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flat assembler > Windows > Changing from MASM to FASM

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andrewk



Joined: 06 Dec 2003
Posts: 5
Are there any tutorials for those experienced with MASM who are interested in using FASM? I found a few tutorials on this site however they explained the basics of assembly language rather than delving into where FASM differs from MASM in its usage and syntax.

To get myself acquainted I had a play around with a simple MessageBox example this afternoon. I wanted to use the commandline Win32 version of FASM as its easier to integrate this with my editor than the Windowed version (which appears to have a different syntax for its import section). I originally started with the following:
Code:
format PE GUI 4.0

include '%fasminc%/windows.inc'

section '.data' data readable writeable

  MsgCaption db "Iczelion's tutorial no.2",0
  MsgBoxText db "Win32 Assembly is Great!",0

section '.code' code readable executable

  entry start

  start:
    push    MB_OK
       push    MsgCaption
  push    MsgBoxText

      push    0
   call    [MessageBox]
        push    0
   call    [ExitProcess]

section '.idata' import data readable writeable

  dd 0,0,0,RVA kernel_name,RVA kernel_table
  dd 0,0,0,RVA user_name,RVA user_table
  dd 0,0,0,0,0

  kernel_table:
    ExitProcess dd RVA _ExitProcess
    dd 0
  user_table:
    MessageBox dd RVA _MessageBoxA
    dd 0

  kernel_name db 'KERNEL32.DLL',0
  user_name db 'USER32.DLL',0

  _ExitProcess dw 0
    db 'ExitProcess',0
  _MessageBoxA dw 0
    db 'MessageBoxA',0

section '.reloc' fixups data readable discardable
    

I added a simple macro that would work in a similar fashion to the MASM Invoke directive. So, after the 'include' line above I entered:
Code:
macro invoke proc, [arg]
{
    reverse push arg
    common call  [proc]
}
    

I then changed the push... call ... instructions to:
invoke MessageBox, NULL, .... (etc.).

However one thing had me scratching my head. This was the syntax of the import data section. RVA is obviously an acronym for Relative Virtual Address. It appears to me that this section creates some kind of lookup table for the procedure names used in the code. Could someone elaborate further? I have a few references on the PE file format, but would like to understand in depth how this section works.

Thanks.


Last edited by andrewk on 20 Jan 2004, 22:24; edited 1 time in total
Post 20 Jan 2004, 03:58
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comrade



Joined: 16 Jun 2003
Posts: 1115
Location: Russian Federation
invoke macro is already defined, see includes

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comrade (comrade64@live.com; http://comrade.ownz.com/)
Post 20 Jan 2004, 04:26
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Tomasz Grysztar
Assembly Artist


Joined: 16 Jun 2003
Posts: 6927
Location: Kraków, Poland
There are no differences in syntax between different fasm releases (DOS/Win32 console/Win32 GUI/Linux), every version will process every your source in the same way, giving you the same output. There are only a bit different sets of examples coming with each release - with Win32 console version you've got some low-level PE coding examples, while with Win32 GUI version you've got the macro-driven simplified Win32 programming examples. So to see how the "invoke", "import" and similar macros are used, see the examples that are included in the Win32 GUI release.
Post 20 Jan 2004, 06:18
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andrewk



Joined: 06 Dec 2003
Posts: 5
Thanks for your replies. I had a quick re-read of the PE file format information I had and discovered the format of the idata section maps exactly to its representation in the PE file format. This is actually very cool. Privalov, the more I play around with FASM the more I like it - thanks for such a great piece of software.
Post 20 Jan 2004, 22:48
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