As I recall, RR's Ada compilers used to come with that as an example Ada program. That was a long time ago, but it's conceivable it's on www.rrsoftware.com Byte magazine used to use it as a benchmark, but that was a really long time ago. I just looked on BIX and found three versions from 1988, for the Mac, the TI Color Computer, and, in Small-C, for the 8088-80386. The idea is straightforward: Create a boolean array with True for each potential prime from 2 .. N. Then take each known prime in turn (2 is a known prime) and mark all its multiples as False (not prime). The ones left True when you are done are not divisibly by any number less than themselves (except 1 of course) so they must be primes. It's pretty straightforward to code.