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Index > Compiler Internals > About fasm coding ADD AL,CL with 02C1
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hopcode



Joined: 04 Mar 2008
Posts: 563
Location: Germany
hopcode 22 Jun 2009, 01:07
Perhaps, i am neglecting something on Intel Docs,that
i cannot find about encoding. Do you know
in which cases fasm does encode like in my manual encoding ?

Code:
ADD EAX,ECX ; 01C8  fasm encoding  <-------
ADD AL,CL   ; 00C8   "    " <------

ADD EDX,ECX ; 01CA   "    "
ADD CL,AL   ; 00C1   "    "
ADD ECX,EAX ; 01C1   "    "

ADD EAX,ECX ; 03C1  manually encoded <-------
ADD AL,CL   ; 02C1  manually encoded <-------


Thanks for help
Post 22 Jun 2009, 01:07
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MazeGen



Joined: 06 Oct 2003
Posts: 977
Location: Czechoslovakia
MazeGen 29 Jun 2009, 08:06
If I'm not wrong, you can't force fasm to encode ADD AL, CL as 02C1.
Post 29 Jun 2009, 08:06
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Tomasz Grysztar



Joined: 16 Jun 2003
Posts: 8351
Location: Kraków, Poland
Tomasz Grysztar 29 Jun 2009, 09:59
Yes. This is sometimes used as a kind of "footprint" to determine, which assembler was used to assemble the code.
Post 29 Jun 2009, 09:59
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hopcode



Joined: 04 Mar 2008
Posts: 563
Location: Germany
hopcode 16 Jan 2010, 08:22
Tomasz Grysztar wrote:
...a kind of "footprint" to determine, which assembler was used...
I cannot believe yet Shocked that latest Antivirus Software (kaspersky/norton) detect virus in compiled output,again,today yet...

Is there no way to change/personalize opcode generation without using macros with fasm ?
That is very boring, and it is not possible that for a stupid virus f****d Twisted Evil "writer" should we, fasm developers, pay to him our professionality.

Am i asking too much /unreal things ?

Thanks in advance,
hopcode
Post 16 Jan 2010, 08:22
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ouadji



Joined: 24 Dec 2008
Posts: 1081
Location: Belgium
ouadji 02 Mar 2010, 22:28

sorry, but i don't understand the problem Confused

00 C8 = ADD AL,CL

and

02 C8 = ADD CL,AL

no ?

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Post 02 Mar 2010, 22:28
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ass0



Joined: 31 Dec 2008
Posts: 518
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ass0 03 Mar 2010, 00:26
Yeah I don't understand too.

This means 00 C8 == 02 C8?

How many ways there are to leave a "footprint" for this instruction?

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Post 03 Mar 2010, 00:26
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revolution
When all else fails, read the source


Joined: 24 Aug 2004
Posts: 20302
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revolution 03 Mar 2010, 01:19
ouadji wrote:

sorry, but i don't understand the problem Confused

00 C8 = ADD AL,CL

and

02 C8 = ADD CL,AL

no ?
No.

00 C8 == 02 C1

Both encode add al,cl
Post 03 Mar 2010, 01:19
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ass0



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ass0 03 Mar 2010, 01:41
Oh thanks, i misspelled "02 C8" i meant "02 C1".

But why three values (add, al, cl) are encoded like that?, Is it a "bit vector" encoding?

0xc8 == 0b11001000
0xc1 == 0b11000001

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Post 03 Mar 2010, 01:41
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revolution
When all else fails, read the source


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revolution 03 Mar 2010, 02:09
ass0: Have a look at the Intel/AMD manuals to see the encoding formats.
Post 03 Mar 2010, 02:09
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ass0



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ass0 03 Mar 2010, 02:37
OK, merci.

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Post 03 Mar 2010, 02:37
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ouadji



Joined: 24 Dec 2008
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ouadji 03 Mar 2010, 08:59
Quote:
No.
00 C8 == 02 C1
Both encode add al,cl
I didn't read correctly above.
not 00C8/02C8 ... but 00C8/02C1 !
I checked it, indeed both encode "add al,cl".
thanks revolution.

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Post 03 Mar 2010, 08:59
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