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procedure VxDCall; external kernel32 index 1;  // Wegen SUBST
{...}
function SubstQuery(DriveLetter: Char): string;
var drvno: byte;
    buff: array[0..256] of char;
    lbuff: array[0..256] of char;
begin
  if Win32Platform = VER_PLATFORM_WIN32_NT then begin
    lbuff[0]:=DriveLetter;
    lbuff[1]:=':';
    lbuff[2]:=#0;
    buff[0]:=#0;
    QueryDosDevice(lbuff,buff,256);
    Result:=StrPas(buff);
    if Copy(Result,1,4) = '\??\' then
      Result:=Copy(Result,5,Length(Result))
    else
      Result:='';
  end else begin
    drvno:=Ord(UpperCase(DriveLetter)[1])-64;
    buff[0]:=#0;
    asm
       pushad
       push    es
       xor     ebx, ebx
       mov     bh,2
       mov     bl,drvno
       lea     edx,buff
       push    0       //ECX (unused)
       push    71AAh
       push    2A0010h
       call    VxDCall
       pop     es
       popad
    end;
    Result:=StrPas(buff);
    if Result = '' then Exit;
    // convert to longfilename
    asm
       // expand long path
       pushad
       push    ds
       push    es
       xor     ebx, ebx
       lea     esi, buff
       lea     edi, lbuff
       mov     ecx,0
       mov     cl, 2
       mov     ch, 0
       push    ECX
       push    7160h
       push    2A0010h
       call    VxDCall
       pop     es
       pop     ds
       popad
    end;
    Result:=StrPas(lbuff);
  end;
end;

function SubstRemove(DriveLetter: Char): Boolean;
var drvno: byte;
    Drive,Path: string;
begin
  if Win32Platform = VER_PLATFORM_WIN32_NT then begin
    SetLength(Drive,3);
    Drive[1]:=DriveLetter;
    Drive[2]:=':';
    Drive[3]:=#0;
    Path:=SubstQuery(DriveLetter);
    Result:=DefineDosDevice(DDD_REMOVE_DEFINITION,PChar(Drive),PChar(Path));
  end else begin
    drvno:=Ord(UpperCase(DriveLetter)[1])-64;
    asm
       pushad
       push    es
       xor     ebx, ebx
       mov     bh,1
       mov     bl,drvno
       push    0       //ECX (unused)
       push    71AAh
       push    2A0010h
       call    VxDCall
       pop     es
       popad
    end;
    Result:=SubstQuery(DriveLetter) = '';
  end;
end;