digital-logic - FullAdder - 减法 - 溢出指示

Question

我有一个真正的全加器表。现在我想完成加法器，以便它也可以用于减法（带有溢出指示器）。（OF = XYS'+ X'Y's）

x y cin --- cout s
0 0 0   ---   0  0
0 0 1   ---   0  1
0 1 0   ---   0  1
0 1 1   ---   1  0
1 0 0   ---   1  1
1 0 1   ---   1  0
1 1 0   ---   1  0
1 1 1   ---   1  1

cout = xy + xc + yc s = x'y'c+ x'yc'+ xy'c'+ xyc

我在想真正的减法表是这样的：

sub s x y --- overflow
0   0 0 0 --- 0
0   0 0 1 --- 0
0   0 1 0 --- 0
0   0 1 1 --- 1
0   1 0 0 --- 1
0   1 0 1 --- 0
0   1 1 0 --- 0
0   1 1 1 --- 0

这张桌子对吗？

或者我需要使用这个

0   0 0 0 --- 0
0   0 0 1 --- 0
0   0 1 0 --- 0
0   0 1 1 --- 1    === can any one explain why i got 1 in the overflow here?
0   1 0 0 --- 1
0   1 0 1 --- 0
0   1 1 0 --- 0
0   1 1 1 --- 0
1   0 0 0 --- 0
1   0 0 1 --- 0
1   0 1 0 --- 1                  is this the correct true table ?
1   0 1 1 --- 0
1   1 0 0 --- 0
1   1 0 1 --- 1
1   1 1 0 --- 0
1   1 1 1 --- 0

这个真实的表格是否正确，功能（of = overflow）OF = XYS'+ X'Y's
How to make AU for this one？需要帮助lz