problem with using a register as a pointer

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fatdogs12

Joined: 18 May 2008
Posts: 3

fatdogs12 19 May 2008, 01:24

I have been trying to create my own arrays, I have figured out how to do this, but not using (from what I can tell) traditional means.

for example to create an array I can do:

Code:


mov ebx, 0
mov ebx, 20

mov dword [array1], ebx
mov dword [array1+1], ebx
        

.data
array1 rb 10

This will do the trick, but I was hoping I would be able to just throw the address of the array into a register and access it that way. (like a pointer) Is that possible? (something like below):

Code:


mov eax, array1

mov [eax], 20
add eax, 1

mov [eax], 20

Thanks in advance

19 May 2008, 01:24

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20462
Location: In your JS exploiting you and your system

revolution 19 May 2008, 02:06

fatdogs12 wrote:

Code:


mov ebx, 0
mov ebx, 20

mov dword [array1], ebx
mov dword [array1+1], ebx
        

.data
array1 rb 10

Don't you want to use a different register for the second value? Or do you want both values to be 20, in which case the first 'mov ebx,0' is unnecessary. And since you are writing dwords you might need to make your offset for the second store +4?

fatdogs12 wrote:

Code:


mov eax, array1

mov [eax], 20
add eax, 1

mov [eax], 20

Thanks in advance

You need a size identifier to make it assemble. Since you are using an offset of +1 then I assume you want a byte size store:

Code:

mov byte[eax],20

19 May 2008, 02:06

asmrox

Joined: 19 Jan 2008
Posts: 160

asmrox 19 May 2008, 07:59

why dont u u se stack for array?

sub esp,10
mov ebp,esp

mov byte [ebp+9],0

19 May 2008, 07:59

edfed

Joined: 20 Feb 2006
Posts: 4353
Location: Now

edfed 19 May 2008, 09:07

difference with stack and arrays:

stack elements are defined at run time ( but can be defined during compile)

arrays elements are defined during compilation ( but can be defined at run time)

stack is in global memory but used for a context-save mechanism.
memory is global, and is general purpose.

the stack frames used in C and hll are what they are: something from an old age.

can you set up a stack like a file?

open file
read file at stack
use this stack reliabily between all procedures?

there are mechanisms in IA 32 to use stack frames in a lot of ways, but none of them is natural or simple.

the simplest is the global memory.

19 May 2008, 09:07

vid
Verbosity in development

Joined: 05 Sep 2003
Posts: 7105
Location: Slovakia

vid 19 May 2008, 09:30

Quote:

stack is in global memory but used for a context-save mechanism.
memory is global, and is general purpose.

what do you mean by this?

Quote:

the stack frames used in C and hll are what they are: something from an old age.

not true anymore. new compilers can live finely without stack frames - something quite hard to do in asm

Quote:

the simplest is the global memory.

But not good for everything. It is okay for small hobby projects, but not for real bigger stuff. With "everything as global" you can't use recursion, position-independent code, shared code, multithreading, etc... eventually you will find out the problem with it (ever wondered why there is no shared DLL version of FASM?)

19 May 2008, 09:30

edfed

Joined: 20 Feb 2006
Posts: 4353
Location: Now

edfed 19 May 2008, 09:42

Quote:

what do you mean by this?

i try to mean that stack is a little part of the possibilities of Global memory.

it i not hard to code without stack frames.

for sure, all "global" is not good, but as global, i assume there are "local" arreas inside the global, like segments, pages and stack frames.

see that stack frames are local, but... they are temporary, not permanent, and they are not able to address variables inside an other stack frame.
then, using stack for arrays is not a good idea.

the global/local addressing is a real problem is OOP, as each object will be charged somewhere in the "Global" memory, and accessed via other objects or code.

y=ax²+bx+c is the equation of the memory problem.

19 May 2008, 09:42

vid
Verbosity in development

Joined: 05 Sep 2003
Posts: 7105
Location: Slovakia

vid 19 May 2008, 11:09

Quote:

i try to mean that stack is a little part of the possibilities of Global memory.

Not true. There are some things only globals can (pre initialized values), but there are many things that locals can and globals can't. See list in previous post.

19 May 2008, 11:09

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