two's complement question
I'm learning about binary and two's complement.
I cannot replicate the result from the following problem with my current knowledge of two's complement - subtraction:
"subtracting -5 from 15 is really adding 5 to 15, but this is hidden by the two's complement representation:"
The question is: in the last 4 bits (left nibble), how can you borrow from 0 if it's 0? The example above seems to be borrowing from the next colum where the top bits are all 0's. Can someone explain how this is happening?
11110 000 (borrow)
0000 1111 (15)
− 1111 1011 (−5)
0001 0100 (20)