What is RDIMM Registered Memory and Why ECC is Needed

What is RDIMM, what is a register for
Random access memory can be abbreviated as RAM. It is also called random access memory, RAM. RAM can also be affectionately called “RAM”. RAM is logically composed of memory cells. Each cell stores the number of bits equal to a power of two.

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

The RAM controller in the processor refers to the registers, while the registers send information to the memory chips. This 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 ordinary, 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. Reading / writing is performed by the memory controller in the processor or the north bridge of the motherboard. Working with this node naturally 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 microcircuit contains the firmware of the memory module. This firmware determines the operation of simpler microcircuits.

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

Such memory is more expensive than regular, 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 strips. In this case, the RAM chips use a serial data transfer interface between the memory modules and the “RAM” controller. Unlike standard RAM modules, they use not 240-pin, but 96-pin out of 240 possible pins. This organization of work allows organizing more channels on the motherboard using memory controllers. Up to 6 channels. These memory modules are incompatible with conventional rails.

A serial interface is a data transfer interface that transfers data over a single 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 right on the “RAM” bar.

With such an organization of the RAM modules, it is possible to install up to 8 “RAM” strips into one memory channel. This allows, in the case of DDR2 RAM, to achieve a RAM capacity of up to 192 Gigabytes 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, registered RAM. But, since the total amount of memory in this case increases, the overall system performance also increases.

A brief history of RAM
Below is a brief history of the development of types of RAM. 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, but the memory delays were slightly higher. Unfortunately, this memory could not stand the competition in the market and had 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 random access memory with double data transfer rate. This RAM standard was released to the market in 2000. This memory is still in use. At the same time, a standard is being developed to achieve higher operating speeds.

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

ECC memory is error correcting RAM. Physically, these “RAM” strips contain additional memory chips for parity bits that correct data read / write errors. When the data check detects a mismatch between the parity bit and the calculated 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, a module for 8 memory chips has 1 additional chip for parity bits.

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 register RAM with error correction:

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 processors, FX. In the case of Intel Desktop processors, 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 in this memory use a serial interface. Due to this, not all contacts on one RAM strip are involved, which allows installing up to 192 gigabytes of “RAM” into one server.

In this article, it was possible to learn about the peculiarities of the structure of RAM, that there is a memory with error correction, that it is divided into memory with and without registers. Many more register memories can be installed in a computer. But once again it is worth noting that register memory is used exclusively in servers.