Not to be confused with ECC memory, although memory modules often use both technologies.
Registered memory modules have a register between the DRAM modules and the system's
memory controller.
They place less electrical load on the memory controller and allow single systems to remain
stable with more memory modules than they would have otherwise.
When compared with registered memory, conventional memory is usually referred to as unbuffered
memory or unregistered memory.
When manufactured as a dual in-line memory module, a registered memory module is called
an RDIMM, while unregistered memory is called UDIMM.
Registered memory is often more expensive because of the lower number of units sold
and additional circuitry required, so it is usually found only in applications where the
need for scalability and robustness outweighs the need for a low price – for example,
registered memory is usually used in servers.
Although most registered memory modules also feature error-correcting code memory, it is
also possible for memory to be Registered and not ECC, or ECC modules to not be registered.
Non-registered ECC memory is supported and used in motherboards which do not support
the very large amounts of memory used by servers.
Performance Normally, there is a performance penalty for
using registered memory.
Each read or write is buffered for one cycle between the memory bus and the DRAM, so the
registered RAM can be thought of as running one clock cycle behind the equivalent unregistered
DRAM.
With SDRAM, this only applies to the first cycle of a burst.
However, this performance penalty is not universal.
There are other factors involved in memory access speed.
For example, the Intel Westmere 5600 series of processors access memory using interleaving,
wherein memory access is distributed across three channels.
If two memory DIMMs are used per channel, this "...results in a reduction of maximum
memory bandwidth for 2DPC configurations with UDIMM by some 5% in comparison to RDIMM.".
This is because "...when you go to two DIMMs per memory channel, due to the high electrical
loading on the address and control lines, the memory controller uses a “2T” or “2N”
timing for UDIMMs.
Consequently every command that normally takes a single clock cycle is stretched to two clock
cycles to allow for settling time.
Therefore, for two or more DIMMs per channel, RDIMMs will have lower latency and better
bandwidth than UDIMMs."
Compatibility Usually the motherboard must match the memory
type: registered memory will not work in a motherboard not designed for it, and vice-versa.
Some PCs will accept registered memory, but registered and unregistered memory cannot
be mixed.
There is much confusion between registered and ECC memory; it is widely thought that
ECC memory will not work in a motherboard without ECC support, although the compatibility
issues arise when trying to use registered memory in a desktop PC motherboard.
Buffered memory Buffered memory is an older term for registered
memory.
Buffered refers to an electronic buffer placed between the memory and the memory controller.
In general registered RAM won't fit in a slot for unregistered RAM, however it will with
SDRAM.
However, some systems with large maximum memory capacities use Fully Buffered DIMM modules.
In normal registered/buffered memory only the control lines are buffered, whereas in
fully buffered memory the data lines are buffered as well with all transfers performed in a
serial fashion; the additional logic present on each FB-DIMM module transforms serial input
into parallel signals required to drive memory chips.
Load Reduced DIMM modules are similar to registered/buffered memory, in a way that LRDIMM modules buffer
both control and data lines while keeping the parallel nature of all signals.
As such, LRDIMM memory provides large overall maximum memory capacities, while addressing
some of the performance and power consumption issues of FB-DIMM memory induced by the required
conversion between serial and parallel signal forms.
Both FB-DIMM and LRDIMM memory types are designed primarily to control the amount of electrical
current flowing to and from the memory chips at any given time.
They are not compatible with registered/buffered memory, and motherboards that require them
usually will not accept any other kind of memory.
References
External links Memory Decisions 2cpu.com
Do I Need ECC and Registered Memory Unbuffered versus Registered ECC Memory