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Houdini 4 comes with two 64-bit executable files: Houdini_4_x64A.exe and Houdini_4_x64B.exe. Depending on your hardware one or the other version can be up to 5% faster; on AMD and older Intel Core processors version A will usually be faster, on more recent Intel Core processors version B will usually be preferable. If you care about this minute difference - which amounts to maximum 5 Elo - you can run a speed test (simply analyze any position) and use the executable file that produces the highest node speed on your computer.