IBM eServer xSeries 255 Option Installation Manual page 60

When you install DIMMs, install them in the order listed in Table 3.
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IBM xSeries 255 Type 8685: Option Installation Guide
See the User's Guide on the IBM xSeries Documentation CD for further details
about enabling the Hot Spare Memory feature.
Table 3. DIMM installation order
Interleaving Number of DIMMs
Two-way
Two-way
Two-way
Four-way
Four-way
Four-way
Install DIMMs in these connectors
2 D2, D8 (standard)
4 D2, D8 (standard) (Hot Spare memory
bank for two-way interleaving)
D4, D10
6 D2, D8 (standard) (Hot Spare memory
bank for two-way interleaving)
D4, D10
D6, D12
4 D2, D8, D1, D7
8 D2, D8, D1, D7 (Hot Spare memory bank
for four-way interleaving)
D4, D10, D3, D9
12 D2, D8, D1, D7 (Hot Spare memory bank
for four-way interleaving)
D4, D10, D3, D9
D6, D12, D5, D11