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 dd 127f5678h,0abcdabcdh
    l equ $-$$
    b resb l

; our code starts here
segment code use32 class=code
    start:
        ; ...
        mov ECX,l
        mov ESI,a
        mov EDI,b
        CLD
        
        label:
            LODSB
            cbw
            mov BX,AX
            LODSB
            cbw
            mov DX,AX
            LODSB
            cbw
            add BX,AX
            LODSB
            cbw
            add AX,DX
            rol EAX,16
            mov AX,BX
            STOSD
            sub ECX,4
            jg label
        ; exit(0)
        push    dword 0      ; push the parameter for exit onto the stack
        call    [exit]       ; call exit to terminate the program