What are the differences between memory standards?

System memory or RAM has so far been the most innocent in terms of naming and understanding what kind of RAM we need for our computer. But it is also true that the status quo has not changed drastically for a long time. We got DDR5, but it didn't change the way we choose RAM that much. We were still mostly concerned with speeds, latency times and compatibility with the system we were looking to build/buy. For example, AMD's AM4 platform does not support DDR5, so the only solution is to upgrade to the newer AM5. With Intel, however, you have to be careful that you bought at least a 12th generation chip or later.

However, the shape and design of the module, which we call RAM, has not changed for a long time. Here we know several terms: DIMM, UDIMM, SODIMM, RDIMM... This year we got another designation - CUDIMM.

First, let's clear up what the labels we already know mean.

What is a DIMM (Dual In-line Memory Module)?

DIMM stands for Dual In-line Memory Module and is actually the official name for what most of us refer to as a memory module. A DIMM is a rectangular circuit with installed memory chips and a connecting edge covered with pins intended for connection to the motherboard.

The reason it is called a DIMM and not a SIMM (Single In-line Memory Module) is that the metal contacts (pins) are on both sides of the module and represent different circuits that provide a 64-bit data path. DDR5 and DDR4 DIMMs have 288 pins, but the notch is moved to a slightly different location, so you cannot insert DDR5 DIMMs into a DDR4 RAM slot or vice versa.

The term DIMM therefore refers to the physical composition of the memory module, but does not tell us anything about the capacity or performance of the memory. For example, a DIMM may have memory chips on only one side of the module, or they may be on both sides. For compatibility, DIMMs typically adhere to the specifications of the JEDEC standards organization, which means that DIMMs are compatible with most of today's systems and platforms.

What is a UDIMM?

Unbuffered RAM or UDIMM (Unbuffered Dual In-Line Memory Module) is the most common type of system memory. Simply put, it's just a type of DIMM, and in most cases you won't even see this designation in the RAM specs because the names DIMM and UDIMM are interchangeable. "Unbuffered" means that UDIMMs have no register between the memory controller and the memory chips. The controller, located in the processor, communicates directly with the memory chips.

This design has its advantages and disadvantages. Since there is no middleman, the latency is lower, which means faster performance. The production is also simpler and therefore cheaper. As already mentioned, they are the most widespread type of memory module and are very popular in gaming computers and computers on which users perform intensive tasks. However, since there is no interface between the controller and the chips, the signal reliability is worse compared to modules that use a register.

UDIMMs also in most cases do not have an Error Correction Code (ECC) system, so there is a higher probability of data corruption.

On the other side of the scale, we find RDIMMs or memory modules with a register missing from UDIMMs.

What is an RDIMM?

Servers, workstations and other similar devices require stability and reliability in the first place. The absence of a register or buffer (as with UDIMM) is even more noticeable with larger RAM capacities, which is why RDIMM or Registered Dual In-line Memory Module is most often used for servers. This intermediate register helps stabilize and manage the electrical load on the memory modules. The advantage of this is that RDIMMs support larger memory capacities than UDIMMs.

ECC is the second most important component in the stability mosaic required by servers. Data corruption is a constant with RAM, but ECC can recognize and repair them, thus preventing catastrophic data loss. The price you pay for much better reliability is latency, which isn't that important for devices that require RDIMMs.

What is a SODIMM?

If you've ever wanted to upgrade the RAM in a laptop or mini PC, you may have made the rookie mistake of buying a standard DIMM or UDIMM and later found out there was a physical limitation. You picked up your computer's manual and noticed the SODIMM label next to the RAM specification.

SODIMMs (Small Outline Dual In-line Memory Modules) are purpose-built for use in space-constrained computers such as laptops and mini-computers. SODIMM RAM is about half the size of standard DIMM RAM, but the structure remains similar, as does the performance.

But what are CUDIMMs and CSODIMMs?

CUDIMM stands for Clocked Unbuffered Dual In-line Memory and is the latest memory standard. CUDIMM modules are designed to improve memory signal integrity by including a clock driver (CKD) on the DIMM module itself. The CKD is a small integrated circuit that recovers the timing signals that are essential for synchronizing the processor, memory controller, and memory modules. The timing signal is now generated directly on the DIMM and no longer in the processor, which improves stability at high RAM speeds. Simply put, the presence of a clock driver changed how DDR5 RAM behaves at higher speeds.

You may have noticed the similarity with RDIMMs. The difference between them is that in addition to the timing signal, the RDIMM also stores the command and address bus in the buffer, while the CUDIMM stores only the timing signal. Therefore, servers will continue to be equipped with RDIMM modules, and CUDIMMs will live in harmony with classic UDIMM modules.

JEDEC recommends that CUDIMM modules be used for speeds above 6400 MT/s.

Fortunately, the new CUDIMM RAM has the same number of connectors (288) as the existing DDR5 UDIMM standard, so compatibility should not be a problem at least from this point of view. At the moment, CUDIMM does not work on AMD's 7000 series processors, and on the 8000 and 9000 series it works in this "bypass" mode where CKD does not work, so there is currently no good reason to install new RAM in an AMD computer, except of course, if you are "sucked".

With the blue team, the latest Intel Arrow Lake chips are compatible, and quite a few modules are already available for purchase. Kingston, Crucial, Biwin and G.Skill are among the first manufacturers that already offer CUDIMM modules with a capacity of up to 48 GB. The price for such capacity is around 400-500 euros, and they can reach speeds of up to 9600 megatransfers per second (MT/s).

For laptops and mini computers, the first CSODIMM modules are also already available, which also contain a clock driver and all the advantages we have already listed.

What are CAMM2 and LPCAMM2?

DIMM modules got another competition - CAMM2 (Compression Attached Memory Module). The main difference is how the CAMM2 connects to the motherboard. The connectors on the CAMM2 are arranged in a grid, similar to newer processors and motherboards, and no longer in a row (or two rows) as we are used to with DIMMs. Due to the new design of CAMM2, the memory lies flat on the motherboard and no longer perpendicular. The first obvious advantage is the space gained, which is good news for installing larger air coolers. At least for now, it looks like motherboards that will support CAMM2 will only have room for one CAMM2 module - and no more than we're used to with DIMM RAM. If you want to upgrade the capacity/speed, you will have to replace the entire module.

But since the RAM will be placed closer to the processor, we can look forward to better signal reliability, higher frequency and lower delay times, similar to CUDIMM.

The number 2 at the end of the label indicates the second generation of this standard, originally designed by Dell. Like CUDIMM, CAMM2 is currently irrelevant to most users, and it is also unclear which standard will prevail in the future. On the side of DIMM modules, there is still the price, which is one of the most important factors for many buyers.

Not to be left out of the action, notebooks are also receiving another standard – LPDDR5 CAMM2 or LPCAMM2.

Every millimeter counts in ultra-thin laptops, so manufacturers often choose to solder the RAM directly to the motherboard, which is great for portability but problematic for potential upgrades. LPCAMM2, like CAMM2, is low-profile, takes up much less space than SODIMMs, while allowing the user to easily upgrade.

The Lenovo ThinkPad P1 G7 is among the first to be equipped with the latest RAM.

After a long time, we are in for a big leap in the field of system memory. We will follow with interest which standard will work best among users.