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assemblyx86masmmov

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

Solution

  • 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).