how multiply two large numbers stored in arrays?

I found a very good algorithm to multiply two numbers if one of them is very large... It is very nicely explained in the followng Codechef tutorial!! http://www.codechef.com/wiki/tutorial-small-factorials but the following problem in spoj requires the algorithm to multiply if both of them are large!! http://www.spoj.com/problems/MUL/ please help me if anybody solved the above problem! :)