[…]What I discovered completely changed my understanding of 64-bit Long Mode semantics and challenged many assumptions I was making – pinging a few other experts, it seems they were as equally surprised as I was (even Mateusz”j00ru” Jurczyk wasn’t aware!). Throughout this blog post, you’ll see how x64 processors, even when operating in 64-bit long mode[…]
http://www.alex-ionescu.com/?p=340
See-also:
http://j00ru.vexillium.org/?p=290