As another post is pointing out, there is a "practicality" clause. If you can use a computer to do something that would otherwise take days to compute, it may still be patentable.
It's impractical to multiply two million-digit numbers by hand, but elementary schoolkids know the algorithm to do it. Does that imply that using a computer to multiply numbers of sufficient size would be patentable?
How about a particularly efficient multiplication algorithm that can be demonstrated by hand?
