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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
appear as a dual-rank DIMM to the system. 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
Size
1
Rank
2
Data width
3
Voltage rating
4
Memory speed
5
DIMM type
6
Definition
—
1R = Single-rank
2R = Dual-rank
4R = Quad-rank
x4 = 4-bit
x8 = 8-bit
L = Low voltage (1.35v)
U = Ultra low voltage (1.25v)
Blank or omitted = Standard
12800 = 1600-MT/s
10600 = 1333-MT/s
8500 = 1066-MT/s
R = RDIMM (registered)
E = UDIMM (unbuffered with ECC)
L = LRDIMM (load reduced)
Hardware options installation 48
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