Subtract 10101 from 11101 using 1’s and 2’s complement​

Change the subtraction to addition by changing a-b to a+(-b). So this becomes 11001+(-10101). Negation in one’s complement is simple, just toggle all the bits of a negative number, so we have 11001+(01010). Performing the addition we get 1 00011. Note, the high order bit is the carry out. With one’s complement addition we perform and “end around carry” which means if the carry out is a one, we add one to the result; therefore, 1 00011 becomes 00100.

Let’s check the result by doing the arithmetic in base 10. 11001 is 25 base 10. 10101 is 21 base 10, so we have 25–21=4. 4 base 2 is 0100 which matches the one’s complement arithmetic.

Explanation:

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