This paper deals with the period T of pseudorandom number sequence gener- ated by subtract- with borrow (SWB ). For the selected base b,legs r > s it is shown that T is dependent only on b, r and s and equal to the order of b with respect to m = br- bs + 1 provided that m and A, which is determined by initial values x1,s’’’ ,xr and initial borrow value cr+1, are coprime. For b = 231-1 this paper searches the field 0 < r ≤ 300, r-s ≤ 10 and finds out all the pairs of (r, s) for which m is prime. In addition, the periods of SWB corresponding to b = 231-1 and these pairs (r, s) are aIso calculated. Besed on data obtained, generators of SWB with extremely long period could be designed.
