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Socket 940 plays only a minor role for Athlon 64 FX processors at this point – it is a Server/Workstation platform designed for AMDs Opteron processors.
The first Athlon 64 FX, the FX-51, as well as it's immediate successor, the FX-53 were released on Socket 940 to distinguish AMDs fastest enthusiast desktop processors from the 'normal' Athlon 64's(which were Socket 754 only at that time).
Socket 940 processors have a 128-bit, 'dual-channel' memory-controller and require registered memory – they DO NOT support Cool'n'Quiet.
All current Socket 940 processors have 1MB of L2-cache.
The Athlon 64 FX-51 and FX-53 are the ONLY non-Opteron processors for Socket 940 – ALL future Athlon 64 FX processors will be released for Socket 939(the FX-53 is already available in a Socket 939 version).
Socket 939 is AMDs latest Socket for both Athlon 64 and Athlon 64 FX processors.
Socket 939 processors also have a 128-bit, 'dual-channel' memory-controller, BUT unlike Socket 940 parts they use unbuffered(non-registered) memory which is slightly faster and less expensive than registered memory – ALL Socket 939 processors support Cool'n'Quiet.
Current Socket 939 Athlon 64s('Newcastle', model F) have 512KB of L2-cache while Socket 939 Athlon 64 FX processors('Clawhammer', model 7) have 1MB of L2-cache.
Unlike Socket 939/940 parts, Socket 754 processors only have a 64-bit, 'single-channel' memory-controller and use unbuffered(non-registered) memory – ALL Socket 754 desktop processors support Cool'n'Quiet, mobile parts support PowerNOW!.
Socket 754 belongs to the Athlon 64 and Sempron(Socket 754 version) processors as well as their respective mobile counterparts – there are NO Athlon 64 FX processors for Socket 754.
Current Socket 754 processors have either 1MB, 512KB or 256/128KB(Sempron) of L2-cache.