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There's not quite that much to say about Athlon's MMX-units, except that AMD added the new MMX-instructions that were introduced by Intel with the Pentium-III. Those instructions include min, max, average and shuffle commands and will mainly be interesting for image processing and video compression. Athlon's latency of many MMX-instructions is double of Pentium III's, so that PIII could almost score a point here. However, in Intel's very own Media Bench, the Athlon was still scoring better and only in FutureMark's MultimediaMark99 Pentium III can beat Athlon. This is not too surprising however, because MMMark99 has so far only been optimized for Intel's SSE (streaming SIMD extensions) and not for 3DNow!. Thus we cannot really count this benchmark, especially since it tests more than only the MMX-part of a CPU, it includes ISSE as well.
AMD added some 5 new 3DNow!-instructions to Athlon's instruction set, which can make quite a difference when used, as seen in 3Dmark99 with AMD's special Athlon-DLL. Using this DLL with a K6-3 or K6-2 led to crashes, which proved that this DLL did not properly recognized the cpus. With the usage of the Athlon-DLL, 3Dmark99 processor-results were increased by some whopping 20%. Running Quake2 with 3DNow!-support turned on and off doesn't show as much of a difference as it does with K6-2 or K6-3. The reason for this is most likely the fact that Athlon has a latency penalty on 3DNow!-instructions of 100% over K6-3, due to its high speed design in comparison to a low to mid-speed design of the K6-series. This means that Athlon doesn't benefit as much from 3DNow! as the K6-2/3-series did. No special point for that then ...