The half subtractor circuit needs two binary inputs and two binary outputs.

The input variables designate the subtrahend and minuend bits.

The output variables produce difference and borrow.

Let 'x' and 'y' be the input bits.

Truth table:

$\begin{align*} \text{Difference} &= x'y+xy'\\[2ex] &=x\oplus y \end{align*}$

$\text{borrow}=x'y$

Working:

• If both inputs are same, then the difference and borrow are zero.
• If minuend is greater than subtrahend, then the difference is '1' and borrow is '0'.

• If minuend is less than subtrahend, then the difference is '1' and borrow is '1'.

   

Logic Diagram of Half Subtractor