What is the Difference between RDIMM vs. UDIMM?

A computer’s RAM capacity is one of the determinants of a system’s performance. Apart from the CPU, the RAM is an important component that drives data transfer, acting as the temporary data storage while the CPU or the HDD are still processing the preceding data. The right amount and the right type of RAM are therefore essential for the system to run smoothly.

There have been many developments in RAM technology to cater to the growing demand for faster data processing. Big data analysis, the rise of different social media platforms, and various applications developed for this cloud-dependent generation are just among the many reasons why the wafer-thin memory stick in our computers should be able to fit in large chunks of data in its compact form. Today, the memory module we see in most computer systems is called the DIMM (Dual In-line Memory Module). There are numerous types of DIMM, but we will only be discussing two of its common types in this article, the RDIMM and the UDIMM. Before we get to the details of these two, let’s first have an overview of their common denominator, the DIMM.

What is a DIMM?

SIMM (Single In-line Memory Module) is a type of memory module that supports 32-bit of data. It used to dominate the memory market in the 1980s, but it gradually became outdated with the advent of 64-bit CPUs. With the introduction of Intel’s Pentium CPUs that used 64-bit data width and the emergence of SDRAM (Synchronous DRAM), two SIMMs need to be installed in-line to accommodate the 64-bit of data transfer. Deemed to be redundant, SIMM was slowly replaced by DIMM, which has a 64-bit data path. DIMM is basically a double-sided SIMM, and a single DIMM can take the place of two SIMMs. With twice the data path of its predecessor, DIMMs transfer data at a faster rate and, to this day, remains the standard memory module in most computers.

Not all DIMMs are created equal. The conventional DIMM is also referred to as the UDIMM (Unregistered DIMM) and the RDIMM (Registered DIMM). Read on as we tackle the differences between these two memory modules, each memory’s uses, and whether we can mix these two DIMMs in the same system.

RDIMM vs. UDIMM

Registered DIMM or RDIMM, as its name suggests, has a register inside the DIMM, placed between the DRAM modules and the system’s memory controller. The register buffers the addresses, command lines, and clocks while data goes directly to and from the memory controller and the DRAMs. The memory controller directs instructions pertaining to addresses, commands, and clocks to the memory register rather than directly accessing the DRAMs. This improves signal integrity and lessens the memory controller’s electrical load, allowing the system to support more memory modules while maintaining stability. RDIMMs also include parity detection, further strengthening its reliability. Once issues are detected with the address and control signals that pass through the register, the RDIMM will revert an error signal back to the memory controller.

On the other hand, UDIMM does not have any buffer or register onboard. All instructions from the memory controller go directly to the DRAMs. Without any buffer, address, and control signals add electrical loading to the memory controller; thus, systems supporting UDIMMs will only be able to handle a limited number of DIMMs. UDIMMs also lack RDIMM’s parity error detection (save for those with ECC, but that would be for another topic); the memory controller only detects address and control signal errors at a later stage, when corruption has already occurred.

Bandwidth and Latency

RDIMMs render an additional clock cycle and more power due to the additional step of traversing the address and command lines to the registry. This results in higher latency and lesser bandwidth for RDIMMs. Consequently, for one DIMM on every memory channel, UDIMMs hold the advantage for latency and bandwidth. But it’s a totally different story when there is more than one DIMMs per memory channel where the clock cycle for UDIMMs changes due to high electrical loading on the address and control lines. A single clock cycle doubles to allow for settling time resulting in higher latency and lesser bandwidth than RDIMMs. RDIMM’s superior performance is therefore unveiled when there is more than one DIMM per memory channel, delivering an 8.7% faster data transfer rate than UDIMMs for two DIMMs per channel.[1] Although the buffering on RDIMMs results in more clock cycles and more power than the traditional DIMMs, they generally provide greater performance when two or more DIMMs are used per memory channel.

UDIMM is also limited to a maximum of two DIMMs per memory channel, so RDIMMs would be a more suitable memory module to use for systems that support three or more memory channels.

Uses

Because of lesser electrical loads in memory controllers, RDIMMs provide better stability, scalability, and reliability than UDIMMs. RDIMMs however, are more expensive than UDIMMs given the kind of performance it offers. For this reason, RDIMMs are mostly used in servers where high-capacity and high-performing RAMs are a necessity.

In contrast, UDIMMs are utilized in systems with low memory requirements like our normal laptops and desktops. UDIMM sizes range from 2GB to 8GB. Its cheaper price, smaller capacity, and mid-range performance make it ideal for regular computer systems.

Mixing RDIMM and UDIMM

A motherboard is designed to support either an RDIMM or a UDIMM but not both since these two DIMMs use different technologies and architectures. Using both won’t harm the system, but it will be unusable. The system will show an error once the motherboard detects an incompatible memory installed and will only function again once you put in the correct memory module.

Which Is Better?

When it comes to performance and signal integrity, RDIMM is way better than UDIMM with the additional register in its architecture where addresses and command lines to it are routed to. The lesser electrical load on the memory controller also makes RDIMM more reliable and stable than UDIMM. However, despite RDIMM’s advantages, it still won’t be practical to use it in regular laptops and desktops, considering its hefty price tag and the memory requirements of such systems. UDIMMs are more suitable for these system types, but when it comes to high-end servers, RDIMMs are a better fit, avoiding memory performance bottlenecks.

However, it is worth noting that RDIMMs are not designed to replace UDIMMs despite the latter’s lower performance. Instead, RDIMM and UDIMM should be treated as two different types of DIMMs designed for different purposes and for different types of computer systems.

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