AMD was faced with a tradeoff during the development of the dual core Athlon 64 X2. In order to maintain backwards compatibility with earlier Socket-939 motherboards, they could not change the pinout of their dual core processors.
While maintaining the same pinout resulted in the ability to upgrade virtually any Socket-939 platform to a dual core Athlon 64 X2, it meant that the dual core processors were left with no more memory bandwidth than their single core counterparts. The single-core Socket-939 Athlon 64s feature a 128-bit wide DDR memory controller, which when operating at DDR400 speeds, it gives the A64 a maximum of 6.4GB/s of memory bandwidth. Sharing the same memory controller, the dual core Athlon 64 X2s also feature the same 6.4GB/s of memory bandwidth, despite the fact that there are now twice as many cores vying for the same amount of memory bandwidth.
AnandTech
While maintaining the same pinout resulted in the ability to upgrade virtually any Socket-939 platform to a dual core Athlon 64 X2, it meant that the dual core processors were left with no more memory bandwidth than their single core counterparts. The single-core Socket-939 Athlon 64s feature a 128-bit wide DDR memory controller, which when operating at DDR400 speeds, it gives the A64 a maximum of 6.4GB/s of memory bandwidth. Sharing the same memory controller, the dual core Athlon 64 X2s also feature the same 6.4GB/s of memory bandwidth, despite the fact that there are now twice as many cores vying for the same amount of memory bandwidth.
AnandTech
