My fasm hello world; how to do sectionless variables?

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mindcooler

Joined: 01 Dec 2009
Posts: 423
Location: Västerås, Sweden

mindcooler 06 Feb 2010, 23:36

This is my FASM hello world, copies an entire volume to an image file.

Code:

include 'win32wx.inc' ; you can simply switch between win32ax, win32wx, win64ax and win64wx here

.data
        pbuffer dd 0
        readcount dd 0
        writecount dd 0
.code

_buffer = 65536

  start:
        invoke  CreateFileW,'\\.\G:',GENERIC_READ,FILE_SHARE_READ or FILE_SHARE_WRITE,0,\
                OPEN_EXISTING,FILE_ATTRIBUTE_NORMAL,0
        mov esi,eax

        invoke  CreateFileW,'\\.\C:\dump.img',GENERIC_WRITE,FILE_SHARE_READ or FILE_SHARE_WRITE,0,\
                CREATE_ALWAYS,FILE_ATTRIBUTE_NORMAL,0
        mov edi,eax

        invoke  VirtualAlloc,0,_buffer,$3000,4
        mov [pbuffer],eax
  @:
        invoke ReadFile,esi,[pbuffer],_buffer,readcount,0
        invoke WriteFile,edi,[pbuffer],[readcount],writecount,0

        cmp [readcount],$10000
        je @

        invoke  ExitProcess,0
.end start

How do I get rid of the .data and .idata sections from my .exe? I would like to have a variables section for address calculation, but I do not want it taking up space in my exe when I just as well can allocate it myself.

Can this be done with zero-initialized variables? How about initialized ones? And code-initialized ones?

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06 Feb 2010, 23:36

cod3b453

Joined: 25 Aug 2004
Posts: 618

cod3b453 07 Feb 2010, 00:10

Change

Code:

        pbuffer dd 0 
        readcount dd 0 
        writecount dd 0

Code:

        pbuffer rd 1 
        readcount rd 1 
        writecount rd 1

these will then be allocated at run time (zeroed)

Last edited by cod3b453 on 07 Feb 2010, 00:29; edited 1 time in total

07 Feb 2010, 00:10

mindcooler

Joined: 01 Dec 2009
Posts: 423
Location: Västerås, Sweden

mindcooler 07 Feb 2010, 00:16

Excellent, thank you.

If I need initialized variables, how can I avoid allocating an entire section for them? Is initalization by code the way to go?

07 Feb 2010, 00:16

LocoDelAssembly
Your code has a bug

Joined: 06 May 2005
Posts: 4623
Location: Argentina

LocoDelAssembly 07 Feb 2010, 00:26

cod3b453, actually you should use "1" instead of "0".

I recommend to also change pbuffer to "buffer rb _buffer" (no longer a pointer) and not use VirtualAlloc anymore.

07 Feb 2010, 00:26

cod3b453

Joined: 25 Aug 2004
Posts: 618

cod3b453 07 Feb 2010, 00:28

Sorry yes I forgot to change the values, thanks LocoDelAssembly Cool

07 Feb 2010, 00:28

windwakr

Joined: 30 Jun 2004
Posts: 827

windwakr 07 Feb 2010, 00:33

mindcooler wrote:

Excellent, thank you.

If I need initialized variables, how can I avoid allocating an entire section for them? Is initalization by code the way to go?

I think FASM includes an example that has data and code in the same section, I think it's the "Beer" one.

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07 Feb 2010, 00:33

mindcooler

Joined: 01 Dec 2009
Posts: 423
Location: Västerås, Sweden

mindcooler 07 Feb 2010, 00:54

LocoDelAssembly wrote:

cod3b453, actually you should use "1" instead of "0".

I recommend to also change pbuffer to "buffer rb _buffer" (no longer a pointer) and not use VirtualAlloc anymore.

Ah, letting the loader do the job for me!

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07 Feb 2010, 00:54

mindcooler

Joined: 01 Dec 2009
Posts: 423
Location: Västerås, Sweden

mindcooler 07 Feb 2010, 00:56

windwakr wrote:

I think FASM includes an example that has data and code in the same section, I think it's the "Beer" one.

Yes, it keeps constant data at the end of the code segment, but it fails for variable data.

Thinking allocating zeroed space and copying constants to it might be a good idea.

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07 Feb 2010, 00:56

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