Acer Network Device Altos R520 - Manual (Page 81)

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Standard memory configuration
The system is capable of supporting a minimum of only one FBDIMM
installed on the DIMM A1 slot. However, for system performance
reasons, we recommend that at least two memory modules must be
installed per branch. The modules must cover the same slot position on
both channels and FBDIMM pairs must be identical with respect to size,
speed, and organization. FBDIMMs that cover adjacent slot positions
do not need to be identical.
When adding four FBDIMMs to a standard non-mirrored
configuration, the memory modules must be populated in DIMM slots
A1 and B1 first then DIMM slots C1 and D1. It allows both memory
branches to operate in parallel and simultaneously to achieve
equivalent performance.
Mirrored memory configuration
Memory mirroring is implemented in the MCH and the system BIOS.
You can configure the system to maintain mirrored copy of the data in
memory. When operating in mirrored mode, both branches operate in
lock step. In mirrored mode, branch 1 contains a replicate copy of the
data in branch 0. The minimum FBDIMM configuration to support
memory mirroring is four FBDIMMs, populated as shown in the figure
below:
All four memory modules must be identical with respect to size, speed,
and organization.