1 Given A1A0 * B1B0 == S3S2S1S0
4 S1= low( (A1-A0)*(B0-B1)) +low( A1*B1) +high(A0*B0)
5 S2= high((A1-A0)*(B0-B1)) +high(A1*B1) +low( A1*B1)
8 Assume we know S1 and S0, and can calulate A1*B1 and high((A1-A0)*(B0-B1))
14 k4= high((A1-A0)*(B0-B1))
16 k1= low((A1-A0)*(B0-B1)) +k2 +high(A0*B0)
20 S1-k2= low((A1-A0)*(B0-B1)) +high(A0*B0)
22 We potentially have a carry or a borrow from S1