What is Registered RDIMM and why is ECC needed?

Random access memory can be abbreviated as RAM. It is also called random access memory, random access memory, RAM. RAM can also be affectionately called “RAM”. RAM is logically made up of memory cells.

Registered memory (Registered DIMM, RDIMM) is called RAM modules that have a separate register on the “board” for the addresses of the “RAM” and commands.

The RAM controller in the processor accesses the registers, while the registers send information to the memory chips. Such an organization of “RAM” allows you to increase the number of modules per RAM channel by reducing the electrical load on the memory controller. The controller is located either in the northbridge of the motherboard or in the processor . Also, the capacity of memory modules is halved if the module contains two registers.

Registered memory differs from conventional, unbuffered “RAM” by higher delays in reading and writing information in RAM modules. This is due to the fact that the modules contain an additional intermediate node – a buffer. Read/write is done by the memory controller in the processor or motherboard northbridge. Working with this node, of course, requires additional work time. But at the same time, we note that the load on the processor decreases, since the buffer is responsible for direct work with memory banks.

Each RAM module contains an SPD (Serial Presence Detect) chip. This chip contains the firmware of the memory module. This firmware determines the operation of simpler microcircuits.

Registered and buffered memory are the same
Registered memory is buffered memory. As indicated above, a register is a buffer for addresses and commands when working with memory. The processor or motherboard northbridge sends data, memory addresses, and commands. Registers execute commands at specified addresses.

Such memory costs more than conventional, unbuffered memory. It is used exclusively in servers , because it allows you to get more memory per processor in the server.

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What is FB-DIMM
FB-DIMM, Full Buffered Dual Inline Memory Module – fully buffered DIMM – these are DDR2 RAM sticks. The RAM dice at the same time use a serial data transfer interface between the memory modules and the “RAM” controller. Unlike standard RAM modules, they do not use 240-pins, but 96-pins out of 240 possible pins. This organization of work allows you to organize more channels on the motherboard using memory controllers. Up to 6 channels. These memory modules are not compatible with conventional RAM sticks.

A serial interface is a data transfer interface in which data is transmitted over one wire or track on a printed circuit board one after the other. There may be several such wires (tracks), but the principle of data transfer does not change.

Advanced Memory Buffer, AMB is a microcircuit that organizes the operation of FB-DIMM memory modules. This microcircuit is located directly on the “RAM” bar.

In one memory channel with such an organization of the operation of RAM modules, it is possible to install up to 8 “RAM” strips. This allows, in the case of DDR2 RAM, to achieve a RAM capacity of up to 192 GB per server.

Due to the fact that the AMB chip adds its own delays to the operation of the memory module, these dies work somewhat slower than RDIMM modules, register RAM. But, since the total amount of memory in this case increases, the overall performance of the system also increases.

A Brief History of RAM
The following is a brief history of the development of RAM types. We start it with the release of SDRAM memory. This happened in 1996. The bandwidth of this RAM was 1.1 GBps.

The next memory in the table is RDRAM. It was released in 1998. It was a completely new RAM architecture. A completely new standard from Rambus. Several generations of memory have been released. It was distinguished by higher frequencies, stable timings, only at the same time the delays in the functioning of the memory were slightly higher. Unfortunately, this memory could not stand the competition in the market and was forced to leave the scene of the memory market.

The next in the table are the RAM DDR lines. Double Data Rate Synchronous Dynamic Random Access Memory – synchronous dynamic memory with random access and double data transfer rate. This RAM standard was released to the market in 2000. This memory is still in use today. At the same time, the standard is being developed to achieve higher speeds of operation.

Registered and ECC memory are not the same
Registered, buffered memory and ECC-DIMM are different things. In the case of registered RAM, modules contain memory registers. They are an intermediate link for the commands and addresses of the “RAM”.

ECC memory is error correcting RAM. Physically, these “RAM” strips contain additional memory chips for parity bits that correct errors in reading / writing data. When the data check detects a mismatch between the parity bit and the computed value based on the data contained in the RAM cell, the data is recovered. ECC allows you to recover one bit of data in 1 byte of information. Physically, there is 1 additional chip for parity bits per 8 memory chips on the module.

Examples of common desktop systems
The following are three examples of computers – the first is with conventional, no-ECC memory, the second is a professional workstation, with ECC “RAM”, the third example is a server with error-correcting registered RAM:

Desktop computer with regular, unbuffered, no-ECC memory.
Desktop PC with ECC-RAM.
Server with DDR4 RDIMM ECC memory.
Do conventional processors and motherboards support ECC memory and/or RDIMM
ECC memory is supported by AMD Ryzen, FX processors. In the case of desktop processors from Intel, there is no ECC support. Register memory is used exclusively in servers.

There is a special type of FB-DIMM RAM – this is RAM with an AMB chip. The modules of this memory use a serial interface. Due to this, not all contacts on one RAM bar are involved, which allows you to install up to 192 gigabytes of “RAM” in one server.

In this article, you could learn about the structural features of RAM, that there is memory with error correction, that it is divided into memory with and without registers. Registered memories can be installed much more in a computer. But once again it is worth noting that memory with registers is used exclusively in servers.