bits 32 ; assembling for the 32 bits architecture

; declare the EntryPoint (a label defining the very first instruction of the program)
global start        

; declare external functions needed by our program
extern exit               ; tell nasm that exit exists even if we won't be defining it
import exit msvcrt.dll    ; exit is a function that ends the calling process. It is defined in msvcrt.dll
                          ; msvcrt.dll contains exit, printf and all the other important C-runtime specific functions

; our data is declared here (the variables needed by our program)
segment data use32 class=data
    ; ...
    a db 'message'
    l equ $-a
    b resb l

; our code starts here
segment code use32 class=code
    start:
        ; ...
        mov ESI,l
        label:
        mov AL,[a+ESI-1]
        
        sub AL,'a'-'A'
        mov [b+ESI-1],AL
        dec ESI
        jne label
        ; exit(0)
        push    dword 0      ; push the parameter for exit onto the stack
        call    [exit]       ; call exit to terminate the program