Call the function using the call instruction.

section .text global _start _start: ; Prepare parameters mov eax, 42 ; Call the function call my_function

Return Value

Functions can return values in registers or memory locations.

section .text global _start my_function: mov eax, 10 ; Set return value ret
Full Source:

Here's an example of a function that adds two numbers:

section .text global _start _start: ; Prepare parameters mov eax, 5 mov ebx, 7 ; Call the function call add_function ; Result is now in eax ; Exit program mov eax, 1 ; System call number for exit xor ebx, ebx ; Exit code 0 int 0x80 ; Invoke system call add_function: ; Function code add eax, ebx ; Add parameters ret

In this example, _start prepares two parameters (eax and ebx), calls the add_function subroutine, and the result is in eax after the call. The add_function subroutine adds the parameters and returns using the ret instruction. Adjust the code inside the function according to your specific requirements.


Functions involve defining labeled blocks of code, with parameters passed through registers or the stack. They are called using the call instruction, and return values can be stored in registers or memory locations. The function's execution is typically concluded with the ret instruction, which transfers control back to the calling code.