bootsect.s流程

                                                         

1.首先将bootsect.s程序从7c00处移动到0x9000h，共512字节；设置cs/ds/es/ss为0x90000h，sp:ff00h

2.加载setup.s程序。从软盘中读取1k字节到0x9200h

3.打印Loading system ...

4.加载system:从软盘的setup.s程序后继续加载system文件

5.跳转到setup.s开始处执行