Interesting indeed. Lots of variables here so something is different. Assume same HW and not running a VM under the covers etc... which compiler did you use and what memory model when you built SIMH. Are all the same optimizations turned on? If different HW, could be a SW emulation of HW instruction optimizations not available on the other CPU implementation. For this reason, besides making sure the HW is that I would standardize the compiler if you can. FYI: The Intel C/C++ compiler for both Linux and OS X are the same compiler, although sometimes slightly different libraries. Tends to be a better Apples to Apples test than GCC because of some of the variations introduced into gcc by different players (plus Apple is moving away from gcc to LLVM due the silliness caused the GPL restrictions). [Non-Commerical download can be obtained at: http://software.intel.com/en-us/non-commercial-software-development These are the product bits, but has a free license key - nothing is restricted - just be honest when you sign up]. I'm not sure if we have a free download.about that Mac OS X compiler - I'll try to ask the product manager when she/he gets back after the new year. Anyway - with identical HW, a 64 bit model, using the same compiler with same optimizations for the CPU, I would expect very similar performance with some places were OS X to be slightly and Linux in a few others. In the case of SIMH on a laptop system, my expectation would be that OS X and its I/O system would have a same advantage over Linux on a same HW. Something sounds very messy if its off by a factor of 2. Clem On Mon, Dec 31, 2012 at 8:26 AM, <sampsa at mac.com> wrote: I noticed that Linux SIMH is about twice as fast on OS X. KUHAVX got about 12 VUPS when running natively on a Core i5 iMac, 24 when in a Linux VM on the same hardware. Pretty weird. sampsa