串口驱动本身分为MDD层和PDD层。MDD层对上层的Device Manager提供了标准的流设备驱动接口(COM_xxx)，PDD层实现了HWOBJ结构及结构中若干针对于串口硬件操作的函数指针，这些函数指针将指向PDD层中的串口操作函数。DDSI是指MDD层与PDD层的接口，在串口驱动中实际上就是指HWOBJ，PDD层会传给MDD层一个HWOBJ结构的指针，这样MDD层就可以调用PDD层的函数来操作串口。-Serial driver itself into the MDD layer and the PDD layer. The MDD layer on top of the Device provides standard flow up COM_xxx Device driver interface (PDD layer), HWOBJ structure and the structure of the realization of serial number on the function pointer, hardware operate these function pointer pointing PDD layer to the serial operation function. DDSI refers to the MDD layer and the PDD layer in serial driver interface, actually refers to HWOBJ, PDD layer will be the MDD layer HWOBJ a pointer to the structure, so the MDD layer can be called PDD layer function to operate the serial interface.