and eax, eax?

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Plue

Joined: 15 Dec 2005
Posts: 151

Plue

What does

Code:

and eax, eax
jz something

do?

Does it jump to something if eax is not 0?

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29 Dec 2005, 21:36

vid
Verbosity in development

Joined: 05 Sep 2003
Posts: 7105
Location: Slovakia

vid

no. all basic arithmemtic instructions (add, sub, or, xor, and, test, cmp, and one more :] ) set ZF if result is zero and clear it otherwise. "and eax,eax" ANDs every bit of eax with itself, so if eax is nonzero then it must have at least some bit = 1, and "1 and 1 = 1" => result won't be zero => CF will be cleared => jz won't jump.

This is one way how to test if eax is equal to zero, without modifying it

29 Dec 2005, 21:49

Plue

Joined: 15 Dec 2005
Posts: 151

Plue

vid wrote:

This is one way how to test if eax is equal to zero, without modifying it

So why does not these two codes do the same?

Works:

Code:

format PE GUI 4.0

include '%fasminc%\win32a.inc'

  start:
        invoke GetAsyncKeyState, 65
        and    eax, eax
        ;cmp    eax, 0
        jnz    exit
        invoke Sleep, 1
        jmp    start
  exit:
        invoke MessageBox, 0, msg, msg,0
        invoke ExitProcess,0

  msg     db "pressed",0

data import
  library kernel,'KERNEL32.DLL',\
          user,'USER32.DLL'

  import kernel,\
         ExitProcess, 'ExitProcess',\
         Sleep, "Sleep"

  import user,\
         MessageBox, "MessageBoxA",\
         GetAsyncKeyState, 'GetAsyncKeyState'
end data

Does not work:

Code:

format PE GUI 4.0

include '%fasminc%\win32a.inc'

  start:
        invoke GetAsyncKeyState, 65
        ;and    eax, eax
        cmp    eax, 0
        jnz    exit
        invoke Sleep, 1
        jmp    start
  exit:
        invoke MessageBox, 0, msg, msg,0
        invoke ExitProcess,0

  msg     db "pressed",0

data import
  library kernel,'KERNEL32.DLL',\
          user,'USER32.DLL'

  import kernel,\
         ExitProcess, 'ExitProcess',\
         Sleep, "Sleep"

  import user,\
         MessageBox, "MessageBoxA",\
         GetAsyncKeyState, 'GetAsyncKeyState'
end data

[/code]

_________________
Roses are red
Violets are blue
Some poems rhyme
And some don't.

30 Dec 2005, 11:30

SDragon

Joined: 13 Sep 2005
Posts: 19
Location: Siberia

SDragon

Quote:

Does not work

It works okay on my machine. Try also test eax, eax; it doesn't write to the register.

31 Dec 2005, 02:49

Plue

Joined: 15 Dec 2005
Posts: 151

Plue

Strange it works now.

31 Dec 2005, 11:36

vid
Verbosity in development

Joined: 05 Sep 2003
Posts: 7105
Location: Slovakia

vid

and -> ZF is set if result is zero
cmp -> ZF is set if arguments of equal (eg. if result of subtraction of numbers is zero)

05 Jan 2006, 23:54

shism2

Joined: 14 Sep 2005
Posts: 248

shism2

cmp eax, 0
je exit

06 Jan 2006, 00:49

Matrix

Joined: 04 Sep 2004
Posts: 1171
Location: Overflow

Matrix

hi!
you can use this,
it will work instead of and, smaller in size than cmp...

Code:

or eax,eax
jz .zero

06 Jan 2006, 05:09

RedGhost

Joined: 18 May 2005
Posts: 443
Location: BC, Canada

RedGhost

Code:

test eax, eax
jz label

apparently its better than or, or so i've read in this forum Very Happy

_________________
redghost.ca

06 Jan 2006, 13:41

shism2

Joined: 14 Sep 2005
Posts: 248

shism2

red ghost is correct Smile

Thats what I've read on this forum too lol

06 Jan 2006, 23:26

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