Excerpt from BKDG For AMD Family 10h Processors Page 60.

2.8 DRAM Controllers (DCTs)

The DCTs support DDR2 DIMMs or DDR3 DIMMs. Products may be configurable between DDR2 and DDR3 operation.

A DRAM channel is the group of the DRAM interface pins that connect to one series of DIMMs. The processor supports two DDR channels. The processor includes two DCTs. Each DCT controls one 64-bit DDR DIMM channel.
For DDR products, DCT0 controls channel A DDR pins and DCT1 controls channel B DDR pins. However, the processor may be configured: (1) to behave as a single dual-channel DCT; this is called ganged mode; or

(2) to behave as two single-channel DCTs; this is called unganged mode.
A logical DIMM is either one 64-bit DIMM (as in unganged mode) or two identical DIMMs in parallel to create a 128-bit interface (as in ganged mode). See section 1.5.2 [Supported Feature Variations] on page 20 for information about supported package/DRAM configurations.

For DDR products, when the DCTs are in ganged mode, as specified by [The DRAM Controller Select Low Register] F2x110[DctGangEn], then each logical DIMM is two channels wide. Each physical DIMM of a 2-channel logical DIMM is required to be the same size and use the same timing parameters. Both DCTs must be programmed with the same information (see section 2.8.1 [DCT Configuration Registers] on page 61). When the DCTs are in 64-bit mode, a logical DIMM is equivalent to a 64-bit physical DIMM and each channel is controlled by a different DCT.