![16 GiB-DDR4-RAM-Riegel RAM019FIX Small Crop 90 PCNT](https://upload.wikimedia.org/wikipedia/commons/8/85/16_GiB-DDR4-RAM-Riegel_RAM019FIX_Small_Crop_90_PCNT.png)
Registered (also called buffered) memory modules have a
register
Register or registration may refer to:
Arts entertainment, and media Music
* Register (music), the relative "height" or range of a note, melody, part, instrument, etc.
* ''Register'', a 2017 album by Travis Miller
* Registration (organ), the ...
between the
DRAM
Dynamic random-access memory (dynamic RAM or DRAM) is a type of random-access semiconductor memory that stores each bit of data in a memory cell, usually consisting of a tiny capacitor and a transistor, both typically based on metal-oxid ...
modules and the system's
memory controller
The memory controller is a digital circuit that manages the flow of data going to and from the computer's main memory. A memory controller can be a separate chip or integrated into another chip, such as being placed on the same die or as an int ...
. They place less electrical load on the memory controller and allow single systems to remain stable with more
memory module
In computing, a memory module or RAM (random-access memory) stick is a printed circuit board on which memory integrated circuits are mounted. Memory modules permit easy installation and replacement in electronic systems, especially computers such ...
s 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 DIMM () (Dual In-line Memory Module), commonly called a RAM stick, comprises a series of dynamic random-access memory integrated circuits. These memory modules are mounted on a printed circuit board and designed for use in personal computers, ...
(DIMM), a registered memory module is called an RDIMM, while unregistered memory is called UDIMM or simply DIMM.
Registered memory is often more expensive because of the
lower number of units sold and additional
circuitry
An electronic circuit is composed of individual electronic components, such as resistors, transistors, capacitors, inductors and diodes, connected by conductive wires or traces through which electric current can flow. It is a type of electrical ...
required, so it is usually found only in applications where the need for
scalability
Scalability is the property of a system to handle a growing amount of work by adding resources to the system.
In an economic context, a scalable business model implies that a company can increase sales given increased resources. For example, a ...
and
robustness
Robustness is the property of being strong and healthy in constitution. When it is transposed into a system, it refers to the ability of tolerating perturbations that might affect the system’s functional body. In the same line ''robustness'' ca ...
outweighs the need for a low price for example, registered memory is usually used in
server
Server may refer to:
Computing
*Server (computing), a computer program or a device that provides functionality for other programs or devices, called clients
Role
* Waiting staff, those who work at a restaurant or a bar attending customers and su ...
s.
Although most registered memory modules also feature
error-correcting code memory
Error correction code memory (ECC memory) is a type of computer data storage that uses an error correction code (ECC) to detect and correct n-bit data corruption which occurs in memory. ECC memory is used in most computers where data corruption c ...
(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.
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
In electronics and especially synchronous digital circuits, a clock signal (historically also known as ''logic beat'') oscillates between a high and a low state and is used like a metronome to coordinate actions of digital circuits.
A clock sign ...
behind the equivalent unregistered DRAM. With
SDRAM
Synchronous dynamic random-access memory (synchronous dynamic RAM or SDRAM) is any DRAM where the operation of its external pin interface is coordinated by an externally supplied clock signal.
DRAM integrated circuits (ICs) produced from the ...
, 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
Interleaving may refer to:
* Interleaving, a technique for making forward error correction more robust with respect to burst errors
* An optical interleaver, a fiber-optic device to combine two sets of dense wavelength-division multiplexing (DW ...
, 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
Memory bandwidth is the rate at which data can be read from or stored into a semiconductor memory by a central processing unit, processor. Memory bandwidth is usually expressed in units of bytes per second, bytes/second, though this can vary for ...
for 2DPC (DIMMs per channel) configurations with UDIMM by some 5% in comparison to RDIMM". (p. 14). This occurs 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.
Compatibility
Usually, the
motherboard
A motherboard (also called mainboard, main circuit board, mb, mboard, backplane board, base board, system board, logic board (only in Apple computers) or mobo) is the main printed circuit board (PCB) in general-purpose computers and other expand ...
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.
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
![20210402 Registered versus load-reduced DIMM memory](https://upload.wikimedia.org/wikipedia/commons/9/92/20210402_Registered_versus_load-reduced_DIMM_memory.svg)
Registered (Buffered) DIMM (R-DIMM) modules insert a
buffer
Buffer may refer to:
Science
* Buffer gas, an inert or nonflammable gas
* Buffer solution, a solution used to prevent changes in pH
* Buffering agent, the weak acid or base in a buffer solution
* Lysis buffer, in cell biology
* Metal ion 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.
The cost is increased
memory latency
''Memory latency'' is the time (the latency) between initiating a request for a byte or word in memory until it is retrieved by a processor. If the data are not in the processor's cache, it takes longer to obtain them, as the processor will hav ...
, as a result of one 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.
Load Reduced DIMM (LR-DIMM) 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.
Fully Buffered DIMM
Fully Buffered DIMM (or FB-DIMM) is a memory technology that can be used to increase reliability and density of memory systems. Unlike the parallel bus architecture of traditional DRAMs, an FB-DIMM has a serial interface between the memory contro ...
(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.
By reducing the number of pins required per memory bus, CPUs could support more memory buses, allowing higher total
memory bandwidth
Memory bandwidth is the rate at which data can be read from or stored into a semiconductor memory by a central processing unit, processor. Memory bandwidth is usually expressed in units of bytes per second, bytes/second, though this can vary for ...
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
External links
Memory Decisions{{Webarchive, url=https://web.archive.org/web/20190519094153/http://www.2cpu.com/contentteller.php?ct=articles&action=pages&page=memory_decisions.html , date=2019-05-19 , February 8, 2004
Do I Need ECC and Registered Memory(.doc document)
Basics of LRDIMMLRDIMM vs RDIMM: Signal integrity, capacity, bandwidth
Computer memory