That’s correct, the mode is set in the BIOS.
We inherited a Dell PowerEdge 710 server.  It has a total of 48 GB Ram.  There are a total of 10 memory sticks in the machine.  Five memory sticks in the A bank and five in the B bank.  Eight of the memory sticks are 4 GB 2Rx4 PC3 1060R 09 10 E1 D2.  Two are 8GB 2R4 PC3L 1060R 9 12 E2.  The 4GBs are in the A1 A4 A2 A5 B1 B4 B2 B5 and the 8GBs in the A7 and A8.  YES, I know this is an unsupported configuration, as I said we inherited it (MSP company).  Every time we reboot it, we get the error message about an unsupported memory configuration.  The message Press (Ctrl-B) for Remote Access Setup in 5 seconds appears next.  Then the message Press F1to continue or F2 to  run the system setup program.  Is the press f1 to continue, caused by the unsupported memory configuration or a problem with the Remote Access Setup? Given that we need to have at least 2 sticks of the same type 2 GB 4 GB, 8 GB in the same memory channels, we need to do one of several things: buy 4: 4GB and replace the 8GB by removing it and putting 2: 4B in A3 A6 and 2: 4GB in B3 B6, buy 4:4GB and remove all the memory and put the 4GB in A4 A7 A5 A8 A6 A9 and B4 B7 B5 B8 B6 B9, or by 2: 8GB, remove the 8GB sticks and put the 8 GB memory in A3 A6 and B3 B6, correct? Are there any changes to the memory configuration in the BIOS we might need to change? 
PowerEdge R710 Memory configuration