What is the difference between MOV and LEA?

• 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.

The instruction MOV reg,addr means read a variable stored at address addr into register reg. The instruction LEA reg,addr means read the address (not the variable stored at the address) into register reg.

Another form of the MOV instruction is MOV reg,immdata which means read the immediate data (i.e. constant) immdata into register reg. Note that if the addr in LEA reg,addr is just a constant (i.e. a fixed offset) then that LEA instruction is essentially exactly the same as an equivalent MOV reg,immdata instruction that loads the same constant as immediate data.

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.