What's the purpose of the LEA instruction?

Question

For me, it just seems like a funky MOV. What's its purpose and when should I use it?

Answer 1

Maybe just another thing about LEA instruction. You can also use LEA for fast multiplying registers by 3, 5 or 9.

LEA EAX, [EAX * 2 + EAX]   ;EAX = EAX * 3
LEA EAX, [EAX * 4 + EAX]   ;EAX = EAX * 5
LEA EAX, [EAX * 8 + EAX]   ;EAX = EAX * 9

Answer 2

The 8086 has a large family of instructions which accept a register operand and an effective address, perform some computations to compute the offset part of that effective address, and perform some operation involving the register and the memory referred to by the computed address. It was fairly simple to have one of the instructions in that family behave as above except for skipping that actual memory operation. This, the instructions:

mov ax,[bx+si+5]
lea ax,[bx+si+5]

were implemented almost identically internally. The difference is a skipped step. Both instructions work something like:

temp = fetched immediate operand (5)
temp += bx
temp += si
address_out = temp  (skipped for LEA)
trigger 16-bit read  (skipped for LEA)
temp = data_in  (skipped for LEA)
ax = temp

As for why Intel thought this instruction was worth including, I'm not exactly sure, but the fact that it was cheap to implement would have been a big factor. Another factor would have been the fact that Intel's assembler allowed symbols to be defined relative to the BP register. If fnord was defined as a BP-relative symbol (e.g. BP+8), one could say:

mov ax,fnord  ; Equivalent to "mov ax,[BP+8]"

If one wanted to use something like stosw to store data to a BP-relative address, being able to say

mov ax,0 ; Data to store
mov cx,16 ; Number of words
lea di,fnord
rep movs fnord  ; Address is ignored EXCEPT to note that it's an SS-relative word ptr

was more convenient than:

mov ax,0 ; Data to store
mov cx,16 ; Number of words
mov di,bp
add di,offset fnord (i.e. 8)
rep movs fnord  ; Address is ignored EXCEPT to note that it's an SS-relative word ptr

Note that forgetting the world "offset" would cause the contents of location [BP+8], rather than the value 8, to be added to DI. Oops.

Answer 3

it because instead you write the code

mov dx,offset something

you can simply write

lea dx,something

Answer 4

The LEA instruction can be used to avoid time consuming calculations of effective addresses by the CPU. If an address is used repeatedly it is more effective to store it in a register instead of calculating the effective address every time it is used.

Answer 5

LEA : just an "arithmetic" instruction..

MOV transfers data between operands but lea is just calculating

Answer 6

Despite all the explanations, LEA is an arithmetic operation:

LEA Rt, [Rs1+a*Rs2+b] =>  Rt = Rs1 + a*Rs2 + b

It's just that its name is extremelly stupid for a shift+add operation. The reason for that was already explained in the top rated answers (i.e. it was designed to directly map high level memory references).