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Index > Macroinstructions > align & struc

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S.T.A.S.



Joined: 09 Jan 2004
Posts: 173
Location: Ru#27
S.T.A.S.
Hi all!
I've got very simple variant of "assume" macro:
Code:
macro   .assume  reg, structure
 {       virtual at reg
              .#reg structure
     end     virtual  }    


And there's a problem with some structures:
Code:
struc foo
 {      align 4
     db      ? }

virtual at ebx
.ebx  foo
end virtual    
error: section is not aligned enough for this operation.
"rb 4-($ and 3)" instead of align gives "invalid expression."

I see $ isn't allowed because of register is used, but data alignment should be very nece in some cases.

Any solutions or ideas how to use "align" inside struc?
Thanks in advance.
Post 28 Mar 2004, 10:56
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Tomasz Grysztar
Assembly Artist


Joined: 16 Jun 2003
Posts: 7404
Location: Kraków, Poland
Tomasz Grysztar
All these errors are to signalize, that fasm is not able to determine the value of $ (as it doesn't know what the value of ebx would be), and whether it is aligned on some boundary or not. But when you want just to align relatively to register base (assuming that this base will be also aligned correctly), you can use some trick like this:
Code:
macro falign value
 {
   virtual
     inc byte [dword $]
     load addrimm from $-4
   end virtual
   rb (value-1)-(addrimm+value-1) mod value
 }    
Post 28 Mar 2004, 11:13
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S.T.A.S.



Joined: 09 Jan 2004
Posts: 173
Location: Ru#27
S.T.A.S.
Privalov, thanks a lot!!
Hmm.. I guess this is not just "some trick", but some kind of magic beyond my mind Smile
Post 28 Mar 2004, 12:31
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Tomasz Grysztar
Assembly Artist


Joined: 16 Jun 2003
Posts: 7404
Location: Kraków, Poland
Tomasz Grysztar
It just generates some virtual instruction (in this case "inc byte") with the address operand forced to use 32-bit immediate, and then loads this immediate from the generated instruction code to get only the immediate part of the address (ignoring the register base). This address is then used instead of $ for alignment (the last line is the same as good old "align" macro from the fasm's docs, with "$" replaced by "addrimm").
Post 28 Mar 2004, 12:36
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S.T.A.S.



Joined: 09 Jan 2004
Posts: 173
Location: Ru#27
S.T.A.S.
Thanks again for detailed explanation!
Now I see:
inc [ebx+disp32] & inc [disp32] -
of cource, the last DWORD of opcode is the same in both cases..
It seems easy when I see HOW this works, but it's not so easy to INVENT such nice trick..
I guess with new "store" directive there'll be more magic in FASM Cool
Post 28 Mar 2004, 15:55
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