窗口置顶功能,防止其他窗口挡住指定窗口,使其能一直显示在最前端-Top features a window to prevent other windows to block the designated window, so that they can always show in the most front-end
设计的文件系统实现下述的基本操作:建立文件;删除文件;打开文件;关闭文件; 可对文件设置保护措施(读,写,执行)。-the file system is designed to implement basic operations as follow:
create file
delete file
open file
close file
set security measure