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azdps



Joined: 10 Jul 2009
Posts: 3
Location: Tucson, Arizona
azdps
i've been reading up on bitwise operations xor, or , and, not and i'm not understanding them like i should. i don't see how i would use them in an application. can someone describe the most common uses for these. specifically or, and and not.
Post 14 Jul 2009, 17:34
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revolution
When all else fails, read the source


Joined: 24 Aug 2004
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revolution
AND, OR and NOT are the fundamental operations that the logic gates can do. With these operations you can make adders (subtracters/comparitors) and in turn from those build multipliers and dividers etc.
Post 14 Jul 2009, 17:41
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vid
Verbosity in development


Joined: 05 Sep 2003
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vid
azdps: In practice, you use OR to set (set to 1) particular bit, AND to clear (set to 0) particular bit, and XOR to switch particular bit to opposite.

How it works:
- any bit that is ORed by 0 remains as is. Any bit that is ORed by 1 is set to 1, regardless of its previous value. This allows you set bits you want to 1 and leave other bits unchanged.
- any bit that is ANDed by 1 remains as is. Any bit that is ANDed by 0 is set to 0, regardless of its previous value. This allows you set bits you want to 1 and leave other bits unchanged.
- any bit that is XORed by 0 remains as is. Any bit that is XORed by 1 is set to the other value (1 to 0, 0 to 1). This allows you to flip bits you want, and leave other bits unchanged.

revolution: can't you do all 16 operations just with NOP?
Post 14 Jul 2009, 18:08
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revolution
When all else fails, read the source


Joined: 24 Aug 2004
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revolution
16 operations? With NOP?
Post 14 Jul 2009, 18:12
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azdps



Joined: 10 Jul 2009
Posts: 3
Location: Tucson, Arizona
azdps
thanks for the replys. vid, i understand how they work. i really just don't understand any practical uses for them. i would like to see some examples of the most comon uses so that I look at them and realize that... hey.. i understand how i would be using them. right now i can code in assembly without them but i don't use them except for xor.


Last edited by azdps on 14 Jul 2009, 20:01; edited 1 time in total
Post 14 Jul 2009, 18:16
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asmcoder



Joined: 02 Jun 2008
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asmcoder
[content deleted]


Last edited by asmcoder on 14 Aug 2009, 14:48; edited 1 time in total
Post 14 Jul 2009, 18:26
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Tomasz Grysztar



Joined: 16 Jun 2003
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Location: Kraków, Poland
Tomasz Grysztar
vid wrote:
revolution: can't you do all 16 operations just with NOP?

revolution wrote:
16 operations? With NOP?

Very Happy

Perhaps he did mean NOR - but this way we've got the joke of the day. Wink
Post 14 Jul 2009, 18:39
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windwakr



Joined: 30 Jun 2004
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windwakr

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Post 14 Jul 2009, 18:58
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r22



Joined: 27 Dec 2004
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r22
@azdps
Code:
;;arg1 N rcx value between 0 and 63
;;return rax as 2^N
Math_2powN:
;;clamp N to make sure it's between 0 and 63
AND rcx,63 ;63 = 111111b AND'd N <= 63
;;clear RAX set = 0
XOR rax,rax ;anything xor'd with itself = 0
;;set RAX (which is 0) to 1
OR rax,1 ;0 OR'd 1 = 1
;;shift RAX to the correct power
SHL rax,cl ; CL is [0,63) and RAX = 1 and SHL shifts 1 by the power N
ret 0
    
Post 14 Jul 2009, 19:58
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vid
Verbosity in development


Joined: 05 Sep 2003
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vid
Quote:
Perhaps he did mean NOR - but this way we've got the joke of the day.

heh-heh, yeah Smile

But I think there was a thread about various instruction lengths for NOP operation, up to max instruction length = 16. If that is true, I wasn't that far either.... Embarassed
Post 14 Jul 2009, 20:17
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revolution
When all else fails, read the source


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revolution
Have you ever seen a length=16 instruction? Hint: A trick question. Read the manuals about maximum possible instruction length.
Post 14 Jul 2009, 20:28
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Fanael



Joined: 03 Jul 2009
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Fanael
Maximum possible instruction length? 15 bytes, isn't it?
Post 14 Jul 2009, 20:50
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vid
Verbosity in development


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vid
You forgot one very important thing - NOP is the only operation that can be encoded in 0 bytes, giving you 16 possibilities Laughing

But seriously, I think I have read that CPU at most fetches 16 byte instruction, if there is duplicated prefix or something? I never really looked into this anyway.
Post 14 Jul 2009, 21:14
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LocoDelAssembly
Your code has a bug


Joined: 06 May 2005
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LocoDelAssembly
I opened a thread about this. If I remember right the behavior was #UD exception for 16-byte instruction and #GP exception for instructions bigger than that (in both cases repeating prefixes of course).
Post 14 Jul 2009, 21:26
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windwakr



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windwakr

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Post 14 Jul 2009, 21:52
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LocoDelAssembly
Your code has a bug


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LocoDelAssembly
windwakr, exactly. So, a swap between #UD and #GP in my post above is required.
Post 14 Jul 2009, 22:02
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ramguru



Joined: 26 Feb 2005
Posts: 19
Location: who cares...
ramguru
AND can erase bits you don't want & leave those you do
For example
WM_MOUSEMOVE
(The low-order word specifies the x-coordinate of the cursor.)
(The high-order word specifies the y-coordinate of the cursor.)
so in order to get x, you erase high-order word:
Code:
mov    ecx, [lParam]
and    ecx, 0FFFFh
    


OR can append some bits while leaving other intact
For example, to form a file size variable in 64bit env.

Code:
invoke GetFileSize, [file], ADDR sizeDQ
mov    rcx, [sizeDQ]
shl    rcx, 32
or     rcx, rax
mov    QWORD[fsize], rcx
    


XOR is handy, because it works like "switch"
lets say you want to fill a long structure in a loop, like:
any_number, 0, any_number, 0, any_number, 0, ...
Code:
xor    ecx, ecx
xor    eax, eax
@fillstruct:
    xor    eax, 12345678h ; any_number
    mov    DWORD[long_struct + ecx*4], eax
    inc    ecx
    cmp    ecx, 100
    jnz    @fillstruct
    


NOT could help to make some branchless code
lets say you want to implement something like this
if (x > 0)
y = y * 1;
else
y = y * 0;

Code:
mov    eax, [y]
mov    ecx, [x]

not    ecx
shr    ecx, 31
xor    ecx, 1
mul    ecx

mov    DWORD[y], eax
    
Post 14 Jul 2009, 22:21
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azdps



Joined: 10 Jul 2009
Posts: 3
Location: Tucson, Arizona
azdps
thanks ramguru! interesting that you registered in 2005 and this is your first post Shocked
Post 15 Jul 2009, 04:12
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