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samuri25404
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 Posted: Thu Jun 07, 2007 1:53 pm Post subject: [Help] LEA |
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What is the LEA function used for? I understand that it puts the offset of one thing into an address, like below.
Now the value of esi is a. But couldn't you just use the mov function to do it? Or does the lea function do something else?
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Labyrnth
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| Posted: Thu Jun 07, 2007 5:06 pm Post subject: Re: [Help] LEA |
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| samuri25404 wrote: | What is the LEA function used for? I understand that it puts the offset of one thing into an address, like below.
Now the value of esi is a. But couldn't you just use the mov function to do it? Or does the lea function do something else? |
This loads esi with the address from [eax+a]
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sponge
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| Posted: Thu Jun 07, 2007 5:23 pm Post subject: |
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lea = load effective address
the address at the defined source is stored in your destination.
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samuri25404
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| Posted: Thu Jun 07, 2007 6:01 pm Post subject: |
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| sponge wrote: | lea = load effective address
the address at the defined source is stored in your destination. |
(I knew it meant load effective address)
So basically the address pointed at by the pointer is stored in esi? How would the mov function handle the same thing, or would that be incorrect syntax? Mov would just add the "eax+a" and store that in esi, wouldn't it?
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Labyrnth
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| Posted: Thu Jun 07, 2007 6:11 pm Post subject: |
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The address calculated from [eax+a] is stored in esi
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samuri25404
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| Posted: Thu Jun 07, 2007 6:12 pm Post subject: |
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| Labyrnth wrote: | | The address calculated from [eax+a] is stored in esi |
Alright, thanks! =)
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Chi-Tur
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| Posted: Wed Jun 13, 2007 5:13 pm Post subject: ... |
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i guess that helped... ty
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Programmer
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| Posted: Mon Sep 10, 2007 7:15 pm Post subject: |
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This is a late post, but might help someone?
You can use it to get the address of a variable etc. e.g
| Code: |
Variable: db 00
lea edi, Variable |
The EDI register now contains the memory address of your variable.
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TheSorc3r3r
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| Posted: Tue Sep 11, 2007 4:18 pm Post subject: |
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| Programmer wrote: | This is a late post, but might help someone?
You can use it to get the address of a variable etc. e.g
| Code: |
Variable: db 00
lea edi, Variable |
The EDI register now contains the memory address of your variable. |
Since the default is the lvalue anyway, wouldn't
mov edi, Variable
do the same thing (assuming you're not using masm which makes the default the rvalue).
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Ksbunker
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| Posted: Tue Sep 11, 2007 8:16 pm Post subject: re: |
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Take from Art of Assembly (http://webster.cs.ucr.edu/AoA/DOS/ch06/CH06-1.html#HEADING1-136)
| Code: | | LEA destination, source |
specifically
| Code: | | lea register, memory |
Of Particular significance;
| Quote: | 1. lea ax, [bx]
2. lea bx, 3[bx]
3. lea ax, 3[bx]
4. lea bx, 4[bp+si]
5. lea ax, -123[di] |
The lea ax, [bx] instruction copies the address of the expression [bx] into the ax register. Since the effective address is the value in the bx register, this instruction copies bx's value into the ax register. Again, this instruction isn't very interesting because mov can do the same thing, even faster.
The lea bx,3[bx] instruction copies the effective address of 3[bx] into the bx register. Since this effective address is equal to the current value of bx plus three, this lea instruction effectively adds three to the bx register. There is an add instruction that will let you add three to the bx register, so again, the lea instruction is superfluous for this purpose.
The third lea instruction above shows where lea really begins to shine. lea ax, 3[bx] copies the address of the memory location 3[bx] into the ax register; i.e., it adds three with the value in the bx register and moves the sum into ax. This is an excellent example of how you can use the lea instruction to do a mov operation and an addition with a single instruction.
The final two instructions above, lea bx,4[bp+si] and lea ax,-123[di] provide additional examples of lea instructions that are more efficient than their mov/add counterparts.
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