Doing it in Sections

You can elect to start small when constructing a virtual reality; most humans can't possibly visualize every contingency of a billion object reality, and there isn't much point to building a billion object space if you can't fill it. The easiest way to do this is to do a small section first, then add another part of the reality adjacent to the first, until the entire memory is filled. You can then extend new sections to the next memory. The Arasaka Castle reality in Osaka was constructed in this way; the upper management has a full team of programmers.