example asm code with function call

test.asm

; example ASM code
; compile with 'nasm -f elf test.asm && gcc -m32 -o test test.o'

bits 32         ; use 32 bit architecture
global main     ; declare 'main' funcion (called by the OS)
extern printf   ; 'printf' from libc

; read-only variables
section .data
strDesu     db "desu desu purrRRrRr %d", 10, 0  ; string, must end with 0, the 10 is "\n"
strRet      db "function returned %d", 10, 0

; uninitialized variables (writeable)
section .bss
var         resd 1  ; d -> 4 bytes

; program code
section .text
main:
    ; var = 33
    mov dword [var], 33
    ; calculate the answer
    add dword [var], 9
    ; call the function
    push dword [var]
    call testfunc
    add esp, 4*1    ; pop without writing to a register
    ; (now eax holds the return value)
    ; printf(strRet, <return value of testfunc>)
    push eax            ; second param
    push dword strRet   ; first param
    call printf
    add esp, 4*2    ; remove args from stack, equivalent to 'pop' twice
    ; return exit status 0
    xor eax, eax    ; eax = 0
    ret             ; returning from main ends the program

testfunc:
    ; initialize stack frame
    push ebp
    mov ebp, esp
    ; printf(strDesu, <argument>)
    push dword [ebp+8]  ; second arg, take from value passed to function
    push dword strDesu  ; first arg
    call printf
    add esp, 4*2    ; remove args from stack, equivalent to 'pop' twice
    ; return constant 3
    mov eax, 3      ; by convention eax holds the return value
    leave           ; cleanup stack frame
    ret             ; return to caller