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pete



Joined: 20 Apr 2009
Posts: 110
pete
I wonder if there is a possibility of checking if the lower halfbyte (f.i. of al) is zero without modifying the higher halfbyte. At the moment i'm doing it very complicated this way:

Code:
    push    eax
 shl     al,4
        shr     al,4
        cmp     al,0
        pop     eax
 jz      @f
    


Thank you in advance for any suggestions!
Post 15 May 2009, 08:53
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revolution
When all else fails, read the source


Joined: 24 Aug 2004
Posts: 17663
Location: In your JS exploiting you and your system
revolution
Code:
test al,0xf    
Post 15 May 2009, 09:03
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pete



Joined: 20 Apr 2009
Posts: 110
pete
Oh gosh [am i dumb]. Thank you revolution!
By the way: this was just to test your skills, of course i already knew the answer.
Post 15 May 2009, 09:09
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r22



Joined: 27 Dec 2004
Posts: 805
r22
pete, if you haven't already
spend some time with the Intel/AMD Instruction Set manuals and then the Optimization manuals they are very enlightening.

This thread really highlights the fact that it takes a lot more effort to become competent in ASM then it does in an HLL.

Code:
//pete's original snippet in a HLL (without resorting to GOTO ln)
//ironically/humorously it's less lines in ASM
unsigned char savedByte = originalByte;
originalByte <<= 4;
originalByte >>= 4;
if( originalByte == 0 ){
    originalByte = savedByte;
    ...
}else{
    original = savedByte;
    ...
}

//rev's micro-snippet in a HLL
if( originalByte & 0xf == 0 ){ ... }

//what a novice/beginner might use in a HLL
if( originalByte << 4 == 0 ){ ... }
//or if ignorant of binary (&) and shift (<<) probably modulus
if( originalByte % 16 == 0 ){ ... }
    


Without full knowledge of an HLL you could muddle along and your ignorance would hardly be noticable to someone experienced in the language, BUT in ASM even the slightest lapse of knowledge can be glaringly obvious.

Facinating realization, don't you think?
Post 15 May 2009, 20:53
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windwakr



Joined: 30 Jun 2004
Posts: 827
Location: Michigan, USA
windwakr
Ya, r22 is right. The manuals are very helpful. In case you don't know where to find them, heres the link.
I always have the instruction set manuals handy, to look through, to find what an instruction does, etc.

I think you should start out by just going through the instruction set part of the "fasm.pdf" file, and then use the intel manuals once you are familiar with most of the instructions.

_________________
----> * <---- My star, won HERE
Post 15 May 2009, 21:13
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bitRAKE



Joined: 21 Jul 2003
Posts: 3043
Location: vpcmipstrm
bitRAKE
Code:
aam 16    
...works if the contents can be modified. Better than DIV because it sets the flags based on AL's contents, but invalid in long mode. Sad

Four bits are called a nibble. Very Happy
Post 16 May 2009, 02:15
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revolution
When all else fails, read the source


Joined: 24 Aug 2004
Posts: 17663
Location: In your JS exploiting you and your system
revolution
bitRAKE wrote:
Code:
aam 16    
...works if the contents can be modified. ... invalid in long mode.
Code:
shl al,4    
... works if the contents can be modified and IS valid in long mode.
Post 16 May 2009, 09:48
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Borsuc



Joined: 29 Dec 2005
Posts: 2466
Location: Bucharest, Romania
Borsuc
Why not and al,15? Confused (or 0xf, or whatever)
Post 16 May 2009, 19:40
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revolution
When all else fails, read the source


Joined: 24 Aug 2004
Posts: 17663
Location: In your JS exploiting you and your system
revolution
Because the topic turned silly at the fourth post, so posting sensible suggestions like and al,15 is not expected. Razz
Post 16 May 2009, 19:45
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Borsuc



Joined: 29 Dec 2005
Posts: 2466
Location: Bucharest, Romania
Borsuc
How about this then?

Code:
mov bl, 0
mov cl, 4
@@:
shr al, 1
adc bl, 0
loop @B    
check bl

Cool
Post 16 May 2009, 19:58
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pete



Joined: 20 Apr 2009
Posts: 110
pete
Aha, four bytes are called a nibble.
Your code is welcome, Borsuc ;) But why is it

Code:
mov cl,4
    


instead of
Code:
mov cl,1
shr cl,2
    


???

Thanks for the link, windwakr! I already knew this site, stumbled across it some time but haven't used it yet; this will change!
Post 18 May 2009, 06:18
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revolution
When all else fails, read the source


Joined: 24 Aug 2004
Posts: 17663
Location: In your JS exploiting you and your system
revolution
pete wrote:
instead of
Code:
mov cl,1
shr cl,2    
Maybe you mean this?
Code:
mov cl,1
shl cl,2    
Post 18 May 2009, 06:24
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pete



Joined: 20 Apr 2009
Posts: 110
pete
Erm, yes...
Post 18 May 2009, 06:28
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DOS386



Joined: 08 Dec 2006
Posts: 1901
DOS386
pete wrote:
instead of
Code:
mov cl,1
shr cl,2
    


Because shr cl,2 doesn't work on 8086 Idea nor shl cl,2 btwww Idea
Post 18 May 2009, 07:58
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revolution
When all else fails, read the source


Joined: 24 Aug 2004
Posts: 17663
Location: In your JS exploiting you and your system
revolution
Well luckily no one has an 8086 (or an 8088) anymore. And anyone that does have one will find it doesn't work. And anyone that finds it does work will not any good use for it. And anyone that does have a good use for it is not living in the real world. Wink
Post 18 May 2009, 08:14
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Borsuc



Joined: 29 Dec 2005
Posts: 2466
Location: Bucharest, Romania
Borsuc
What about using virtual machines for fooling around with a bad algo? Laughing
Post 18 May 2009, 17:28
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Azu



Joined: 16 Dec 2008
Posts: 1159
Azu
pete wrote:
Aha, four bytes are called a nibble.
I thought half a byte is called a nibble? Aren't four bytes called a dword?
Post 23 May 2009, 01:32
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DOS386



Joined: 08 Dec 2006
Posts: 1901
DOS386
> I thought half a byte is called a nibble? Aren't four bytes called a dword?

Sure, pete is wrong. 4 Bytes = DWORD = ?INT32
Post 23 May 2009, 09:13
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revolution
When all else fails, read the source


Joined: 24 Aug 2004
Posts: 17663
Location: In your JS exploiting you and your system
revolution
Depends upon the CPU, ARM call a 4 byte value a word. Razz
Post 23 May 2009, 12:38
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pal



Joined: 26 Aug 2008
Posts: 227
pal
A nibble (some people spell it nybble I think) is 4 bits. It is not half of a byte as a byte is not necissarily 8 bits. I.e. nibble = half of an octet.

There are other ones too.

http://en.wikipedia.org/wiki/Byte wrote:

Similarly to the terms bit, byte, and nibble, other terms of bit groups of varying sizes have been used over time. All of these are jargon, are obsolete, or are not very common.

* 1 bit: sniff
* 2 bits: lick, crumb, quad, quarter, tayste, tydbit
* 4 bits: nibble, nybble
* 5 bits: nickel, nyckle
* 10 bits: deckle, dyme bag
* 16 bits: plate, playte, chomp, chawmp (on a 32-bit machine)
* 18 bits: chomp, chawmp (on a 36-bit machine)
* 32 bits: dinner, dynner, gawble (on a 32-bit machine)
* 48 bits: gobble, gawble (under circumstances that remain obscure)


But it is not half a byte necissarily.
Post 23 May 2009, 13:47
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