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What is the difference between "Mov" and "Lea

 
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sjl002
Master Cheater
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Joined: 31 Aug 2013
Posts: 305
PostPosted: Fri Jan 29, 2016 5:53 am    Post subject: What is the difference between "Mov" and "Lea Reply with quote
Hi.
This is my big question :
What is the difference between "Mov" and "Lea"?
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hhhuut
Grandmaster Cheater
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Joined: 08 Feb 2015
Posts: 607
PostPosted: Fri Jan 29, 2016 6:33 am    Post subject: Reply with quote
A small example:

ebx = 0x12345678
value at that address = 0x500

Code:
mov eax,ebx

Now eax = 0x12345678

Code:
mov eax,[ebx]

Now eax = 0x500

Code:
lea eax,ebx

Invalid instruction!

Code:
lea eax,[ebx]

Now eax = 0x12345678

So basically, as the names already suggest, "mov" copies the value of the source operand (right operand) to the target operand (left operand). If the source is a register (like the first code snippet) then the target operand equals the source operand.
If the source operand is a memory location (either static or referenced via registers) then "mov" will copy the memory value stored at that specific location (like code snippet 2).

"lea" stands for "load effective address" and only loads the address of a memory location into the target operand (which I think is mandatory to be a register).

So as you see, code snippet 1 and code snippet 4 have the exact same effect, but it is not possible to get the value "0x500" stored at [ebx] into eax when using "lea" ...

Hope that cleared things up.
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Gniarf
Grandmaster Cheater Supreme
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Joined: 12 Mar 2012
Posts: 1285
PostPosted: Fri Jan 29, 2016 6:41 am    Post subject: Reply with quote
Mov eax, [edx+1234] means "read what is at address edx+1234 and put it in eax".
lea eax, [edx+1234], means "eax=edx+1234", it would be the equivalent of "mov eax, edx+1234" if were a valid asm instruction.

Lea is often used to perform multiple basic math operations in one instruction (like lea eax,[ebx+edx*4+123] instead of one mov, one imul and two add)
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