MikeThanks for the response - a very interesting topic. Another FSX enigma!!As I say above I did not notice any difference on my rig (solely for FSX) with HT on or off.Have you ever checked this out wrt to RAM usage: I came across this RAMMap for Vista/ Win 7 from Microsoft which might be useful for FSX! The link is: http://technet.microsoft.com/en-us/sysinternals/ff700229.aspx The blurb: Use RAMMap to gain understanding of the way Windows manages memory, to analyze application memory usage, or to answer specific questions about how RAM is being allocated. RegardsPeterH

I agree that I don't think there is such a thing as "hyperthread aware", except that from the PT comments FSX is or was designed to run as a single core app if it is on a single core machine regardless of HT setting. I suspect on a multi-core machine, FSX takes no notice of the state of HT. It will allow for whatever set of cores (real or HT) is set in the affinity mask. Of course, the usefulness of more cores (real or HT) is a function of how many threads FSX can spawn that can be run concurrently. Since to some extent HT virtual cores must share processor hardware, running concurrent threads on an HT machine probably will not perform as well as on a machine with an equivalent number of real cores. But that's only at the processor level, obviously many other parts of the machine must be shared as well, so there's a limit on how much concurrent execution of threads can have on overall throughput, certainly less than linear.edit: should add that my impression from the BP=0 investigation is that there is one FSX thread that is extremely processor intense, and that thread needs to run on a single core that isn't shared via HT. Thee are maybe two or three other threads that also need to run quite a bit, (but not 100%). Beyond that I got the impression that more cores aren't going to do much.Besides RAMMap you might want to check out vmmap. scott s..