Unbuffered and registered DIMMs
DDR3 DIMMs are available as both Unbuffered (UDIMMs) and Registered (RDIMMs). Both UDIMMs
and RDIMMs support error correcting code (ECC). There are three types of DDR3 memory:
• PC3-8500R (RDIMM) —1066 or 800 MT/s data rate, depending on memory configuration and
processor
• PC3-10600E (UDIMM) —1333, 1066, or 800 MT/s data rate, depending on memory
configuration and processor
• PC3-10600R (RDIMM) —1333, 1066, or 800 MT/s data rate, depending on memory
configuration and processor
Administrators can use either RDIMMs or UDIMMs, but RDIMM and UDIMM memory cannot be
mixed within a single server. When choosing memory configurations using DDR3 memory, the
following guidelines are useful:
• UDIMM configurations are limited to a maximum of two UDIMMs per memory channel because the
memory controller must drive the address and command signals to each DRAM chip on a channel.
This results in a 24 GB maximum configuration. Because they require fewer components, UDIMMs
are typically less expensive than RDIMMs.
• RDIMM configurations can provide larger memory capacity because the memory controller only
drives the address and command signals to a single register chip, thereby reducing the electrical
load on the memory controller. Users requiring large memory footprints can install three 8-GB
RDIMMs per channel for a total of 144 GB.
• For smaller memory configurations, installing only one or two DIMMs per memory channel can
potentially increase memory performance. In many instances this allows administrators to clock the
memory channel at a higher data rate.
Table 1 summarizes the choices available for DDR3 memory.
Table 1. DDR3 memory options
Type DIMM
capacity
(GB)
Rank Max. data rate in
MT/s (1 or 2
DIMMs/channel
Max. data rate in MT/s
(3 DIMMs/channel)
PC3-8500R
RDIMM 4 quad 1066 800
PC3-8500R
RDIMM 8 dual 1066 800
PC3-10600R
RDIMM 2 dual 1333 800
PC3-10600R
RDIMM 4 dual 1333 800
PC3-10600R
RDIMM 8 dual 1333 800
PC3-10600E
UDIMM 1 single 1333 N/A
PC3-10600E
UDIMM 2 dual 1333 N/A
An RBSU setting allows PC3-10600R memory modules to run at 1333 MT/s with two DIMMs per
channel and without performance degradation. Only HP branded DIMMs have been fully validated to
operate at this speed. Third-party DIMMs may not meet stringent design requirements; therefore, HP
does not recommend configuring third-party DIMMs for 1333 MT/s data rates with two DIMMs per
channel. At this writing, operating the memory at 1333 MT/s is supported only on select ProLiant SL
G6 server trays using the Xeon 5500 Series processors operating at 95W.
15
