NEC Express 5800/320Lb - Manual (Page 261)

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Note the followings to install or replace DIMM.
The DIMMs with the same number are linked among the groups. When a DIMM is added
to one group, another identical DIMM should be installed to the socket with the same
number in another group. This rule is applied to the case of removal.
The linked DIMMs should be of the same product with same performance.
DIMMs should be installed in sockets, starting from the lowest socket number.