The Phenom II triples the shared L3 cache size from 2MB (in the original Phenom line) to 6MB, leading to benchmark performance gains as high as 30%. In another change from the original Phenom, Cool 'n Quiet applies to the processor as a whole, rather than on a per-core basis. AMD implemented this in order to address the mishandling of threads by Windows Vista, which can cause single-threaded applications to run on a core that idles at half its clock rate.
Socket AM2+ versions of the Phenom II (920, 940) lack forward-compatibility with Socket AM3. Socket AM3 versions of the Phenom II are backwards-compatible with Socket AM2+, though this is contingent on motherboard manufacturers supplying BIOS updates.
Source [Wikipedia]
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