Comparing FASM to TASM

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Alexander

Joined: 22 Dec 2008
Posts: 40

Alexander

If I compare fasm to tasm I miss something.

For example, in tasm I was able to set the target processor like

Code:

It was also possible to set the model.

Code:

         small, flat, tiny, ...

I know we are in P-Mode or Long Mode now, but is there a possibility in fasm too?

Is it possible to set up a stack like it was in tasm?

In tasm I had a few segments like cs, ds, fs, ...
but in fasm, which do I have here?

data
code
idata
edata

and more?

And how can segments in fasm be valid combined with permissions like
readable, writeable, ...

In fasm manual there are a few sentences about this but how do I combine segements with permissions.

What means fixups?

An other question not related with tasm: How do I get my code ready of 64-Bit processors? I now a 64-Bit System like Vista64 is able of running a 32-Bit app on the top of Vista64. But when I call a 32-Bit Win32 dll like a dll created with fasm from .NET an error is thrown by .NET, so the .NET app has to be set manually to 32-Bit. Is fasm supported on both IA64 and x86_64 (AMD64)?

Is there any good documentation out there for "upgrading" from tasm, or is there any good tutorial for fasm, please?

I'm looking forward to your answers
Alexander Very Happy

19 Jan 2009, 16:32

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 18841
Location: In your JS exploiting you and your system

revolution

Alexander wrote:

For example, in tasm I was able to set the target processor like
Code:
         .8086
         .186
         .286P
         .486P
         .586P
         .587 
         ...
    

Have a look at "Macros to choose target CPU"

Alexander wrote:

It was also possible to set the model.
Code:
         small, flat, tiny, ...
    
I know we are in P-Mode or Long Mode now, but is there a possibility in fasm too?

Is it possible to set up a stack like it was in tasm?

In tasm I had a few segments like cs, ds, fs, ...
but in fasm, which do I have here?

data
code
idata
edata

and more?

include 'win32[a|u]x.inc' gives you this kind of thing. For DOS models check out the exe and com examples in the download.

Alexander wrote:

And how can segments in fasm be valid combined with permissions like
readable, writeable, ...

In fasm manual there are a few sentences about this but how do I combine segements with permissions.

This is a job for the linker. If you use fasm to directly create an exe file (something which tasm cannot do) then you need to place the code/data in whatever segment is appropriate.

Alexander wrote:

What means fixups?

A method of embedding relocation information into an exe file to allow the OS to reposition the code from its compiled base if needed.

Alexander wrote:

An other question not related with tasm: How do I get my code ready of 64-Bit processors? I now a 64-Bit System like Vista64 is able of running a 32-Bit app on the top of Vista64. But when I call a 32-Bit Win32 dll like a dll created with fasm from .NET an error is thrown by .NET, so the .NET app has to be set manually to 32-Bit. Is fasm supported on both IA64 and x86_64 (AMD64)?

Sure, use64 gives you the 64bit code generation mode.

19 Jan 2009, 18:26

Alexander

Joined: 22 Dec 2008
Posts: 40

Alexander

revolution,

thank you for your answer.

Is it possible to choose the target cpu without macros?

Another, question: Do you know which segements are allowed and which permissions I can give them?

Would you please be so kind to give me a detailed list?

19 Jan 2009, 18:54

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 18841
Location: In your JS exploiting you and your system

revolution

Alexander wrote:

Is it possible to choose the target cpu without macros?

No. Not unless you edit the fasm source code and make your own special version.

Alexander wrote:

Another, question: Do you know which segements are allowed and which permissions I can give them?

Would you please be so kind to give me a detailed list?

All segments are allowed, And you can give them any permissions you require. Perhaps I don't understand your question properly?

19 Jan 2009, 19:01

Alexander

Joined: 22 Dec 2008
Posts: 40

Alexander

Quote:

All segments are allowed, And you can give them any permissions you require. Perhaps I don't understand your question properly?

So, is it possible to give a relocation information a readable and writeable
or to give a data segment an executable attribute?

What combinations are commonly used in fasm on windows (and linux) and which of them make sense?

Thank you.

19 Jan 2009, 19:13

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 18841
Location: In your JS exploiting you and your system

revolution

Alexander wrote:

So, is it possible to give a relocation information a readable and writeable
or to give a data segment an executable attribute?

Yes, I suggest you try various things out. Experience is the best teacher.

Alexander wrote:

What combinations are commonly used in fasm on windows (and linux) and which of them make sense?

Have a look at win32[a|x](p).inc' to see which sections are defined by .data and .code. Play with it and try it for yourself.

19 Jan 2009, 19:23

Alexander

Joined: 22 Dec 2008
Posts: 40

Alexander

Ok, I'll do so.

19 Jan 2009, 19:27

Alexander

Joined: 22 Dec 2008
Posts: 40

Alexander

revolution,

I haven't found anything in Win32WX or Win32AX. I've just found .text, .data, .idata.
Question

But what is about edata or relocation information?

Is there anywhere on the web a complete list on how to combine segments with permissions?

I found the following in the manual: http://flatassembler.net/docs.php?article=manual#2.4.2)

But thats a little bit weak, could anyone please give me some more information about which segments are allowed and how to combine attribute or permissions with segments?

Another question: Is it possible to create a stack with "enter", or how can I create one as I could in tasm?

Thanks
Alexander

20 Jan 2009, 12:38

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 18841
Location: In your JS exploiting you and your system

revolution

Code:

macro .data { section '.data' data readable writeable }

macro .code { section '.text' code readable executable }

...

section '.idata' import data readable writeable

Those are the flags used; readable, executable, etc. See the fasm manual about the section directive.

I think you expect that there is some "correct" way to select flags, but there is no such thing. You can set whatever flags you want that suit your purpose. There are some "standard" flags used and "standard" sections defined but these are by no means what you must do. You could if you wanted put all of your code+data+imports+fixups+exports+whatever in one section with all the flags set. some AVs might get upset about it but it would still be a valid exe.

20 Jan 2009, 12:41

Alexander

Joined: 22 Dec 2008
Posts: 40

Alexander

So, a stupid question, but is it really possible to share my code segment, as I could do with a data segment?

20 Jan 2009, 12:48

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 18841
Location: In your JS exploiting you and your system

revolution

Alexander wrote:

So, a stupid question, but is it really possible to share my code segment, as I could do with a data segment?

Try it. I see no reason why you couldn't do it.

20 Jan 2009, 12:50

Alexander

Joined: 22 Dec 2008
Posts: 40

Alexander

Ok, so it would be possible. But isn't that a little bit unsecure or are there any needs to do such things?

20 Jan 2009, 12:52

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 18841
Location: In your JS exploiting you and your system

revolution

Alexander wrote:

Ok, so it would be possible. But isn't that a little bit unsecure or are there any needs to do such things?

Perhaps, who knows what is insecure these days. With virus writers getting smarter every day anything is possible. Personally I wouldn't worry about it too much.

20 Jan 2009, 12:57

Alexander

Joined: 22 Dec 2008
Posts: 40

Alexander

A last question about the PE-Format is it possible to set another Version than "format PE GUI 4.0"? Supports fasm any older PE-Formats?

20 Jan 2009, 12:59

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 18841
Location: In your JS exploiting you and your system

revolution

Alexander wrote:

A last question about the PE-Format is it possible to set another Version than "format PE GUI 4.0"? Supports fasm any older PE-Formats?

Try it. yes it is possible, but you really should try it for yourself to by sure that I am not lying.

20 Jan 2009, 13:03

Alexander

Joined: 22 Dec 2008
Posts: 40

Alexander

revolution,

thank you for your patience regarding my questions.

Alexander

20 Jan 2009, 13:05

vid
Verbosity in development

Joined: 05 Sep 2003
Posts: 7105
Location: Slovakia

vid

IMO it is always wise to stick to standard section settings. I'd say FASM is one of very few compilers (if not only one) capable of producing non-standard PEs, and there are LOT of tools which have troubles with them. Not just antiviruses, you can have trouble with debuggers, disassemblers, protectors, or even some windows loaders. Especially if you combine flags to some complete nonsense.

If you want to use FASM like other assemblers (with linker), you must set "format MS COFF" instead of "format PE" (eg. use FASM just to produce .obj files, not .exe files directly), and then use linker on these object files same way you are used to from TASM.

20 Jan 2009, 13:07

Alexander

Joined: 22 Dec 2008
Posts: 40

Alexander

vid,

where can I find the standard sections settings? I've looked nearly everywhere, but I haven't found anything. Embarassed

20 Jan 2009, 13:13

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 18841
Location: In your JS exploiting you and your system

revolution

Alexander wrote:

where can I find the standard sections settings? I've looked nearly everywhere, but I haven't found anything.

It is already answered in this thread. See here, just 10 posts above this one.

Last edited by revolution on 20 Jan 2009, 13:27; edited 1 time in total

20 Jan 2009, 13:25

Alexander

Joined: 22 Dec 2008
Posts: 40

Alexander

Ok, revolution.
I'm sorry.

20 Jan 2009, 13:27

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