If you want to compute c = a * b where b and c are double word numbers and a is a single word number, then you can do that as follows:
unsigned a;
unsigned hb, lb;
unsigned hc, lc;
{hc,lc} = macs(lb, a, hb*a, 0)
Where {hb,lb} are the high and the low word of b, and {hc,lc} are the high and the low word of c.