I don't understand it Would you please let me know more? How could you know about that ? Is there any reference to let me know regarding your answer?It depends how many cycles you want the result in. You could use just one if you have multiple cycles, or you might want 4 if you want a 16-bit result.
I think remembering some secondary school mathematics can help. (a+b)(c+d) = ac + ad + ...know how to know this