Acer Altos R910 - Manual (Page 183)

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b Use the up and down arrow keys to select BIOS Setup Utility,
then press Enter. The BIOS main menu displays on the
monitor.
2 Use the up and down arrow keys to select the Memory menu,
then press Enter. The Memory menu appears.
3 Use the up and down arrow keys to select Configure Memory
RAS and Performance, then press Enter. The Configure Memory
RAS and Performance menu appears.
4 Set the Desired Memory Configuration to Mirror.
5 Select View Configuration Details.
Make sure Configuration Possible indicates Yes. If the option
indicates No, you may need to install more memory to meet the
requirements of the selected configuration.
6 Press F10 to save changes and exit.
7 Press Y to confirm. The server reboots to activate the changes.