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Memory subsystem architecture
The memory subsystem in this server is divided into channels. Each processor supports four channels, and
each channel supports two DIMM slots, as shown in the following table.
Channel
1
2
3
4
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 node 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. There are four ranks of DRAM on the DIMM, but the LRDIMM
buffer creates an abstraction that allows the DIMM to appear as a dual-rank DIMM to the system. The
LRDIMM buffer isolates the electrical loading of the DRAM from the system to allow for faster operation. This
allows higher memory operating speed compared to quad-rank RDIMMs.
DIMM identification
To determine DIMM characteristics, use the label attached to the DIMM and the following illustration and
table.
42
Memory subsystem architecture
Population order
A
E
B
F
C
G
D
H
Slot number
8
7
6
5
1
2
3
4
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