What Registered, Unbuffered and Load-Reduced DIMMs are, why they cannot be mixed, and how to identify each type from the part number.
Published August 2026 · AltoChip team
DDR4 server memory comes in three main module types. UDIMM (Unbuffered DIMM) connects the memory controller directly to the DRAM chips — simple and cheap, used in desktops and entry servers, but limited in capacity and count per channel. RDIMM (Registered DIMM) adds a register chip that buffers command and address signals, letting a server run more, larger modules reliably — this is the standard type in mainstream servers, and what most 32GB DDR4 server modules are. LRDIMM (Load-Reduced DIMM) goes further and buffers the data lines too, enabling the highest capacities (64GB, 128GB and up) at the cost of slightly higher latency and price.
RDIMM, UDIMM and LRDIMM are electrically different. A platform initializes memory according to the module type it detects, and mixing types in one system — even one channel — will simply fail to POST. When you buy server memory, the module type must match what your platform already uses. This is the first thing to confirm in any RFQ.
The module type is encoded in the part number. A few common patterns: Samsung M393… is RDIMM, M386… is LRDIMM, M378… is UDIMM. SK hynix RDIMMs typically look like HMA84GR7… (the R indicates Registered). Micron server RDIMMs commonly start with MTA36ASF… A label also states voltage (1.2V for DDR4), speed grade and rank — more on those in our speed-grade guide.
Decommissioned servers overwhelmingly yield RDIMMs — which is why tested server-pull RDIMM is the deepest, best-value pool of pre-owned memory. Confirm module type, capacity, speed grade and rank against your platform, and insist on per-module evidence: a test report and a serial-number list. That is exactly what ships with every AltoChip order — see Testing & Evidence.