[SOLVED] Pointing to the wrong address

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Apos

Joined: 11 Jan 2012
Posts: 17
Location: Paris, France (I'm not from France though.)

Apos 14 Jan 2012, 19:14

I have played with the 'dup' command in order to create arrays.

When I do something like this:

Code:

int_var         db      2 dup(0)

...

     mov ecx, int_var        ;ecx points to int_var
      mov byte [ecx], 50      ;int_var[0] = 50
    inc ecx                 ;ecx now points to int_var[1]
       mov byte [ecx], 30      ;int_var[1] = 30
    dec ecx                 ;ecx now points to int_var[0] again (But it doesn't.)

ecx ends up pointing to something else than int_var[0] which is 50. I thought that doing it that way would work but it doesn't.

However, when I do it without putting the 30, it works.

Code:

int_var             db      2 dup(0)

...

     mov ecx, int_var        ;ecx points to int_var
      mov byte [ecx], 50      ;int_var[0] = 50
    inc ecx                 ;ecx now points to int_var[1]

   dec ecx                 ;ecx now points to int_var[0] again

_________________
"A designer knows he has achieved perfection not when there is nothing left to add, but when there is nothing left to take away." - Antoine de Saint-Exupéry

Last edited by Apos on 14 Jan 2012, 23:16; edited 1 time in total

14 Jan 2012, 19:14

LocoDelAssembly
Your code has a bug

Joined: 06 May 2005
Posts: 4623
Location: Argentina

LocoDelAssembly 14 Jan 2012, 19:29

You must have something wrong in how you check that int_var[0] is 50 (or the code is not an exact enough representation of your real compilable code). One mistake could be that you are pretending int_var[0] to be 4 bytes long (dword, int), so of course, when you write 30 in the second array slot you change the dword interpretation of int_var[0] to something different than 50 (50 + 30*2^8 + x*2^16 + y*2^24 = 7730 with x = y = 0 based on your experience when you don't write 30, but technically both x and y are unknown here).

Last edited by LocoDelAssembly on 14 Jan 2012, 19:34; edited 1 time in total

14 Jan 2012, 19:29

Apos

Joined: 11 Jan 2012
Posts: 17
Location: Paris, France (I'm not from France though.)

Apos 14 Jan 2012, 19:34

Well... The 50 does end up being 7730... What do I have to do in order to only modify int_var[1]?

14 Jan 2012, 19:34

LocoDelAssembly
Your code has a bug

Joined: 06 May 2005
Posts: 4623
Location: Argentina

LocoDelAssembly 14 Jan 2012, 19:37

Then your number to string routine is working with the wrong size, you have byte sized numbers here, not dword. If you want int as per C definition then your code should be like this:

Code:

int_var         dd      2 dup(0)

...

        mov ecx, int_var        ;ecx points to int_var
        mov dword [ecx], 50      ;int_var[0] = 50
        add ecx, 4                 ;ecx now points to int_var[1]
        mov dword [ecx], 30      ;int_var[1] = 30
        sub ecx, 4                 ;ecx now points to int_var[0] again

14 Jan 2012, 19:37

Apos

Joined: 11 Jan 2012
Posts: 17
Location: Paris, France (I'm not from France though.)

Apos 14 Jan 2012, 19:42

That works. Thanks!

I think I used to do something like that in C when I was going to the next address. Can't remember correctly though... Skipping 4 rings a bell.

_________________
"A designer knows he has achieved perfection not when there is nothing left to add, but when there is nothing left to take away." - Antoine de Saint-Exupéry

14 Jan 2012, 19:42

AsmGuru62

Joined: 28 Jan 2004
Posts: 1739
Location: Toronto, Canada

AsmGuru62 15 Jan 2012, 15:27

I use a simple macro to write values into arrays:

Code:

macro vSet32 vect,index,value
{
     mov     [vect + index*4], value
}
...
array dd 32 dup(0)
...
mov    esi, array
vSet32    esi, 7, eax     ; array[7]=eax
vSet32        esi, 11, 627    ; array[11]=627

It is better to use a register as an index, because this instruction gets 3 bytes longer once an index exceeds 32. However, you also need to load that index into a register, which also takes room.

You can make a macro for each data type, so you can't make that same mistake.

15 Jan 2012, 15:27

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