What is the assembly source code of the following C code?

[James Taylor]

What is the assembly source code of the following C code?

Quote

 

char string[1001];

   while(string[index] != '\0') {
    diff = string[index] - string[index - 1];
    if(diff < 0) {
    count += (26 + diff);
    } else {
    count += diff;
    }
    index += 1;
    }hints: 

 

 

Quote

 

loc_4013B4:
1. lea     edx, [esp+13h]
2. mov     eax, [esp+408h]
3. add     eax, edx
4. movzx   eax, byte ptr [eax]
5. movsx   edx, al
6. mov     eax, [esp+408h]
7. sub     eax, 1
8. movzx   eax, byte ptr [esp+eax+13h]
9. movsx   eax, al
sub     edx, eax
mov     eax, edx
mov     [esp+400h], eax
cmp     dword ptr [esp+400h], 0
jns     short loc_401401[/php]

note: 

 

Quote

 

line 1 loading address of array from stack.
what the 2 and 3 and others line are doing?

Thanks

[deepzero]

The C code doesnt fully match the assembler code. It's only the first two lines of the loop body.

Please check if you copied the asm correctly and/or clarify what you want to know.

Generally, always make sure to let us know all types involved (count, diff) and locations (loc_401401).

Edited July 14, 2018 by deepzero

[James Taylor]

47 minutes ago, deepzero said:

The C code doesnt fully match the assembler code. It's only the first two lines of the loop body.

Please check if you copied the asm correctly and/or clarify what you want to know.

Generally, always make sure to let us know all types involved (count, diff) and locations (loc_401401).

I want to understand the following in C in assembly. (indexing)

Quote

char string[1001];

   while(string[index] != '\0') {
    diff = string[index] - string[index - 1];

I want to understand the following as well. 

 

Quote

add     eax, edx

 

Edited July 14, 2018 by James Taylor

[evlncrn8]

deja vu... it was explained once, if you didnt understand it then you wont now...  base.. index ? .. 

some days i wonder if some people are truly beyond help.. today is one of them

[deepzero]

1. lea     edx, [esp+13h]
2. mov     eax, [esp+408h]
3. add     eax, edx
4. movzx   eax, byte ptr [eax]

Let's look at above code snippet.

* There is an array at esp+13h.

* There is a counter variable at esp+408h

 

Also remember that lea doesnt actually load the dereferences value, but rather stores the address.

lea edx, [esp+13]  ------->   edx = esp+13

So we have:

First line: load the starting address of the array into edx -> edx = esp+13

Second line: load the value of the counter variable into eax -> eax = [esp+408]

third line: add them together. we are adding the base address of the array (edx = esp+13) and the value of the counter variable (eax = [esp+408]). So we store in eax a pointer into the array at the specified index.

fourth line: load a byte from the pointer we just calculated. So we load a byte from the byte array at address esp+13 at index [esp+408].

 

Conceptually, the four lines can be summed up in C as : int diff = string[index].

Where string is the array esp+13, and index is the int-variable at esp+408.

-----------

Maybe this pseudo-assembly makes it more clear. The four lines can be rewritten as:

mov eax, byte ptr [esp + 13h + dword ptr [esp+408h]]

again, esp+13 is the base address of the array, dword ptr [esp+408] is the index variable.