Trouble reversing a string in assembly

Question

I am trying to reverse a string in assembly. However my code does not seem to work correctly. I added a newline string for better readability.

I am using linux and n

Answer 1

The way to reverse a string by swapping characters is to swap the first and last, then the second and next to last, etc. In C, you would write:

for (i = 0; i < len/2; ++i)
{
    c = s[i];
    s[i] = s[len-i-1];
    s[len-i-1] = c;
}

In assembly language, the easiest way is to set up the ESI and EDI registers to point to the start and end of the string, then loop. At each iteration, you increment ESI and decrement EDI. The result looks something like this:

mov ecx, helloLen
mov eax, hello
mov esi, eax  ; esi points to start of string
add eax, ecx
mov edi, eax
dec edi       ; edi points to end of string
shr ecx, 1    ; ecx is count (length/2)
jz done       ; if string is 0 or 1 characters long, done
reverseLoop:
mov al, [esi] ; load characters
mov bl, [edi]
mov [esi], bl ; and swap
mov [edi], al
inc esi       ; adjust pointers
dec edi
dec ecx       ; and loop
jnz reverseLoop