说明：  多用户 ：usr1, …, usr8 (1-8个用户) 多级目录：可有多级子目录； 具有login (用户登录) 系统初始化（建文件卷、提供登录模块） 文件的创建： create 文件的打开：open 文件的读：read 文件的写：write
(Multi-user: usr1, ..., usr8 (1-8 users) multi-level directory: can have the subdirectory level have a login (user login) system initialization (construction paper roll, providing login module) file creation: create Open file: open the document read: read a file write: write)