移位寄存器设计 整个电路由一个主时序进程完成；在每一个时钟的上升沿，根据mode[1:0]的值进行清零、左移或右移操作，在主时序进程中由case语句完成；移位操作由for….loop语句完成8位十六进制数逐位移动。-Shift register design the entire circuit is completed by a master timing process each rising edge of the clock, according to the value of t