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Teehee



Joined: 05 Aug 2009
Posts: 568
Location: Brazil
Teehee
Code:
mov eax,1
lea eax,[eax+4]
; eax = 5    


if eax has 1, + 4, so eax = 5, ok. But loading as [xxx] it seams you are returning a value pointed for an address, and not the result itself, like in:

Code:
mov eax,1
mov eax,[eax+4]
; eax = any value at address 5    


for me this is more readable:

Code:
mov eax,1
lea eax,eax+4
; eax = 5    


get my point? Razz what do you think?

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Post 31 May 2011, 19:39
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edfed



Joined: 20 Feb 2006
Posts: 4237
Location: 2018
edfed
i think the reason you think that is because you still not pointed that lea means load, and not move.

every load instructions in x86 uses a pointer to an adress as an operand.

for example, lgdt loads the content of gdtr pseudo descriptor in a ram location.
Code:
lgdt  [ebx+eax*8+sys.gdt]
    

for example, will use the conventionnal adress formula as an operand,

there are lea, lgdt, lidt, lldt, ltr, lsl, etc... all these instructions uses a memory operand.

and lea is just the Load Effective Adress, means exactlly what it does, it loads the effective adress using the 32 bits adress operand.
Code:
[segment : offset + pointer + index * scale ]
         : effective adress
    

the segment part is not included in the result of the adress calculation because the result will be relative to a segment.
Post 31 May 2011, 19:52
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ouadji



Joined: 24 Dec 2008
Posts: 1081
Location: Belgium
ouadji

most often, I use "lea" to compute a "result",
and not to compute an address.
Code:
        lea     ebx,[(ecx*2) - 6]
   mul     ebx
    
I like this opcode, very convenient
and much more powerful than it appears at the first glance.

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Post 31 May 2011, 21:20
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Teehee



Joined: 05 Aug 2009
Posts: 568
Location: Brazil
Teehee
now it makes more sense Smile

thanks
Post 31 May 2011, 22:12
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MazeGen



Joined: 06 Oct 2003
Posts: 975
Location: Czechoslovakia
MazeGen
LEA is one of those weird instructions in x86 ISA. It uses source operand that has nothing to do with memory, it is just encoded as a memory operand.
Post 01 Jun 2011, 14:04
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