push ebp
mov ebp, esp
mov ebx, [ebp+08]
cld
mov ecx,-1
mov edi,ebx
mov al,0
repne scasb
mov eax,edi
dec eax
xor eax,ebx

;ToLowerCase
mov ecx,eax
mov edi,ebx
ToLower:
or byte[edi],0x20
inc edi
loop ToLower
pop ebp

input: HOLA SOY PINK :)
ouput:hola soy pink :)