lea: bug or feature?

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Dunduk

Joined: 08 Sep 2003
Posts: 38
Location: Russia

Dunduk 18 Oct 2004, 06:37

Hi all
Piece of code:

Code:

  .code
    ...
    mov  edi,var1
    lea  esi,[edi+var2-var1]
    ...
   .data
     var1 dd ?
     var2 dd ?

Fasm compiles it OK.
But this:

Code:

    lea  esi,[edi-var1+var2]

and even this:

Code:

    lea  esi,[edi-var2]

generates error "invalid use of symbol"... What's wrong?

18 Oct 2004, 06:37

Madis731

Joined: 25 Sep 2003
Posts: 2139
Location: Estonia

Madis731 18 Oct 2004, 07:02

Works for me, I don't know where the problem is?
I have to see the full code...

18 Oct 2004, 07:02

Joshua

Joined: 12 Jul 2003
Posts: 56
Location: Belgium

Joshua 18 Oct 2004, 07:40

This has to do with relocations, search the board for a more complete answer.
A workarouns is this:

Code:

virtual
    dd var2
    load var2.offset dword from $-4
end virtual
lea esi,[edi-var2.offset]

18 Oct 2004, 07:40

Dunduk

Joined: 08 Sep 2003
Posts: 38
Location: Russia

Dunduk 18 Oct 2004, 08:04

Yes, this is solution, but code:

Code:

    lea  esi,[edi+var2]

works fine without any "virtual" tricks. What's the difference between these two cases?

18 Oct 2004, 08:04

JohnFound

Joined: 16 Jun 2003
Posts: 3499
Location: Bulgaria

JohnFound 18 Oct 2004, 08:39

var2 - var 1 is positive value.
var1 - var2 is negative.
AFAIR, Windows can't relocate negative offsets. For detailed explanation look here: http://board.flatassembler.net/topic.php?t=37

Regards.

18 Oct 2004, 08:39

Dunduk

Joined: 08 Sep 2003
Posts: 38
Location: Russia

Dunduk 18 Oct 2004, 08:50

Thanks for answer JohnFound. I've tried to find answer on forum but failed. Thanks for the link too.

18 Oct 2004, 08:50

Dunduk

Joined: 08 Sep 2003
Posts: 38
Location: Russia

Dunduk 18 Oct 2004, 09:30

That's me again.
I've thought a little and found that (var1-var2) is negative number, but not offset. And

Code:

    lea  esi,[edi-number]

is legal instruction... What the difference between (-var1+var2) and (var2-var1)?

18 Oct 2004, 09:30

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