What is the difference between MOV and LEA?
I would like to know what the difference between these instructions is:
MOV AX, [TABLE-ADDR]
and
LEA AX, [TABLE-ADDR]
Solution 1:
-
LEA
means Load Effective Address -
MOV
means Load Value
In short, LEA
loads a pointer to the item you're addressing whereas MOV loads the actual value at that address.
The purpose of LEA
is to allow one to perform a non-trivial address calculation and store the result [for later usage]
LEA ax, [BP+SI+5] ; Compute address of value
MOV ax, [BP+SI+5] ; Load value at that address
Where there are just constants involved, MOV
(through the assembler's constant calculations) can sometimes appear to overlap with the simplest cases of usage of LEA
. Its useful if you have a multi-part calculation with multiple base addresses etc.
Solution 2:
In NASM syntax:
mov eax, var == lea eax, [var] ; i.e. mov r32, imm32
lea eax, [var+16] == mov eax, var+16
lea eax, [eax*4] == shl eax, 2 ; but without setting flags
In MASM syntax, use OFFSET var
to get a mov-immediate instead of a load.