Reverse external array in assembly using swapping method - x86 MASM

Question

I am working on a project where we need to pass an array of type char as a parameter and reverse the array. I feel like I am very close to getting it done, but I am stuck on the actual swapping process.

For my swapping function in my .asm, I used the same method I would in c++ (use an unused register as a temp, then swap the front and the back.) What I am not understanding is how would I go about changing the actual content at that address. I assumed performing the following would "change" the content at the destination address:

mov eax,[edx]

However, this did not work as planned. After I ran a for loop to iterate through the array again, everything stayed the same.

If anyone can point me in the right direction, it would be great. I have provided the code below with as much comments as I could provide.

Also, I am doing all this in a single .asm file; however, my professor wants me to have 3 separate .asm document for each of the following functions: swap, reverse, and getLength. I tried to include the other 2 .asm document in the reverse.asm, but it kept giving me an error.

Assembly Code Starts:

.686
.model flat

.code

_reverse PROC 
    push ebp
    mov ebp,esp ;Have ebp point to esp

    mov ebx,[ebp+8] ;Point to beginning of array
    mov eax,ebx
    mov edx,1
    mov ecx,0
    mov edi,0
    jmp getLength

getLength:
    cmp ebp, 0          ;Counter to iterate until needed to stop
    je setup

    add ecx,1
    mov ebp,[ebx+edx]
    add edx,1
    jmp getLength

setup:                  ;This is to set up the numbers correctly and get array length divided by 2
    mov esi,ecx
    mov edx,0
    mov eax,ecx
    mov ecx,2
    div ecx

    mov ecx,eax
    add ecx,edx         ;Set up ecx(Length of string) correctly by adding modulo if odd length string
    mov eax,ebx
    dec esi

    jmp reverse

reverse:                ;I started the reverse function by using a counter to iterate through length / 2
    cmp edi, ecx
    je allDone

    mov ebx,eax         ;Set ebx to the beginning of array
    mov edx,eax         ;Set edx to the beginning of array
    add ebx,edi         ;Move ebx to correct index to perform swap
    add edx,esi         ;Move edx to the back at the correct index

    jmp swap            ;Invoke swap function

swap:
    mov ebp,ebx         ;Move value to temp
    mov ebx,[edx]       ;Swap the back end value to the front
    mov edx,[edx]       ;Move temp to back

    inc edi             ;Increment to move up one index to set up next swap
    dec esi             ;Decrement to move back one  index to set up for next swap

    jmp reverse         ;Jump back to reverse to setup next index swapping

allDone:
    pop ebp
    ret

_reverse ENDP

END

C++ Code starts:

#include <iostream>
#include <string>

using namespace std;

extern "C" char reverse(char*);

int main()
{
    const int SIZE = 20;
    char str1[SIZE] = { NULL };

    cout << "Please enter a string: ";
    cin >> str1;

    cout << "Your string is: ";

    for (int i = 0; str1[i] != NULL; i++)
    {
        cout << str1[i];
    }

    cout << "." << endl;

    reverse(str1);

    cout << "Your string in reverse is: ";

    for (int i = 0; str1[i] != NULL; i++)
    {
        cout << str1[i];
    }

    cout << "." << endl;

system("PAUSE");
return 0;
}

Answer 1

So after many more hours of tinkering and looking around, I was finally able to figure out how to properly copy over a byte. I will post my .asm code below with comments if anybody needs it for future reference.

I was actually moving the content of the current address into a 32 bit registers. After I changed it from mov ebx,[eax] to mov bl,[eax] , it copied the value correctly.

I will only post the code that I was having difficulty with so I do not give away the entire project for other students.

ASM Code Below:

swap:
mov bl,[edx]        ;Uses bl since we are trying to copy a 1 byte char value
mov bh,[eax]        ;Uses bh since we are trying to copy a 1 byte char value

mov [edx],bh        ;Passing the value to the end of the array
mov [eax],bl        ;Passing the value to the beginning of the array

inc eax             ;Moving the array one index forward
dec edx             ;Moving the array one index backwards

dec ecx             ;Decreasing the counter by one to continue loop as needed

jmp reverse         ;Jump back to reverse to check if additional swap is needed

Thanks for everyone that helped.

Answer 2

mov eax,[edx] (assuming intel syntax) places the 32 bits found in memory at address edx into eax. Ie, this code retrieves data from a memory location. If you'd like to write to a mem location, you need to reverse this, ie mov [edx], eax

After playing with some 16 bit code overnight for sorting, I've the following two functions that may be of use. Obviously, you can't copy/paste them - you'll have to study it. However, you'll notice that it is able to swap items of arbitrary size. Perfect for swapping elements that are structures of some type.

; copies cx bytes from ds:si to es:di
copyBytes:
    shr     cx, 1
    jnc     .swapCopy1Loop
    movsb
.swapCopy1Loop:
    shr     cx, 1
    jnc     .swapCopy2Loop
    movsw
.swapCopy2Loop:
    rep     movsd
    ret

;               bp+0        bp+2   bp+4
;void swap(void *ptr1, void *ptr2, int dataSizeBytes)
swapElems:
    push    bp
    mov     bp, sp
    add     bp, 4
    push    di
    push    si
    push    es

    mov     ax, ds
    mov     es, ax

    sub     sp, [bp+4]      ; allocate dataSizeBytes on the stack, starting at bp-6 - dataSizeBytes

    mov     di, sp
    mov     si, [bp+0]
    mov     cx, [bp+4]
    call    copyBytes

    mov     si, [bp+2]
    mov     di, [bp+0]
    mov     cx, [bp+4]
    call    copyBytes

    mov     si, sp
    mov     di, [bp+2]
    mov     cx, [bp+4]
    call    copyBytes

    add     sp, [bp+4]

    pop     es
    pop     si
    pop     di
    pop     bp
    ret     2 * 3