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Registered memory

January 01, 1970

Not to be confused with ECC memory, although memory modules often use both technologies.

Registered memory (also called buffered memory) is computer memory that has a register between the DRAM modules and the system's memory controller. A registered memory module places less electrical load on a memory controller compared to an unregistered one. Registered memory allows a computer system to remain stable with a higher number of memory modules than it would have otherwise.

One 64 GiB DDR5-4800 ECC 1.1 V registered DIMM (RDIMM)
Example of an unregistered DIMM (UDIMM)

When conventional memory is compared with registered memory, the 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 circuitry required and lower number of units sold, 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 (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.[1]

Performance edit

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 5600 series of processors access memory using interleaving, wherein memory access is distributed across three channels. If two memory DIMMs are used per channel, there is a reduction of maximum memory bandwidth for this configuration with UDIMM by some 5% in comparison to RDIMM.[2][3]

Compatibility edit

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.[4] 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 often supports ECC and is described as ECC RAM) in a PC motherboard that does not support it.

Buffered memory types edit

 
Comparison: Registered Memory (R-DIMM) and Load Reduced DIMM (LR-DIMM)[5]

Registered (Buffered) DIMM (R-DIMM) modules insert a buffer between the pins of the command and address buses on the DIMM and the memory chips. A high-capacity DIMM might have numerous memory chips, each of which must receive the memory address, and their combined input capacitance limits the speed at which the memory bus can operate. By redistributing the command and address signals within the R-DIMM, this allows more chips to be connected to the memory bus.[6] The cost is increased memory latency, as a result of one[citation needed] 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.[7]

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 provide large overall maximum memory capacities, while avoiding the performance and power consumption problems of FB-DIMMs, induced by the required conversion between serial and parallel signal forms.[6][8]

Fully Buffered DIMM (FB-DIMM) modules increase maximum memory capacities in large systems even more, using a more complex buffer chip to translate between the wide bus of standard SDRAM chips and a narrow, high-speed serial memory bus. In other words, all control, address and data transfers to FB-DIMMs are performed in a serial fashion, while the additional logic present on each FB-DIMM transforms serial inputs into parallel signals required to drive memory chips.[8] By reducing the number of pins required per memory bus, CPUs could support more memory buses, allowing higher total memory bandwidth and capacity. Unfortunately, the translation further increased memory latency, and the complex high-speed buffer chips used significant power and generated a lot of heat.

Both FB-DIMMs and LR-DIMMs are designed primarily to minimize the load that a memory module presents to the memory bus. They are not compatible with R-DIMMs, and motherboards that require them usually will not accept any other kind of memory modules.

References edit

  1. ^ "Servers and workstations: P9D-V motherboard". Asus. Retrieved December 4, 2014.
  2. ^ "WHITE PAPER - FUJITSU PRIMERGY SERVERS - MEMORY PERFORMANCE OF XEON 5600 (WESTMERE-EP) BASED SYSTEMS" (PDF). Fujitsu Global. 2.0. Fujitsu Technology Solutions GmbH. 2011-06-06. p. 17. Retrieved 2023-05-20. This results in a reduction of maximum memory bandwidth for 2DPC configurations with UDIMM by some 5% in comparison to RDIMM..
  3. ^ Florin, Anghel (2013-10-22). "How to: Difference between RDIMM and UDIMM". Spiceworks. Retrieved 2023-05-20. 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.
    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.
  4. ^ "Dell servers example" (PDF). Dell.
  5. ^ Deffree, Suzanne (September 20, 2011). "Basics of LRDIMM". EDN. from the original on April 2, 2021.
  6. ^ a b Johan De Gelas (2012-08-03). "LRDIMMs, RDIMMs, and Supermicro's Latest Twin". AnandTech. Retrieved 2014-09-09.
  7. ^ "Socket sWRX8 - AMD".
  8. ^ a b "What is LR-DIMM, LRDIMM Memory? (Load-Reduce DIMM)". simmtester.com. Retrieved 2014-08-29.

External links edit

  • Memory Decisions 2019-05-19 at the Wayback Machine, February 8, 2004
  • Do I Need ECC and Registered Memory (.doc document)
  • Basics of LRDIMM
  • LRDIMM vs RDIMM: Signal integrity, capacity, bandwidth

January 01, 1970
registered, memory, confused, with, memory, although, memory, modules, often, both, technologies, also, called, buffered, memory, computer, memory, that, register, between, dram, modules, system, memory, controller, registered, memory, module, places, less, el. Not to be confused with ECC memory although memory modules often use both technologies Registered memory also called buffered memory is computer memory that has a register between the DRAM modules and the system s memory controller A registered memory module places less electrical load on a memory controller compared to an unregistered one Registered memory allows a computer system to remain stable with a higher number of memory modules than it would have otherwise One 64 GiB DDR5 4800 ECC 1 1 V registered DIMM RDIMM Example of an unregistered DIMM UDIMM When conventional memory is compared with registered memory the 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 circuitry required and lower number of units sold 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 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 1 Contents 1 Performance 2 Compatibility 3 Buffered memory types 4 References 5 External linksPerformance editNormally 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 5600 series of processors access memory using interleaving wherein memory access is distributed across three channels If two memory DIMMs are used per channel there is a reduction of maximum memory bandwidth for this configuration with UDIMM by some 5 in comparison to RDIMM 2 3 Compatibility editUsually 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 4 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 often supports ECC and is described as ECC RAM in a PC motherboard that does not support it Buffered memory types edit nbsp Comparison Registered Memory R DIMM and Load Reduced DIMM LR DIMM 5 Registered Buffered DIMM R DIMM modules insert a buffer between the pins of the command and address buses on the DIMM and the memory chips A high capacity DIMM might have numerous memory chips each of which must receive the memory address and their combined input capacitance limits the speed at which the memory bus can operate By redistributing the command and address signals within the R DIMM this allows more chips to be connected to the memory bus 6 The cost is increased memory latency as a result of one citation needed 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 7 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 provide large overall maximum memory capacities while avoiding the performance and power consumption problems of FB DIMMs induced by the required conversion between serial and parallel signal forms 6 8 Fully Buffered DIMM FB DIMM modules increase maximum memory capacities in large systems even more using a more complex buffer chip to translate between the wide bus of standard SDRAM chips and a narrow high speed serial memory bus In other words all control address and data transfers to FB DIMMs are performed in a serial fashion while the additional logic present on each FB DIMM transforms serial inputs into parallel signals required to drive memory chips 8 By reducing the number of pins required per memory bus CPUs could support more memory buses allowing higher total memory bandwidth and capacity Unfortunately the translation further increased memory latency and the complex high speed buffer chips used significant power and generated a lot of heat Both FB DIMMs and LR DIMMs are designed primarily to minimize the load that a memory module presents to the memory bus They are not compatible with R DIMMs and motherboards that require them usually will not accept any other kind of memory modules References edit Servers and workstations P9D V motherboard Asus Retrieved December 4 2014 WHITE PAPER FUJITSU PRIMERGY SERVERS MEMORY PERFORMANCE OF XEON 5600 WESTMERE EP BASED SYSTEMS PDF Fujitsu Global 2 0 Fujitsu Technology Solutions GmbH 2011 06 06 p 17 Retrieved 2023 05 20 This results in a reduction of maximum memory bandwidth for 2DPC configurations with UDIMM by some 5 in comparison to RDIMM Florin Anghel 2013 10 22 How to Difference between RDIMM and UDIMM Spiceworks Retrieved 2023 05 20 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 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 Dell servers example PDF Dell Deffree Suzanne September 20 2011 Basics of LRDIMM EDN Archived from the original on April 2 2021 a b Johan De Gelas 2012 08 03 LRDIMMs RDIMMs and Supermicro s Latest Twin AnandTech Retrieved 2014 09 09 Socket sWRX8 AMD a b What is LR DIMM LRDIMM Memory Load Reduce DIMM simmtester com Retrieved 2014 08 29 External links editMemory Decisions Archived 2019 05 19 at the Wayback Machine February 8 2004 Do I Need ECC and Registered Memory doc document Basics of LRDIMM LRDIMM vs RDIMM Signal integrity capacity bandwidth Retrieved from https en wikipedia org w index php title Registered memory amp oldid 1217913167, wikipedia, wiki, book, books, library,

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