Why do I need to use XOR, AND, OR?

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Kristian_

Joined: 13 Nov 2004
Posts: 38

Kristian_ 15 Nov 2004, 17:27

Hi!
Well, I know how to use them, but for what are they useful? Thank you!

15 Nov 2004, 17:27

Madis731

Joined: 25 Sep 2003
Posts: 2139
Location: Estonia

Madis731 15 Nov 2004, 19:07

You will learn soon, that you can't live without them Wink

Code:

;AND:
    ;Let's say you have useful data in lower part of eax
    and eax,0FFFFh
    ;Now the upper part does not bother you
;OR:
    ;You have something in lower eax and upper ebx
    or  eax,ebx
    ;Now you have all you need in eax
;XOR:
    ;This is not meant this way but mostly it is used to zero out regs
    xor eax,eax
    ;Now eax=0 according to XOR rules

;#Ok, now one real world application
;#You have a user input a string which
;#is in decimal of course. This takes up
;#much space and we can pack it to
;#50% exactly (4 to 2 bytes).
    mov eax,[UserInput] ;Let's pretend we have 32353739h
                        ;Which is "2579" as a string
    and eax,0F0F0F0Fh   ;Now we only have 02050709h
    mov ebx,eax         ;ebx=eax
    shl ebx,4           ;ebx is 20507090h
    or  ebx,eax         ;ebx is 22557799h now (EAX OR EBX)
    ror ebx,4           ;ebx is 92255779h
    mov eax,ebx         ;eax=ebx
    shr eax,8           ;eax is 00922557h
    mov al,bl           ;eax changed to 00922579
    and eax,0FFFFh      ;eax is 00002579 <=BCD result

Bad example you say Confused

I'll give you a buch of others when you grow older and wiser Cool

like an MMX solution for the same problem.

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15 Nov 2004, 19:07

Kristian_

Joined: 13 Nov 2004
Posts: 38

Kristian_ 15 Nov 2004, 19:15

Thank you very much!
Assembly is so interesting!!!

15 Nov 2004, 19:15

Matrix

Joined: 04 Sep 2004
Posts: 1166
Location: Overflow

Matrix 15 Nov 2004, 19:16

they are logical operations of the Arithmetical&Logical Unit of your Processor,

Code:

xor al,bl ; this will exclusive or bits in al, with appropriate bits in bl, then place the result in al

you can get idea on data recovery using XOR
X XOR B = A
A XOR X = B

so

like ABCDEFXYZ (like in raid - 3 letters represent one disk)
where
X=A xor D
Y=B xor E
Z=C xor F
now XYZ on 3rd disk is data recovery information, now if 1 disk loses all data, you can recover it.
more info here:
http://www.thecodeproject.com/csharp/raidfile.asp

so use it when you need it

btw.: i also recommend you reading the basics before asking something, this board is not large enough to contain every basic questions like what is byte? or similar.

you should also take a look @
Flat Assembler Frequently Asked Questions (FAQ) (main)

its linked to other FAQs as well, and they should contain a large amount of links to tutorials. if you find tutorials explaining your problem well you can recommend them @ faq page like:

"i found this great tutorial about xxxx @ http://xxxx.com/whatisit.htm,
and you should add it to your faq."

this way it others have the same question, they will find it easily without searching in the FAQ

Last edited by Matrix on 17 Nov 2004, 04:40; edited 2 times in total

15 Nov 2004, 19:16

veach1

Joined: 16 Jul 2004
Posts: 165

veach1 16 Nov 2004, 07:37

XOR can be used instead of XCHG Smile

Code:

xor ax, bx
xor bx, ax
xor ax, bx

16 Nov 2004, 07:37

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