A look at The Machine

Fortunately, x86-64 requires that the top 16 bits of the virtual address be all the same (either all 1 or all 0), and http://support.amd.com/TechDocs/24592.pdf says that a GPF or Stack fault is generated when this isn't true. So, at least we're somewhat protected from applications doing "bad" things by hiding data up in those bits, unless they're masking them out before use. We've been here before several times, and it looks like AMD took those lessons to heart.