LEA & MOV instruction comparision

 

Instruction 1: 

LEA DX, MESSAGE ; Move the address of MESSAGE in register DX

Instruction 2: 

MOV DX, OFFSET MESSAGE ; Move the address of MESSAGE in register DX

Questions:

  • Are the above instructions equal? They seem to work similarly, but I have just started programming assembly so I can not say.
  • If both of them are similar, then which one of above is the better way to do the above task?

    • Note: I have already read this question

    On my 32-bit system, the instructions match opcodes like this: 

    8d 15 c8 90 04 08       lea    0x80490c8,%edx
ba c8 90 04 08          mov    $0x80490c8,%edx

    So you use a whole extra byte when the code is loaded into memory if you use lea .

    I found a reference to AMD chips at one point having lower latency for lea than for mov , but only by one clock cycle (and this will be immaterial if the data is not in L1 cache). I am not sure if that result holds for recent processors.

    I have found lea useful when trying to add an offset to a base address like this: 

    lea message(,%esi,2), %ecx  # put address of message + 2 x ESI in ECX

    whereas I can't do this: 

    mov $message(,%esi,2), %ecx  # fails on bad syntax

    and this produces the wrong result: 

    mov message(,%esi,2), %ecx  # puts _content_ at address, not address, into ECX

    at least in my assembler (GNU as).

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