Dell PowerEdge R420xr Owner's Manual page 40

Figure 12. Memory Socket Locations
Memory channels are organized as follows:
Processor 1 channel 1: memory sockets A1 and A4
channel 2: memory sockets A2 and A5
channel 3: memory sockets A3 and A6
Processor 2 channel 1: memory sockets B1 and B4
channel 2: memory sockets B2 and B5
channel 3: memory sockets B3 and B6
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