Hardware options installation 41
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. A quad-rank
DIMM is, effectively, two dual-rank DIMMs on the same module. 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. There are four ranks of DRAM on the DIMM, but the LRDIMM
buffer creates an abstraction that allows the DIMM to appear as a logical dual-rank DIMM to the system. This
is called Rank-Multiplication. The LRDIMM buffer also isolates the electrical loading of the DRAM from the
system to allow for faster operation. These two changes allow the system to support up to three LRDIMMs per
memory channel, providing for up to 50% greater memory capacity and 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.
Item Description
Definition
1
Size
—
2
Rank
1R = Single-rank
2R = Dual-rank
3R = Three-rank
4R = Quad-rank
3
Data width
x4 = 4-bit
x8 = 8-bit
