Dell PE R720XD Owner's Manual page 42

Figure 17. Memory Socket Locations
Memory channels are organized as follows:
Processor 1
Processor 2
The following table shows the memory populations and operating frequencies for the supported configurations.
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channel 0: slots A1, A5, and A9
channel 1: slots A2, A6, and A10
channel 2: slots A3, A7, and A11
channel 3: slots A4, A8, and A12
channel 0: slots B1, B5, and B9
channel 1: slots B2, B6, and B10
channel 2: slots B3, B7, and B11
channel 3: slots B4, B8, and B12