Question

已知机器字长8位,X=-61,Y=-79,试利用补码运算求X+Y的值,并判断是否发溢出

Answer 1

X = -61D = - 11 1101B
[X]原 = 1011 1101B
[X]反 = 1100 0010B
[X]补 = 1100 0011B

Y = -79D = -100 1111B
[Y]原 = 1100 1111B
[Y]反 = 1011 0000B
[Y]补 = 1011 0001B

求和：
[X]补 = 1100 0011B
[Y]补 = 1011 0001B (+
------------------------
[X+Y]补 = 10111 0100B
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↑（Cs是符号位的进位，Cp是最高数值位的进位，⊕表示异或）
Cs⊕Cp = 1⊕0 = 1 ，负溢出
-[X+Y]反 =-0111 0011B
-[X+Y]原 =-1000 1100B
X+Y = -1000 1100B = -8CH = -140D