4.4 The Overflow flag

In a subtraction, the sign of the result makes no sense when:

• non-negative subtracted from negative, but the result is non-negative.
• negative is subtracted from non-negative, but the result is negative.

Since the MSb is the sign bit, we can define

O = x_n-1y_n-1d_n-1 + x_n-1y_n-1d_n-1

In this equation, y is subtracted from x, and d is the result of the subtraction.