双向移位寄存器的原理设计程序，对于初学者将会有很大帮助，尤其在设计功能比较复杂的FPGA时，有些问题其实用这个就很简单-The principle of bi-directional shift register the design process, for beginners there will be a great help, especially in the design features of the FPGA more complex, there are some proble