AMD64 CPUs are available in different socket types. You must use a motherboard with the same socket type your CPU has. Different socket types exist because of the different memory controller specs available. So far you can find AMD64 CPUs using the following sockets:

• Socket 754: Single-channel DDR memory controller, i.e. the memory controller accesses memory at 64-bit rate. Used by the first Athlon 64 models, Turion 64 and socket 754 Sempron models.
• Socket 939: Dual-channel DDR memory controller, i.e. the memory controller accesses memory at 128-bit rate. Used by the current models of Athlon 64, Athlon 64 X2, Athlon 64 FX and some Opteron 1xx series models.
• Socket 940: Dual-channel DDR memory controller requiring ECC memories. Used by Opteron and the first models of Athlon 64 FX.
• Socket AM2: Dual-channel DDR2 memory controller. Used by forthcoming models of Athlon 64, Athlon 64 X2 and Athlon 64 FX CPUs.

On the pictures below you can see the physical difference between socket 754 and socket 939, which are the two most popular socket types for AMD64 CPUs.

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Figure 7: A socket 754 CPU.

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Figure 8: A socket 939 CPU.

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Figure 9: Socket 754.

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Figure 10: Socket 939.