Six module classes exist across the DDR4/DDR5 server memory catalogue, and per Kingston's own documentation they are not interchangeable - a mismatched population can leave a server unable to boot, which Intel's own board documentation describes in stronger language still: a "Fatal Error Halt" during memory initialisation. This page states what each class is per the vendors' own documentation, then works through what we found - and could not confirm - checking the three highest-capacity rows in our own catalogue against it.
Updated
| Class | What it is |
|---|---|
| UDIMM | Unbuffered DIMM - the command/address and data signals connect directly to the memory controller with no register or buffer chip in between. The desktop/workstation default; lower capacity and rank ceilings than the buffered classes below. |
| ECC UDIMM | An unbuffered DIMM with on-module error-correcting code circuitry added, for workstations and entry servers that want ECC without the buffering RDIMM adds. |
| RDIMM | Registered DIMM - a register chip buffers the command/address signals, reducing electrical load on the memory controller and allowing more ranks and higher capacities per channel than UDIMM. The server-platform standard. |
| 3DS RDIMM | 3D-stacked (TSV, through-silicon-via) RDIMM - multiple DRAM die are stacked within one package to reach a higher capacity per chip than a monolithic die allows. Intel's own board documentation names this as its own distinct type from plain RDIMM, not a variant of it. |
| LRDIMM | Load-reduced DIMM - adds a memory buffer chip that also isolates the data lines, not just command/address, which is what lets LRDIMM reach higher rank counts and capacities than RDIMM on the same platform. |
| MRDIMM | Multiplexed Rank DIMM - a DDR5-generation type that presents two ranks through a multiplexer, used on some current server platforms. Its own distinct class, not a drop-in match for the others. |
Sources 1-4 below.
Kingston's own documentation states it in one sentence: "Different memory types (UDIMM, RDIMM, LRDIMM, MRDIMM): Cannot be mixed in the same system. If mixed, the server may not boot."
Intel's own server board memory population rules put a sharper point on the failure mode: "Mixing of DDR4 DIMM types (RDIMM, LRDIMM, 3DS RDIMM, 3DS LRDIMM) within or across processor sockets produces a Fatal Error Halt during memory initialization." A sibling board family's documentation states the identical rule in almost the same words, and both name 3DS RDIMM and 3DS LRDIMM as their own distinct types - not variants of plain RDIMM and LRDIMM.
A module's class stops being a footnote once its capacity is high enough that reaching that capacity requires 3DS stacking or load reduction in the first place - which is common at 128GB and near-universal at 256GB in the current DDR5 generation. At that point the class question is not "which one is better," it is "which one physically works in this platform at all," and Intel's own "Fatal Error Halt" language is the honest description of getting it wrong.
This site currently tracks exactly three DDR5 rows at 128GB or above: one 128GB module and two 256GB modules. Per the discipline above - record the class from the datasheet or listing spec, or label it unverified - here is what checking each one against its named manufacturer's own site actually found.
The listing behind our 128GB row is sold by a reseller, TSS, as "compatible with SK Hynix HMCT04MEERA132N" - phrasing that does not itself confirm the unit is a genuine SK Hynix-manufactured module rather than a compatible or relabeled part. TSS's own listing states the rank architecture as "Quad Rank (4Rx4 / 2S2Rx4 3DS/TSV stacked architecture)" - a real, specific claim, but a reseller's listing-spec claim rather than a manufacturer datasheet fact, and we render it at that tier rather than upgrading it silently.
We also checked the part number itself against a document attributed to SK Hynix's own DDR5 part-number decoder scheme (retrieved via a third-party distributor mirror, not skhynix.com directly, which we could not locate this exact part number on). The decoder resolves the module-type character in this part number to "RDIMM" - consistent with a registered module rather than load-reduced. But the packaging-option code that would confirm or rule out 3DS stacking specifically does not appear among the example codes in the version of the decoder we could retrieve, so we cannot confirm the 3DS claim against SK Hynix's own numbering scheme, even though the reseller's listing states it.
Neither 256GB part number we track (V-Color TR564G48D446Q and TR564G64D460Q) appears anywhere on v-color.net, which is V-Color's sole official web presence - confirmed by site search and by exporting V-Color's full published product catalogue and searching it directly. Their dedicated DDR5 ECC RDIMM server-memory product page lists confirmed part numbers only up to 96GB.
A pattern worth flagging rather than resolving: every real, currently-listed V-Color part number we could confirm uses the segment "564G" to denote a 64GB module (for example, the genuinely listed "TR564G48D440-64G"). Both of our tracked 256GB part numbers share that identical "564G" segment. We do not know what that means for these two specific parts - V-Color's own site simply does not list them - and we are stating the open question rather than asserting either a 256GB single module or something else.
Per the discipline this page states above: all three of our currently-tracked rows at 128GB and above render class unverified and are excluded from any compatibility statement we make. That is not a comment on whether these are good or bad products - it is a statement that we could not confirm the fact this page exists to explain the importance of, and we would rather say so than guess at a class distinction Intel's own documentation ties to a fatal boot failure.
Even a correctly classed module still needs its exact capacity and class supported by the target platform's own memory documentation - never inferred from a CPU generation or a chipset name. This site's own OEM-verified platform data shows several current DDR5 servers document support for 256GB per DIMM (Dell PowerEdge R660 and R760, HPE ProLiant DL360/DL380 Gen11, Lenovo ThinkSystem SR630/SR650 V3), while the Dell PowerEdge R760xs is documented at a 128GB per-DIMM ceiling - genuinely different numbers on genuinely similar-looking platforms. Check your exact server's own page rather than assuming from its generation name.
DDR4 16GB · 32GB · 64GB · DDR5 16GB · 32GB · 64GB · 96GB · 128GB · 256GB - the last two currently track zero in-stock listings and render our standard thin-content state rather than a full buying guide, consistent with everything on this page about not overstating what we can currently verify or sell.
All server RAM · RAM price index · Shortage tracker · Server Memory Finder - enforces per-DIMM and total capacity maxima from OEM data · Part-number lookups for exact-module verification · Dell, HPE, Lenovo and Supermicro platform memory rules.
Per Kingston's own documentation, RDIMM (registered DIMM) buffers the command/address signals through a register chip, while LRDIMM (load-reduced DIMM) also buffers the data lines through a memory buffer chip - which is what lets LRDIMM reach higher capacities and rank counts than RDIMM on the same platform. They are not interchangeable: Kingston states plainly that mixing DIMM types, including RDIMM and LRDIMM, can leave a server unable to boot.
3DS (3D-stacked, sometimes written TSV for through-silicon-via) RDIMM stacks multiple DRAM die on top of each other within one package to reach a higher capacity per chip than a monolithic die allows - which is how many 128GB-and-above modules reach their capacity at all. Intel's own server board memory population rules name "3DS RDIMM" and "3DS LRDIMM" as their own distinct types alongside plain RDIMM and LRDIMM, and state that mixing DIMM types produces a Fatal Error Halt during memory initialization.
MRDIMM (Multiplexed Rank DIMM) is a DDR5-generation module type that presents two ranks through a multiplexer to increase effective bandwidth, used on some current-generation server platforms. Like every other class on this page, it is its own distinct type - not a drop-in match for RDIMM, 3DS RDIMM or LRDIMM.
No. Kingston's own documentation states these types cannot be mixed in the same system, and on DDR5 platforms specifically notes the module key (a physical notch position) may physically prevent installing the wrong type in the first place - a mechanical safeguard for exactly this mistake.
Because at these capacities a module is more likely to require 3DS stacking or load-reduction to reach its stated capacity at all, which means the class question stops being a performance footnote and becomes the difference between a module that initialises and one that halts the server on boot - per Intel's own "Fatal Error Halt" language for exactly this failure mode.
We record what we can establish from the manufacturer's own datasheet or the listing's own stated specification, and where neither settles the question, the row is labelled unverified and excluded from any compatibility claim rather than guessed. Our own check of our three highest-capacity rows, detailed on this page, is a worked example of exactly that discipline - two vendors' own sites did not confirm what we needed, so we say so rather than filling the gap.
Specification claims on this page were checked against the pages below on 4 August 2026. Where a widely-repeated figure did not survive that check, we report what the source says now and say so in the text rather than repeating the familiar number.