c或c++编写一个多级菜单。至少三级.一个参考程序如下。程序涉及到一些底层库函数的调用和cpu内部寄存器的使用，需要有相当软硬件水准去理解。-c or c++ to write a multi-level menu. At least three. A reference procedure is as follows. Procedures involve some underlying library function calls and cpu internal registers use