If you have to cater for many platforms, you have to know a great deal about their conventions, especially if you have to mix assembler with your high level languages. Windows XP 64 is no exception, and Microsoft have given us a whole new set of calling conventions that make use of the expanded instruction set. Rather than explain it myself, I will allow such Microsoft luminaries as Raymond Chen and Josh Williams to do it for me. They both explain the same thing from their own point of view, but two explanantions are better than one:)