计数器用一个开关K控制，当K=1时，可逆加法计数器进行计数，K=0时可逆减法计数器进行计数，即可构成一个具有清零和预置功能的可逆加减法计数器-counter with a control switch K, K = 1, the reversible addition counter for counting, K = 0 reversible subtraction counter for counting, with constitutes a reset and preset functi