Backwards virtual

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Dragontamer

Joined: 24 Aug 2003
Posts: 84

Dragontamer 27 Aug 2004, 21:59

Just wondering if such a thing exists.

I have been using virtual mainly for procedures and local variables, and i usually implement it like this:

Code:

myproc:
virtual at esp
  i dd ?
  x dd ?
end virtual

push ebp
mov ebp, esp
sub esp,8

mov [i], eax
mov [x], 50

mov esp, ebp
pop ebp
ret

I was hoping for a backwards virtual, something like this:

Code:

myproc:
backwards_virtual at ebp
  i dd ?
  x dd ?
end virtual

push ebp
mov ebp, esp

mov [i], eax
mov [x], 50

mov esp, ebp
pop ebp
ret

For one, i won't have to worry about the amount and sizes on the stack, and because it is backwards, i won't have to do sub esp, (sizeof the virtual block) so that the code follows the backward stack.

Thx.

27 Aug 2004, 21:59

vid
Verbosity in development

Joined: 05 Sep 2003
Posts: 7105
Location: Slovakia

vid 28 Aug 2004, 09:41

seems you didn't really comprehend what is "virtual", only one of it's usages.
First of all, you have to reserve space on stack for variables (sub esp), otherwise you will overwrite something other or something other will overwrite your variables.

28 Aug 2004, 09:41

Dragontamer

Joined: 24 Aug 2003
Posts: 84

Dragontamer 28 Aug 2004, 16:58

That is the point. If virtual went backwards, the stack wouldn't be a problem.

Anyway, as long as you don't push/pop or call a function in the function, then there really is no loss, and it would be easier to have the second code over the first code.

And if you saw the first code, the one which i do right now, i did do sub esp, 8.

28 Aug 2004, 16:58

vid
Verbosity in development

Joined: 05 Sep 2003
Posts: 7105
Location: Slovakia

vid 28 Aug 2004, 19:11

now i get what you wanted. Look at fasm's proc macro. You can use forward reference to get size of local data and then sustract size from base of virtual block, like:

Code:

;in macro
local ..from, ..size
virtual at ebp - ..size
  ..from = $
  a dd 0
  b dd 0
  ..size = $ - ..from
end virtual

28 Aug 2004, 19:11

vid
Verbosity in development

Joined: 05 Sep 2003
Posts: 7105
Location: Slovakia

vid 28 Aug 2004, 19:13

you wanted this?

Code:

;must be in macro
local ..from,..size
virtual at ebp - ..size
  ..from:
  a dd ?
  b dd ?
  ..size = $ - ..from
end virtual

28 Aug 2004, 19:13

Tommy

Joined: 17 Jun 2003
Posts: 489
Location: Norway

Tommy 28 Aug 2004, 19:28

What about this?

Code:

macro back_virtual_at x {
  local ..start, ..end
  ..1 equ ..start
  ..2 equ ..end
  virtual at x - ..2
    ..1:
}

macro end_virtual {
    ..2 = $ - ..1
  end virtual
  restore ..1
  restore ..2
}

back_virtual_at ebp
  a dd ?
  b dd ?
end_virtual

mov [a],eax
mov [b],ebx

Last edited by Tommy on 28 Aug 2004, 19:47; edited 2 times in total

28 Aug 2004, 19:28

Tomasz Grysztar

Joined: 16 Jun 2003
Posts: 8351
Location: Kraków, Poland

Tomasz Grysztar 28 Aug 2004, 19:34

Better use "restore ..1" instead of "..1 equ" - this way you can even make these macros nestable.

28 Aug 2004, 19:34

Tommy

Joined: 17 Jun 2003
Posts: 489
Location: Norway

Tommy 28 Aug 2004, 19:47

Thanks for pointing that out Privalov! Smile

28 Aug 2004, 19:47

Dragontamer

Joined: 24 Aug 2003
Posts: 84

Dragontamer 29 Aug 2004, 17:18

Wow. Fasm macros impress me again!!

29 Aug 2004, 17:18

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