Do OSes benefit from these things, or just games?
closed as not constructive by Mokubai, Diogo, Renan, Randolph West, Canadian Luke Aug 8 '12 at 21:44
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These are a collection of "extended" instructions in the CPU.
They are used for accelerating common 'media' based operations, such as FFT (Fast Fourier Transform), Matrix transformations, and such.
The MMX and SSE are mainly geared towards working with media streams (audio and video) but have applications elsewhere. 3DNow! as the name suggests is more geared towards 3D transformations.
While the main thing to benefit are media applications and games the instructions can be used in many other applications.
No, it's only for specially-compiled applications, mainly games, graphics, multimedia, etc. applications. OSes don't take advantage of them because they rarely need to perform Single Instruction Multiple Data (SIMD) operations (which means they would apply the same instructions on massive amounts of data).