Hardware options installation 55
Memory subsystem architecture
The memory subsystem in this server is divided into channels. Each processor supports four channels, and
each channel supports three DIMM slots, as illustrated.
This multi-channel architecture provides enhanced performance in Advanced ECC mode. This architecture
also enables Online Spare Memory modes.
DIMM slots in this server are identified by number and by letter:
• Letters identify the population order.
• Slot numbers indicate the DIMM slot ID for spare replacement.
Population order
For memory configurations with a single processor or multiple processors, populate the DIMM slots in the
following order:
• LRDIMM: Sequentially in alphabetical order (A through L)
• RDIMM: Sequentially in alphabetical order (A through L)
After installing the DIMMs, use RBSU to configure Advanced ECC, or online spare memory support.
Single-, dual-, and quad-rank DIMMs
To understand and configure memory protection modes properly, an understanding of single-, dual-, and
quad-rank DIMMs is helpful. Some DIMM configuration requirements are based on these classifications.
A single-rank DIMM has one set of memory chips that is accessed while writing to or reading from the
memory. A dual-rank DIMM is similar to having two single-rank DIMMs on the same module, with only one
rank accessible at a time. A quad-rank DIMM is, effectively, two dual-rank DIMMs on the same module. Only
one rank is accessible at a time. The server memory control subsystem selects the proper rank within the
DIMM when writing to or reading from the DIMM.
Dual- and quad-rank DIMMs provide the greatest capacity with the existing memory technology. For
example, if current DRAM technology supports 8-GB single-rank DIMMs, a dual-rank DIMM would be 16
GB, and a quad-rank DIMM would be 32 GB.
LRDIMMs are labeled as quad-rank DIMMs; however, they function more like dual-rank DIMMs. There are
four ranks of DRAM on the DIMM, but the LRDIMM buffer creates an abstraction that allows the DIMM to
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