减法8位二进制数

Question

I'm trying to add up two 8 bit numbers which one is negative and second one is positive. Here it is what I do:

92-113

so I represent each number as binary

92 - 01011100
113 - 01110001

after changing 0 to 1 and 1 to 0 I get :

10001110 and after adding 1 I have 1000111 which is -113

then I'm adding them up and I get :

11101011

what totally makes no sense, what I probably do wrongly ? Would really love to know where I make mistake as it's really basic knowledge ;/

Answer 1

You're not missing anything - 11101011 is the binary equivalent of -21 (92-113) in 8-bit signed binary.

For singed integer types, the left=most bit determine if the number is positive or negative. To get the additive inverse, you flip all bits but the rightmost. It;s the same process you used to convert 113 to -113.

Doing that yields 00010101 which is 21. So 11101011 is -21.