(1)若真值Y是正数,[Y]补 =Y,再求一次补码还是Y;
(2)若真值Y是负数,[Y]补 =模 +Y
例如字长4位,真值Y = -101b,模 = 10000B
[Y]补 = 模 +Y = 10000b +(-101b) = 1011b