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vng21092
Grandmaster Cheater
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Joined: 05 Apr 2013
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 Posted: Fri Feb 14, 2014 8:57 am Post subject: help with mov command? |
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| How do you move the value of one register to another? Say in a game the current ammo of a gun was located in edi+00000004 and the max value of ammo for that gun (value it reloads to) was located in edi+00000008, I tried using mov [edi+04],[edi+08] but that doesn't work... also, is there a difference between mov [a],b and mov a,[b]? i also tried using a registersmbol and mov [registername]edi+08 nd then mov [edi+04],registername and instead got some wierd value for the ammo.
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DDS
Expert Cheater
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Location: Bill's Planet
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| Posted: Fri Feb 14, 2014 11:25 am Post subject: |
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You can use another register for that.
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push eax //Save the value of register eax
mov eax,[edi+00000008] // Move the value of max ammo into the reg eax
mov [edi+00000004],eax // now eax has the max ammo value, move it to the current ammo address
pop eax // Restore the Original eax value
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And the difference between mov [a],b and mov a,[b] is.
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mov [a],b // Moves the value of the variable b into the address of variable a
mov a,[b] // Moves the value from the address of variable b into the variable a
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And finally, You got a weird value when you did this.
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mov [edi+04],registername
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Because you were moving the address of [registername] into the address of [edi+04] instead of moving the value of the [registername] Address into the address of [edi+04].
I know that this is a little confusing for you right now if you are a newbie
but dont worry everything will make sense if you keep triying. 
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vng21092
Grandmaster Cheater
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| Posted: Fri Feb 14, 2014 12:23 pm Post subject: |
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| ahhh ok got it, thanks a lot
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STN
I post too much
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Joined: 09 Nov 2005
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| Posted: Fri Feb 14, 2014 5:34 pm Post subject: Re: help with mov command? |
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| vng21092 wrote: | | How do you move the value of one register to another? Say in a game the current ammo of a gun was located in edi+00000004 and the max value of ammo for that gun (value it reloads to) was located in edi+00000008, I tried using mov [edi+04],[edi+08] but that doesn't work... also, is there a difference between mov [a],b and mov a,[b]? i also tried using a registersmbol and mov [registername]edi+08 nd then mov [edi+04],registername and instead got some wierd value for the ammo. |
You can't mov(copy) values between two memory addresses in a single instruction. I have seen people get confused with this and its probably because CE automatically takes care of data type calculation for you based on register. e.g mov [address], 4 is actually mov dword ptr [address], 4...you don't see the dword ptr because CE takes care of that.
So the easiest way to understand and remember this is whenever you see [] there should be a dword(datatype) ptr [address] and there can NEVER be TWO dword ptr in a single instruction. So when you tried to mov [edi+04],[edi+08]...what you are actually writing is mov dword ptr [edi+04], dword ptr [edi+08] which is not correct. Got it ?
The solution is moving the value of max ammo in a register and then saving it in the current ammo address like this
push eax
mov eax, dword ptr [edi+08]
mov dword ptr [edi+04], eax
pop eax
or if you are feeling lazy like i always do, this is another solution
push [edi+08]
pop [edi+04]
"also, is there a difference between mov [a],b and mov a,[b]?"
yes like i said above CE handles the datatype which is pretty confusing for newbies to asm. mov [a], b is mov datasize ptr [a], b. Whenever you see [] that means the value inside [] is being accessed and not the register or address (the only exception is with lea instruction but nvm that for now) so
mov [a],b = mov whats at b into what a points at.
mov a, [b] = vice versa
Read this tutorial by Dabhand on basic assembly especially the part about MOV instruction and you will never get confused again.
http://deviatedhacking.com/index.php/topic/1974-basic-assembly-tutorial-opcodes-and-instructions-explanations/
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