Как работать с файлами WinAPI из ассемблера?

var
  Buf: packed array [1..1024*1024] of Byte;
 
procedure TForm1.Button1Click(Sender: TObject);
type
  TDiskGeometry = packed record
    Cylinders: Int64;
    MediaType: Integer;
    TracksPerCylinder: Integer;
    SectorsPerTrack: Integer;
    BytesPerSector: Integer;
  end;
  TDiskGeometryEx = packed record
    Geometry: TDiskGeometry;
    DiskSize: Int64;
    Data: array[0..0] of Byte
  end;
const
  IOCTL_DISK_BASE = 7;
  FILE_READ_ACCESS = 1;
  FILE_ANY_ACCESS = 0;
  METHOD_BUFFERED = 0;
  FILE_DEVICE_FILE_SYSTEM = 9;
  METHOD_NEITHER = 3;
  IOCTL_DISK_GET_DRIVE_GEOMETRY_EX = (IOCTL_DISK_BASE shl 16) or (FILE_ANY_ACCESS shl 14) or ($0028 shl 2) or (METHOD_BUFFERED);
  IOCTL_DISK_GET_LENGTH_INFO = (IOCTL_DISK_BASE shl 16) or (FILE_READ_ACCESS shl 14) or ($0017 shl 2) or (METHOD_BUFFERED);
  FSCTL_ALLOW_EXTENDED_DASD_IO = (FILE_DEVICE_FILE_SYSTEM shl 16) or (FILE_ANY_ACCESS shl 14) or (32 shl 2) or (METHOD_NEITHER);
  SizeToRead = 2048*512;
var
  H: THandle;
  F: File;
  Res: DWord;
  Info: TDiskGeometryEx;
  Size, Position: Int64;
begin
  H := CreateFile('\\.\PhysicalDrive0', GENERIC_READ, FILE_SHARE_READ or FILE_SHARE_WRITE, nil, OPEN_EXISTING, 0, 0);
//  H := CreateFile('\\.\C:', GENERIC_READ, FILE_SHARE_READ or FILE_SHARE_WRITE, nil, OPEN_EXISTING, 0, 0);
  if H = INVALID_HANDLE_VALUE then
    MessageBox(0, 'Drive open error.', 'Error!', MB_OK or MB_ICONERROR or MB_TASKMODAL)
  else
  begin
(*
    // Разрешить чтение последних секторов (только для '\\.\C:', для '\\.\PhysicalDrive0' не надо, выдаст ошибку!)
//    if not DeviceIoControl(H, FSCTL_ALLOW_EXTENDED_DASD_IO, nil, 0, nil, 0, Res, nil) then ShowMessage('Can''t allow ext. IO / LastError: ' + IntToStr(GetLastError));
 
    DeviceIoControl(H, IOCTL_DISK_GET_LENGTH_INFO, nil, 0, @Size, SizeOf(Size), Res, nil);
//    DeviceIoControl(H, IOCTL_DISK_GET_DRIVE_GEOMETRY_EX, nil, 0, @Info, SizeOf(Info), Res, nil);
      ShowMessage(IntToStr(Size{Info.DiskSize} div (1024*1024*1024)) + ' Gb size / LastError: ' + IntToStr(GetLastError));
 
    Position := Size{Info.DiskSize}-SizeToRead;
    SetFilePointer(H, Position, Pointer(Integer(@Position)+4), FILE_BEGIN);
      ShowMessage('LastError: ' + IntToStr(GetLastError));
*)
    ReadFile(H, Buf, SizeToRead, Res, nil);
      ShowMessage(IntToStr(Res) + ' bytes read / LastError: ' + IntToStr(GetLastError));
    CloseHandle(H);
 
    if Res > 0 then
    begin
      AssignFile(F, ExtractFilePath(ParamStr(0))+'mbr.sec');
      ReWrite(F, 1);
      BlockWrite(F, Buf, Res, Res);
      CloseFile(F);
        ShowMessage(IntToStr(Res) + ' bytes written');
    end
    else ShowMessage('Nothing to write!')
  end
end;