.data
string DWORD 1,2,3,4,5
stringLen LENGTHOF string
.code
main PROC
mov ecx, stringLen / 2 ; loop counter
mov esi, OFFSET string ; esi = the memory address of the first letter
mov edi, OFFSET string + stringLen - 4 ; edi = the memory address of the last letter
L1:
mov al, [esi]
xchg [edi], al
mov [esi], al
inc esi
dec edi
loop L1
push 0