"virtual at (REG + VAR)" -- possible?

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Frank

Joined: 17 Jun 2003
Posts: 100

Frank 26 Jan 2004, 17:10

Hi,

"virtual ... end virtual" blocks can be defined at an absolute address ("virtual at 0"), at a register address ("virtual at ebp"), at a register address with a constant offset ("virtual at ebp + 8"), and at a register address with a constant offset that will be computed only later ("virtual at ebp - dynamic_size").

Is it also possible to define a virtual block at a register address plus a variable offset? What I try to do is "virtual at esp + ..stdp", where macros change the value of "..stdp" after every push, pop, invoke, etc.

Thank you,

Frank

26 Jan 2004, 17:10

Tomasz Grysztar

Joined: 16 Jun 2003
Posts: 8354
Location: Kraków, Poland

Tomasz Grysztar 26 Jan 2004, 17:30

Adress for a "virtual" directive is a regular address expression (of the same type as the one you use inside the square brackets) and is calculated at the time when the "virtual" directive is processed.

Code:

x = 4

virtual at ebx+x
  test1 dd ?
end virtual

x = 8

virtual at ebx+ecx*4+x
  test2 dd ?
end virtual

        mov     eax,[test1]             ; it's mov eax,[ebx+4]
        mov     eax,[test2]             ; it's mov eax,[ebx+ecx*4+8]

Numerical constants can be redefined (also in "if" clauses), but they cannot be forward-referenced in such case.

26 Jan 2004, 17:30

Frank

Joined: 17 Jun 2003
Posts: 100

Frank 26 Jan 2004, 18:11

Privalov wrote:

and is calculated at the time when the "virtual" directive is processed.

Thank you for explanation and examples. Too bad, that was the "missing link" for a high-convenience, low-risk, fully automatic, framepointerless macro system.

26 Jan 2004, 18:11

Tomasz Grysztar

Joined: 16 Jun 2003
Posts: 8354
Location: Kraków, Poland

Tomasz Grysztar 26 Jan 2004, 18:19

But you can achieve something near to what you're looking for by using symbolic constants, like:

Code:

virtual at esp
 x dd ?
 x equ x+offs
end virtual

offs = 0

        mov     eax,[x]         ; mov eax,[esp]

offs = 8

        mov     eax,[x]         ; mov eax,[esp+8]

26 Jan 2004, 18:19

aaro

Joined: 21 Jun 2003
Posts: 107
Location: hel.fi

aaro 26 Jan 2004, 20:40

http://board.flatassembler.net/topic.php?t=322&highlight=

26 Jan 2004, 20:40

comrade

Joined: 16 Jun 2003
Posts: 1150
Location: Russian Federation

comrade 26 Jan 2004, 21:14

Why not make section of FASM site for such macro tricks? Kind of like 'assembly gems' but for FASM macros Smile

_________________
comrade (comrade64@live.com; http://comrade.ownz.com/)

26 Jan 2004, 21:14

Tomasz Grysztar

Joined: 16 Jun 2003
Posts: 8354
Location: Kraków, Poland

Tomasz Grysztar 26 Jan 2004, 21:19

This forum is a section of FASM site. Wink

And it was made mainly for such 'gems' - most threads here contain some.

26 Jan 2004, 21:19

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