CUDIMM Technology and Benefits

Covering the basics first, we start with what computer system memory is and the function. RAM, or Random Access Memory, is an essential component in computers, whether it's a laptop, desktop, servers or NUC. Each RAM module is known as a DIMM (Dual In-line Memory Module), with UDIMMs (Unbuffered Dual In-line Memory Modules) being the most common type of DDR5 RAM used in modern desktop PCs. DRAMs main function is to act as short term storage and quick retrieval of data for programs in an operational mode. In a more digestible description, DRAM acts as the go-to medium between much slower long storage solutions and the ultrafast on die processor cache capabilities. Whatever does not fit inside the processors internal cache, must be stored and accessed somewhere else. The faster this "other" storage solution is, the quicker a processor can complete its full computation. The common phase "in system memory" has only one contextual reference and means exactly what is stated. Program data is read from the slower data transfer storage mediums like Mechanical Hard Drives and Solid State Disks to System Memory while the program is running. This allows for the program data to be quick and responsive to the needs of the user as the computer processor no longer had to access the much slower data storage solutions.

The downside of system memory is the volatility of data integrity. The data in question stays in distinct high and low voltages states stored inside memory banks. These banks have rows and columns for organization of data, which makes up the internals of the RAM Integrated Circuit (IC). The idea behind the JEDEC organization and standard is for the preservation of data integrity by creating operational parameters, which is used for the creation of new types of memory and future client needs.

Modern system memory started out as Synchronous Dynamic Random-Access Memory (SDRAM). Data and Commands were limited to one operation per clock cycle. With the release of DDR (Double Data Rate) Commands were still limited to one per clock cycle, but read and writes could now be performed on the raise and fall of the clock cycle. This was the starting point of the actual operating clock speed not being direct 1:1 representation of data transfer rates. A more useful term is Mega Transfers (MT). By using DDR5-4800 as example, it is effectively operating at 4800 MHz, but actually it is instead just 2400 MHz. Another way of saying this is 4800 MT/s. The ambiguous nature of DDR is that each iteration adds another layer of complexity that requires innovation to solve. DDR5 hit a roadblock in its expanse; signal integrity at higher memory speeds.

Que CUDIMM!

Client Clock Driver (CKD) is located in the middle

While DDR5 RAM is already incredibly fast and highly responsive, further advancements are needed to push its performance to even greater heights. One of the primary challenges is signal integrity, which currently limits DDR5 UDIMMs from achieving the same level of stability at the same frequency across different platforms. With the wide variety of capacity and performance orientated options available to consumers on multiple platforms, compatibility issues exponentially increase. At the core, these compatibility issues are often related to electrical challenges like single strength, noise and jitter. This is often the main contributing factor defining the max memory speed of motherboards.


Image courtesy of RAMBUS

This problem had to be addressed to enable the next generation of faster, more reliable memory technologies. Que the Clock Driver! This is the backbone of CUDIMM, CSODIMM, and CAMM. By incorporating a clock redriver directly on the memory module, CUDIMMs enhance signal integrity, leading to improved stability and enabling higher operating frequencies. Its primary function is to buffer the clock between the host controller, ie; CPU Integrated Memory Controller (IMC) and the DRAM. To distinguish these new modules from legacy DDR5 designs without a clock driver, JEDEC introduced a naming convention that adds a "C" for "Clock" resulting in the new module types: CUDIMM (Clocked Unbuffered DIMM) and CSODIMM (Clocked Small Outline Dual Inline Memory Module). CAMM2 (Compression Attached Memory Module V2) is the outlier with the "C" not referencing a Clock Driver IC, but can be included as well. This distinction between UDIMM and CUDIMM is critical because CUDIMM and CSODIMM products are designed to use the same sockets, pins and protocols as traditional DDR5 UDIMMs and SODIMMs. Without proper motherboard support, it will operate in bypass mode by default. This gives limited backwards compatibility, but it can no longer subjected to the same operational frequency it was intended for in this bypass mode.

Questions & Answers

Q: How will I know if CUDIMM is supported for a specific platform?



A: Unfortunately, support for DDR5 CUDIMM depends on the motherboard vendor at this time. If you do not see a statement like "Supports CUDIMM memory modules" in the product specifications or if the memory is absent from the QVL (Qualified Vendor List), it's best to contact the manufacturer for clarification.

Q: How does CUDIMM allow for higher frequency than UDIMM?

A: CUDIMM features a Client Clock Driver (CKD) integrated into the PCB, specifically designed to tackle signal integrity issues caused by factors such as thermal noise, trace length, and RF interference. This additional functionality ensures better stability, making it easier for PC enthusiasts to achieve higher memory frequencies without compromising the convenience of a plug-and-play experience.

Q: How does bypass mode work?


Image courtesy of JESD82-531A.01

A: The CKD during boot is waiting for a "Control Word." If this is not sent, the driver will automatically fall back to bypass mode.

This Control Word can be one of the following:

• Reserved1
• Read Only
• Write Only
• RD/WR
• One Time Programmable
• Sticky - Cleared by power cycle not Reset

Q: Will CUDIMM work on Legacy Intel and AMD platforms?

A: To ensure proper compatibility, the motherboard BIOS must support the Client Clock Driver (CKD). On legacy systems, the clock driver functionality is disabled when a CUDIMM or CSODIMM is installed. This safeguards basic JEDEC backward compatibility, though the memory may not be able to operate at its rated XMP/EXPO profile. The motherboard also may require a BIOS update on the legacy system for CUDIMM to be recognized as such and boot properly in bypass mode.

Q: Can I run the CUDIMM XMP/EXPO memory profile without using the Client Clock Driver (CKD) function?

A: Yes, but depending on the platform and targeted frequency, the system may fail to boot or remain stable after booting up. The DRAM ICs operate at the same set profile frequency regardless of whether the CKD function is enabled. In theory, the CKD function isn't necessary as long as signal integrity is maintained. However, in practice, achieving these much higher frequencies of 8000 MT/s and beyond, introduced with CUDIMM, is unlikely on many motherboards without the CKD function enabled.

Q: Is there a Latency penalty from the Client Clock Driver (CKD)?

A: Technically, yes, but only on a nanosecond scale. Since the signal is redirected before reaching the DRAM ICs, there is inevitably an increase in latency. However, this increase is so minimal that it typically falls within the margin of error in benchmarking.

Q: Why are some CUDIMM memory kits sold at same frequencies as UDIMM?

A: This answer has multiple overlaying answers. Not all consumers need the highest frequencies, but they still want to benefit from CUDIMM technology. CUDIMM allows budget motherboards to support much higher memory speeds than UDIMM. Thus, without the need for extra PCB layers and special shielding, manufacturing costs stay down and consumers can still enjoy the benefits of higher bandwidth memory.