Memory Slots

I found this site http://www.belarc.com/free_download.html This can tell you how meny memory slots are on your computer and how much memory is in each. Does anyone know how they could get that type of info. I looked on MSDN and could not fine anything but total memory in the system. I don''t think windows knows any more then how much total memory. Would this have to be a bios call and if so how can they support so meny different motherboards? Thanks.