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Thread: Local variable with asm

  1. #1

    Local variable with asm

    i am having some fun with asm here

    in witch ways i can store local variables? and the faster one?

    ps: esp is the global stack address right?
    ps: ebx and ebp ?
    From brazil (:

    Pascal pownz!

  2. #2

    Local variable with asm

    Here's an example
    Code:
       push ebp //Push the base pointer
       mov ebp, esp //Set the base pointer to the current stack pointer
       add esp, -4 //Subtract 4 from the stack. Equivalent of PUSH DWORD 0. This will hold the local variable. [EBP-4] and [ESP] now points to the local variable
       lea ecx, [eax*4+1024] //ecx := eax*4+1024
       mov [ebp-4], ecx //store ecx in the local variable
       mov eax, [ebp-4] //store the local variable in result
       mov esp, ebp //restore the old stack pointer
       pop ebp //pop the old base pointer
       ret
    This is ofcourse a pretty naive and unoptimized code sample, but it goes to show how to write assembler
    The equivalent pascal code:

    [pascal]
    function GetIndex(i: integer): integer;
    var x: integer;
    begin
    x := i*4+1024;
    result := x;
    end;
    [/pascal]
    Peregrinus, expectavi pedes meos in cymbalis
    Nullus norvegicorum sole urinat

  3. #3

    Local variable with asm

    weird :shock:

    ebp and esp need to be restored on the function end?

    what about ebx?
    From brazil (:

    Pascal pownz!

  4. #4

    Local variable with asm

    ebx is a general purpose register. It's not different from eax, ecx, edx, esi or edi
    '
    You would probably have to read up on the calling convention you use. This is most likely fastcall, which is the standard in delphi and freepascal.

    This states that the parameters gets passed left to right like this:
    eax,edx,ecx,ebp+4,ebp+8,ebp+12,ebp+16, and so on

    You are free to do whatever with ebx in the function, but if you modify any of the others and they are not used to pass parameters in, then you must restore them to the value they were before you altered them

    This you must do, to any register, except for eax, which is the result register
    Peregrinus, expectavi pedes meos in cymbalis
    Nullus norvegicorum sole urinat

  5. #5

    Local variable with asm

    interesting :}, thanks
    From brazil (:

    Pascal pownz!

  6. #6

    Local variable with asm

    Quote Originally Posted by JSoftware
    You are free to do whatever with ebx in the function, but if you modify any of the others and they are not used to pass parameters in, then you must restore them to the value they were before you altered them
    Actually EBX, ESP and EBP values must be always preserved. Also you're free to change and not restore contents of EAX, EDX and ECX even if they were not used to pass parameters into the function. That's the case with all normal calling conventions (fastcall, stdcall, cdecl) as far as I know. Or maybe I do not understand your post correctly.

  7. #7

    Local variable with asm

    Quote Originally Posted by JSoftware
    You are free to do whatever with ebx in the function, but if you modify any of the others and they are not used to pass parameters in, then you must restore them to the value they were before you altered them
    Actually EBX, ESP and EBP values must be always preserved. Also you're free to change and not restore contents of EAX, EDX and ECX even if they were not used to pass parameters into the function. That's the case with all normal calling conventions (fastcall, stdcall, cdecl) as far as I know. Or maybe I do not understand your post correctly.

    EDIT: How to delete my own double-post? I can't seem to find a buttom for that.

  8. #8

    Local variable with asm

    hi arthur,

    Code:
      push ebp
      mov ebp, esp
      add esp, FFFFFFFCh
      leave
      ret
    note:
    Code:
    add esp, FFFFFFFCh
    this is basically esp := esp -4
    Code:
    FFFFFFFCh = -4
    reason for being -4 is because a cardinal is 4 bytes.

    also the leave is basically to restore the stack for the procedure exit.

    ret is to return to the line that called the procedure

    this is equive something along the lines of.

    Code:
    procedure Blah;
    var
    	xxx: Cardinal;
    begin
    	//do nothing.
    end;
    also a quick note, passing an argument is different to storing a local variable.

    this whole proc would take 8 bytes, it is the smallest and quickest way todo it.

    hope this helps.

    -MM

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