format PE GUI 4.0
entry start
include '%fasminc%\win32a.inc'
struct  SEH
        PrevLink        dd ?
        CurrentHandler  dd ?
        SafeOffset      dd ?
        PrevEsp         dd ?
        PrevEbp         dd ?
ends
IDD_DLG1    = 1
MAXSIZE  = 512
ButtonID       = 3
section '.data' data readable writeable
hInstance   dd ?
CommandLine     dd ?
ofn             OPENFILENAME
FilterString    db 'EXE-files (*.exe), DLL-files (*.dll)',0,'*.exe;*.dll',0
             db 'All Files (*.*)',0,'*.*',0,0
buffer          rb MAXSIZE
hFileRead dd ?
hMapFile        dd ?
pMemory         dd ?
seh             SEH
section '.code' code readable writeable executable
start:
   invoke  GetModuleHandle,0
   mov     [hInstance],eax
     invoke  GetCommandLine
      mov     [CommandLine],eax
   invoke  DialogBoxParam,[hInstance],IDD_DLG1,HWND_DESKTOP,DlgProc,0
  invoke  ExitProcess,eax
     
proc DlgProc hWnd,uMsg,wParam,lParam
    cmp     [uMsg],WM_INITDIALOG
        jz      .initdialog
 cmp     [uMsg],WM_CLOSE
     jz      .wmclose
    cmp     [uMsg],WM_COMMAND
   jz      .wmcommand
  mov     eax,FALSE
   ret
.initdialog:
     mov     [ofn.lStructSize],sizeof.OPENFILENAME
       push    [hWnd]
      pop     [ofn.hwndOwner]
     push    [hInstance]
 pop     [ofn.hInstance]
     mov     [ofn.lpstrFilter],FilterString
      mov     [ofn.lpstrFile],buffer
      mov     [ofn.nMaxFile],MAXSIZE
      jmp     _finish
.wmclose:
        cmp     [hMapFile],0
        je      @f
        call    CloseMapFile
@@:
   invoke  EndDialog,[hWnd],0
  jmp     _finish
.wmcommand:
  mov     eax,[wParam]
        mov     edx,eax
     shr     edx,16
      cmp     dx,BN_CLICKED
       jne     _finish
     cmp     ax,ButtonID
 jne     _finish
     mov     [ofn.Flags],OFN_FILEMUSTEXIST or OFN_PATHMUSTEXIST or OFN_LONGNAMES or OFN_EXPLORER or OFN_HIDEREADONLY
     invoke  GetOpenFileName,ofn
 cmp     eax,TRUE
    jne     _finish
     invoke  CreateFile,buffer,GENERIC_READ,FILE_SHARE_READ,0,OPEN_EXISTING,FILE_ATTRIBUTE_NORMAL,0
      test    eax,eax
     je      _erroropen
  mov     [hFileRead],eax
     invoke  CreateFileMapping,[hFileRead],0,PAGE_READONLY,0,0,0
 test    eax,eax
     je      _errormap
   mov     [hMapFile],eax
      
    ;=======================================
    ;=======================================
    call    CheckFile
FinalExit:
        test    ecx,ecx
        jne     @next
        push    [seh.PrevLink]
        pop     dword[fs:0]
        invoke  UnmapViewOfFile,[pMemory]
        jmp     @f
ValidText       db 'It''s a valid PE file!',0
AppName         db 'PE View',0
@@:
        invoke  MessageBox,HWND_DESKTOP,ValidText,AppName,MB_ICONINFORMATION
 jmp     _finish
@next:
        push    [seh.PrevLink]
        pop     dword[fs:0]
        invoke  UnmapViewOfFile,[pMemory]
        jmp     @f
InvalidText     db 'It''s not a valid PE file!',0
@@:
        invoke  MessageBox,HWND_DESKTOP,InvalidText,AppName,MB_ICONERROR
        jmp     _finish
        ;=======================================
        ;=======================================
_finish:
    mov     eax,TRUE
    ret
errop           db 'Can not open file for reading',0
errmp           db 'Can not map file',0

_erroropen:
        invoke  MessageBox,HWND_DESKTOP,errop,AppName,MB_ICONERROR
        jmp     _finish
_errormap:
        invoke  MessageBox,HWND_DESKTOP,errmp,AppName,MB_ICONERROR
        jmp     _finish
endp

proc CloseMapFile
        invoke  CloseHandle,[hMapFile]
        mov     [hMapFile],0
        invoke  CloseHandle,[hFileRead]
        ret
endp

proc CheckFile
        invoke  MapViewOfFile,[hMapFile],FILE_MAP_READ,0,0,0
        test    eax,eax
        je      _mappingerror
        mov     [pMemory],eax
        push    dword[fs:0]
        pop     [seh.PrevLink]
        mov     [seh.CurrentHandler],SEHHandler
        mov     [seh.SafeOffset],FinalExit
        mov     eax,seh
        mov     [fs:0],eax
        mov     [seh.PrevEsp],esp
        mov     [seh.PrevEbp],ebp
        mov     edi,[pMemory]
        cmp     word[edi],'MZ'
        jne     .notmz
        add     edi,dword[edi+3Ch]
        cmp     dword[edi],00004550h
        je      _valid
        xor     ecx,ecx
        inc     ecx
        ret
.notmz:
        jmp     @f
NotMZ           db 'MZ-signature is not found!',0
@@:
        invoke  MessageBox,HWND_DESKTOP,NotMZ,AppName,MB_ICONERROR        
        xor     ecx,ecx
        inc     ecx
        ret
_mappingerror:
        jmp     @f
MappingError    db 'Can not map file into memory!',0
@@:
        invoke  MessageBox,HWND_DESKTOP,MappingError,AppName,MB_ICONERROR
        xor     ecx,ecx
        inc     ecx
        ret
_valid:
        xor     ecx,ecx
        ret
endp

proc SEHHandler pExcept,pFrame,pContext,pDispatch
        push    edx
        mov     edx,[pFrame]
        mov     eax,[pContext]
        push    dword[seh.SafeOffset]
        add     dword[pContent],0B8h
        pop     dword[pContent]
        sub     dword[pContent],0B8h
        add     dword[pContent],0C4h
        push    dword[seh.PrevEsp]
        pop     dword[pContent]
        sub     dword[pContent],0C4h
        add     dword[pContent],0B4h
        push    dword[seh.PrevEbp]
        pop     dword[pContent]
        sub     dword[pContent],0B4h
        xor     ecx,ecx
        inc     ecx
        xor     eax,eax
        pop     edx
        ret
endp

section '.idata' import data readable writeable
        library user32,'user32.dll',\
            kernel32,'kernel32.dll',\
                comdlg32,'comdlg32.dll'
   include '%fasminc%\api\user32.inc'
      include '%fasminc%\api\kernel32.inc'
    include '%fasminc%\api\comdlg32.inc'
section '.rsrc' resource from 'PEView.res' data readable
    

#define IDD_DLG1 1
#define IDC_GRP1 2
#define IDC_BTN1 3
IDD_DLG1 DIALOGEX 150,88,192,118
CAPTION "PE View"
FONT 10,"Comic Sans MS",400,0,204
STYLE 0x10CA0000
BEGIN
  CONTROL "PE VIEW",IDC_GRP1,"Button",0x50000007,0,0,192,119
  CONTROL "Open File",IDC_BTN1,"Button",0x50012F00,10,41,168,27
END


    

proc SEHHandler pExcept,pFrame,pContext,pDispatch
        push    edx
        mov     edx,[pFrame]
        mov     eax,[pContext]
        push    dword[seh.SafeOffset]
        add     dword[pContent],0B8h
        pop     dword[pContent]
        sub     dword[pContent],0B8h
        add     dword[pContent],0C4h
        push    dword[seh.PrevEsp]
        pop     dword[pContent]
        sub     dword[pContent],0C4h
        add     dword[pContent],0B4h
        push    dword[seh.PrevEbp]
        pop     dword[pContent]
        sub     dword[pContent],0B4h
        xor     ecx,ecx
        inc     ecx
        xor     eax,eax
        pop     edx
        ret
endp
    

It's very hard without assume and without .IF, .ENDIF, .ELSEIF, .WHILE..
Can you advice me a good way for cheking for PE file validation or tell, where I can find a source of this program?..
Thank you
P.S. I am new to FASM, and I use it only for some days