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patulinu



Joined: 21 Jun 2016
Posts: 15
patulinu
I'm learning Assembly. All I know is Delphi/Pascal. Before posting this question, I searched on the web (including this forum), but no success.

I'm trying to declare 2 variables (of different types) so that they use the same memory address.
In FASM, I'd like to do something like this:


Code:
 section '.data' data readable writeable
   wcStruc WNDCLASS ?
   wcRaw   db sizeof.WNDCLASS @wcStruc dup (0)    

Obviously, the @wcStruc part is pseudo-code. That's what I'm trying to tell the compiler but I don't know how.
Is that possible? If so, How?
Post 16 Jul 2016, 16:36
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CandyMan



Joined: 04 Sep 2009
Posts: 311
Location: film "CandyMan" directed through Bernard Rose OR Candy Shop
CandyMan
Code:
section '.data' data readable writeable
   wcStruc WNDCLASS ?
virtual at wcStruc
   wcRaw   db sizeof.WNDCLASS dup 0
end virtual
    

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Post 16 Jul 2016, 19:15
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patulinu



Joined: 21 Jun 2016
Posts: 15
patulinu
CandyMan wrote:
Code:
section '.data' data readable writeable
   wcStruc WNDCLASS ?
virtual at wcStruc
   wcRaw   db sizeof.WNDCLASS dup 0
end virtual    

Thank you so much!

Now, just out of curiosity:
Since all I wanted was initialize the entire structure as raw bytes (and not I can), does the code below do the same or it just initializes the first byte (or field)?

Code:
 wc   WNDCLASSEX (0)    
Post 16 Jul 2016, 19:25
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CandyMan



Joined: 04 Sep 2009
Posts: 311
Location: film "CandyMan" directed through Bernard Rose OR Candy Shop
CandyMan
see fasmw's EXAMPLES directory

Code:
/*
struct WNDCLASS
  style         dd ?
  lpfnWndProc   dd ?
  cbClsExtra    dd ?
  cbWndExtra    dd ?
  hInstance     dd ?
  hIcon         dd ?
  hCursor       dd ?
  hbrBackground dd ?
  lpszMenuName  dd ?
  lpszClassName dd ?
ends
*/
 wc WNDCLASS 0,WindowProc,0,0,NULL,NULL,NULL,COLOR_BTNFACE+1,NULL,_class    

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Post 16 Jul 2016, 20:07
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patulinu



Joined: 21 Jun 2016
Posts: 15
patulinu
CandyMan wrote:
see fasmw's EXAMPLES directory
Code:
 ...
 wc WNDCLASS 0,WindowProc,0,0,NULL,NULL,NULL,COLOR_BTNFACE+1,NULL,_class    

Yes, I have already done that. The example above illustrates how to initialize a structure field by field. The "virtual" approach, on the other hand, allows me to pre-initialize to zero all those fields I'm not interested in, making easier the job of setting the required fields (by code, as I prefer).

In Delphi/Pascal I'm used to the FillChar() function, which doesn't care about the type of the variable. Now I know how to do so in FASM. Wink

Thank you again.
Post 16 Jul 2016, 20:31
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l_inc



Joined: 23 Oct 2009
Posts: 881
l_inc
patulinu
Quote:
The "virtual" approach, on the other hand, allows me to pre-initialize to zero all those fields I'm not interested in

Firstly, "the virtual approach" in the way shown by CandyMan does not allow you to do anything except for declaring the wcRaw label. The zeroes put into the virtual addressing space have no effect on the content of wcStruc.

Secondly, it doesn't make sense to try to initialize the structure with zeroes explicitly, because this is done implicitly anyway. The only difference is the output file size.

Thirdly, "the virtual approach" would work the other way around, i.e. if you put the wcRaw array first, and then put wcStruc declaration in a virtual section with its base at wcRaw, but a bit simpler way is to forget about wcRaw and just straightforwardly store zeroes:
Code:
wcStruc WNDCLASS
times sizeof.WNDCLASS store byte 0 at wcStruc+(%-1)    

But again, this brings nothing, but a (potentially) increased file size.

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Post 17 Jul 2016, 00:26
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patulinu



Joined: 21 Jun 2016
Posts: 15
patulinu
l_inc wrote:

Firstly, "the virtual approach" in the way shown by CandyMan does not allow you to do anything except for declaring the wcRaw label. The zeroes put into the virtual addressing space have no effect on the content of wcStruc.

Secondly, it doesn't make sense to try to initialize the structure with zeroes explicitly, because this is done implicitly anyway. The only difference is the output file size.

Thirdly, "the virtual approach" would work the other way around, i.e. if you put the wcRaw array first, and then put wcStruc declaration in a virtual section with its base at wcRaw, but a bit simpler way is to forget about wcRaw and just straightforwardly store zeroes:
Code:
wcStruc WNDCLASS
times sizeof.WNDCLASS store byte 0 at wcStruc+(%-1)    

But again, this brings nothing, but a (potentially) increased file size.

Thank you for the update. I'm happy to see this forum is a valuable source of knowledge for FASM/Assembler newbies like me.
Post 17 Jul 2016, 02:19
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