movsx eax ,WORD PTR [ebp+20] ?why not esp

[xiahan]

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WndProc proc hWin:DWORD,uMsg:DWORD,wParam:DWORD,lParam:DWORD

    LOCAL hDC       :DWORD
    LOCAL sDC       :DWORD
    LOCAL xDC       :DWORD
    LOCAL oldobj    :DWORD
    LOCAL wwid      :DWORD
    LOCAL Ps        :PAINTSTRUCT
    LOCAL Rct       :RECT
    LOCAL pt        :POINT
    LOCAL buffer1[260]:BYTE  ; these are two spare buffers
    LOCAL buffer2[260]:BYTE  ; for text manipulation etc..

     ......

        case WM_NCHITTEST
   QUESTION!      movsx eax, WORD PTR [ebp+20]        ; get X-Y coordinates
        mov pt.x, eax
        movsx eax, WORD PTR [ebp+22]
        mov pt.y, eax



in the line i marked QUESTION, if the author of the code want to load the lParam
from stack, why not use esp,and the index might be 16? any thing I miss?

[RHL]

  :bg using EBP is more comfortable and easy to read, on x86 processors PUSH instructions always pushed a 32-bit value

[dedndave]

well - the stack frame is set into EBP
and - the processor is optimized to use EBP for stack references
but - the main reason
EBP is stationary
ESP goes up and down whenever you push or pop something

that code isn't a very good example
he should reference the stack parameters by name, unless the assembler epilogue has been disabled
also - he should use MOVZX, not MOVSX (not a big deal, but the values should always be positive)

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    case WM_NCHITTEST
        movzx eax, WORD PTR lParam        ; get X-Y coordinates
        mov pt.x, eax
        movzx eax, WORD PTR lParam+2
        mov pt.y, eax
also - i am not sure WM_NCHITTEST is the best message to handle to track cursor movement
can't see the rest of the code, but this message should exit via DefWindowProc

[xiahan]

Hi,dedndave,actually I think the code should deploy like yours, its more reasonable! :U

besides,where does EBP point to? is it point to the base of the stack? if so, the stack grows backward,if use EBP to reference parameter, should we use something like this mov eax, [ebp - 16]

i will attach the whole code

[dedndave]

http://www.masm32.com/board/index.php?topic=17611.msg150119#msg150119

as i mentioned...
EBP is used as a stationary reference to the stack frame
ESP changes when you PUSH or POP values, EBP does not
that is why EBP is used instead of ESP

[xiahan]

http://www.masm32.com/board/index.php?topic=17611.msg150119#msg150119

as i mentioned...
EBP is used as a stationary reference to the stack frame
ESP changes when you PUSH or POP values, EBP does not
that is why EBP is used instead of ESP

i know what you mean now, cause the existence of PROLOGUE AND EPILOGUE(masm32 convenience), the ebp has the value of esp

just like this

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        OPTION  PROLOGUE:NONE
        OPTION  EPILOGUE:NONE

Funk    PROC    Parm1:DWORD,Parm2:DWORD,Parm3:DWORD

        push    ebx               ;for win32, these 4 registers should generally be preserved
        push    esi               ;EAX, ECX, and EDX can be destroyed and are sometimes used to return results
        push    edi
        push    ebp
        mov     ebp,esp
        sub     esp,12            ;4*LocalDwordCount


and ebp has the value of esp

[xiahan]

 :bg Thank you RHL and dedndave, you are both talking the same thing, while I'm a little un-brilliant,cause I'm a newbie, :lol