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flat assembler > Macroinstructions > [fasmg] pe.inc : data types override

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zhak



Joined: 12 Apr 2005
Posts: 473
Location: Belarus
[fasmg] pe.inc : data types override
I'm a bit lost with overriding data types in pe.inc.

Code:

macro dword? value
    ;display '+'
    local v
    v = value
    if ~ v relativeto 0 & v relativeto PE.RELOCATION
        store $-PE.IMAGE_BASE:dword at PE.relocated_addresses:PE.RELOCATION_INDEX shl 2
        store IMAGE_REL_BASED_HIGHLOW:word at PE.relocation_types:PE.RELOCATION_INDEX shl 1
        dd v - PE.RELOCATION
        PE.RELOCATION_INDEX = PE.RELOCATION_INDEX + 1
    else
        dd v
    end if
end macro

macro dd? definitions&
    iterate value,definitions
        match ?value
            dd ?
        else match n =dup? ?value
            dd n dup ?
        else match n =dup? (?), value
            dd n dup ?
        else match n =dup? vvalue
            repeat n
                dword v
            end repeat
        else
            dword value
        end match
    end iterate
end macro

struc dd? definitions&
    label . : dword
    iterate value,definitions
        match ?value
            dd ?
        else match n =dup? ?value
            dd n dup ?
        else match n =dup? (?), value
            dd n dup ?
        else match n =dup? vvalue
            repeat n
                dword v
            end repeat
        else
            dword value
        end match
    end iterate
end struc



These overrides are needed to build relocations table. That is what I understand. However, it seem I don't fully understand how overrides work. Let's add

Code:

display '+'



to dword macro

Now, how I understand it, if I do dd 1, the flow is: macro dd > macro dword > built-in dd?? and + is displayed once.

But if I do dword 1, the flow seems to be macro dword > macro dd > macro dword >built-in dd?? because + is displayed twice.

How comes it doesn't get stuck in recursion?
Post 07 Apr 2017, 10:17
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Tomasz Grysztar
Assembly Artist


Joined: 16 Jun 2003
Posts: 6603
Location: Kraków, Poland
Re: [fasmg] pe.inc : data types override

zhak wrote:
But if I do dword 1, the flow seems to be macro dword > macro dd > macro dword >built-in dd?? because + is displayed twice.

How comes it doesn't get stuck in recursion?

For the same reason that "dd" does not recurse. You can ignore the initial "macro dword >" layer and "macro dd > macro dword > built-in dd" is just the regular way how instruction re-definition works in fasm and fasmg. Every built-in instruction becomes a variable symbol when you re-define it.

This "dword" macro was not supposed to be used directly, to avoid excess calls like in your example it would be better to make "dword" macro use "emit" instead of "dd".
Post 07 Apr 2017, 10:26
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zhak



Joined: 12 Apr 2005
Posts: 473
Location: Belarus
So, it "digs down" in redefinitions.

Still the following doesn't make sence to me:

Code:

macro dword? value
  display '11 '
  dd value
end macro

macro dword? value
  display '12 '
  dd value
end macro

macro dword? value
  display '13 '
  dd value
end macro

macro dd? value
  display '01 '
  dword value
end macro

macro dd? value
  display '02 '
  dword value
end macro

dword 1



The above does dword 13 > dd 02 > dword 12 > dd 01 > dword 11 > built-in dd, which is correct. Now, if remove dword 13, an error occurs because dword 12 > dd 02 > dword 11 > dd 01 > no more dword defined.
The same applies if reduce further: remove macro dd 02 - works, then remove macro dword 12 - error.

So, dword must be defined times dd defined + 1, in general. But this doesn't apply to the case when both dword and dd are defined once (as in original pe.inc example) -- it works instead of generating an error. Maybe it is silly things I ask, but why exactly does this happen?
Post 07 Apr 2017, 12:02
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Tomasz Grysztar
Assembly Artist


Joined: 16 Jun 2003
Posts: 6603
Location: Kraków, Poland
When "dword" is defined just once, it is not variable and therefore allows recursion. So when "dd" is called from "dword" and it in turn calls "dword", it calls the same "dword" macro, not a previous version like in the case when instruction is variable.
Post 07 Apr 2017, 12:09
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zhak



Joined: 12 Apr 2005
Posts: 473
Location: Belarus
It's clear now. Thanks a lot.
Post 07 Apr 2017, 12:12
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revolution
When all else fails, read the source


Joined: 24 Aug 2004
Posts: 15167
Location: GW170817
I think that when you say "variable", it means the same as "overloaded" in HLL speak. i.e. same name but different code.
Post 07 Apr 2017, 12:14
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Tomasz Grysztar
Assembly Artist


Joined: 16 Jun 2003
Posts: 6603
Location: Kraków, Poland
The "overload" happens when you define a symbol in the child namespace with the same name as one in the parent/global namespace. In fasmg these two mechanisms can be combined in different ways. You can have the overloading symbol in child namespace be variable while still having non-variable global one. For example if you have a variable "dd" macro defined in nested namespace and it calls the "dd" macro recursively, it is going to finally call "dd" in the parent namespace, and if that macro is not variable, it may proceed with a true recursion. You also may have a non-variable macro in local namespace and then it may call itself recursively, resulting in a complete overload of a global macro.

Generally in fasm term "variable symbol" means one that has no single universally accessible value, and therefore cannot be forward-referenced (nor self-referenced, as in case of recursion). The "overloading" happens when there are symbols with the same name but in different namespaces and one takes precedence over the other.
Post 07 Apr 2017, 12:27
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