8位加减器，八位减法器与加法器，用过一个控制端可以自由变换，采用移位加法方式，用途广泛，利用减法位补码加法的理论实现。-8 addition and subtraction, eight subtractor and adder, used a control terminal can freely change the using Shift addition, a wide range of uses, the use of subtraction complement addition t