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{{Short description|Type of computer memory}}
{{Hatnote|Not to be confused with [[ECC memory]], although memory modules often use both technologies.}}
[[File:
[[File:16 GiB-DDR4-RAM-Riegel RAM019FIX Small Crop 90 PCNT.png|thumb|Example for UDIMM RAM]]▼
▲[[File:16 GiB-DDR4-RAM-Riegel RAM019FIX Small Crop 90 PCNT.png|thumb|Example
'''Registered memory''' (also called '''buffered memory''') is [[computer memory]] that has a [[Hardware register|register]] between the [[Dynamic random access memory|DRAM]] modules and the system's [[memory controller]]. A registered memory module places less electrical load on a memory controller than an unregistered one. Registered memory allows a computer system to remain stable with more [[memory module]]s than it would have otherwise.
Registered memory is often more expensive because of the [[Economies of scale|lower number of units sold]] and additional [[Electrical component|circuitry]] required, so it is usually found only in applications where the need for [[scalability]] and [[Robustness (computer science)|robustness]] outweighs the need for a low price{{snd}} for example, registered memory is usually used in [[Server (computing)|server]]s.▼
When conventional memory is compared with registered memory, conventional memory is usually referred to as '''unbuffered memory''' or '''unregistered memory'''. When registered memory is manufactured as a [[dual in-line memory module]] (DIMM), it is called an '''RDIMM'''. Similarly, an unregistered DIMM is called a '''UDIMM''' or simply "DIMM".
▲Registered memory is often more expensive because of the additional [[Electrical component|circuitry]] required and [[Economies of scale|lower number of units sold]]
Although most registered memory modules also feature [[error-correcting code memory]] (ECC), it is also possible for registered memory modules to not be error-correcting or vice versa. Unregistered ECC memory is supported and used in workstation or entry-level server motherboards that do not support very large amounts of memory.<ref>{{cite web
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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 many other factors involved in memory access speed. For example, the Intel [[Westmere (microarchitecture)|Westmere]] 5600 series of processors access memory using [[Interleaved memory|interleaving]], wherein memory access is distributed across three channels. If two memory DIMMs are used per channel,
{{cite web
| url = https://sp.ts.fujitsu.com/dmsp/Publications/public/wp-westmere-ep-memory-performance-ww-en.pdf
| title = WHITE PAPER - FUJITSU PRIMERGY SERVERS - MEMORY PERFORMANCE OF XEON 5600 (WESTMERE-EP) BASED SYSTEMS
| page = 17
| date = 2011-06-06
| version = 2.0
| website = Fujitsu Global
| publisher = Fujitsu Technology Solutions GmbH
| access-date = 2023-05-20
| quote = ''This results in a reduction of maximum memory bandwidth for 2DPC configurations with UDIMM by some 5% in comparison to RDIMM.''
}}.</ref><ref>
{{cite web
| url = https://community.spiceworks.com/how_to/54504-difference-between-rdimm-and-udimm
| title = How to: Difference between RDIMM and UDIMM
| last = Florin
| first = Anghel
| date = 2013-10-22
| website = Spiceworks
| access-date = 2023-05-20
| quote = ''But when you go to 2 DIMMs per memory channel, due to the high electrical loading on the address and control lines, the memory controller use something called a “2T” or “2N” timing for UDIMMs.<br />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.''
}}</ref>
==Compatibility==
Usually, the [[motherboard]] must match the memory type; as a result, registered memory will not work in a motherboard not designed for it, and vice versa. Some PC motherboards accept or require registered memory, but registered and unregistered memory modules cannot be mixed.<ref>
{{cite web | url = http://www.dell.com/downloads/global/products/pedge/en/server-pedge-installing-upgrading-memory-11g.pdf | title = Dell servers example | website = Dell | archive-url = https://web.archive.org/web/20201112034337/http://www.dell.com/downloads/global/products/pedge/en/server-pedge-installing-upgrading-memory-11g.pdf | archive-date = 2020-11-12 | url-status = dead }}</ref> There is much confusion between ''registered'' and ''ECC'' memory; it is widely thought that ECC memory (which may or may not be registered) will not work at all in a motherboard without ECC support, not even without providing the ECC functionality, although the compatibility issues actually arise when trying to use ''registered'' memory (which == {{Anchor|LRDIMM}}Buffered memory types ==
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| date = 2012-08-03 | accessdate = 2014-09-09
| author = Johan De Gelas | publisher = [[AnandTech]]
}}</ref> The cost is increased [[memory latency]], as a result of one{{citation needed|date=March 2021}} additional clock cycle required for the address to traverse the additional buffer. Early registered RAM modules were physically incompatible with unregistered RAM modules, but the two variants of SDRAM R-DIMMs are mechanically interchangeable, and some motherboards may support both types.
| url = https://en.wikichip.org/wiki/amd/packages/socket_swrx8
| title = Socket sWRX8 - AMD
}}</ref>
'''Load Reduced DIMM (LR-DIMM or LRDIMM)''' modules are similar to R-DIMMs, but add a buffer to the data lines as well. In other words, LR-DIMMs buffer both control and data lines while keeping the parallel nature of all signals. As a result, LR-DIMMs
| url = https://www.simmtester.com/News/PublicationArticle/167
| title = What is LR-DIMM, LRDIMM Memory? (Load-Reduce DIMM)
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