Hardware
RDIMM
Also known as: Registered DIMM, Registered Memory, Buffered DIMM
An RDIMM (Registered DIMM) is a server memory module with a register chip that buffers the command and address signals between the CPU memory controller and the DRAM chips, allowing higher memory capacity per channel.
As memory capacity per channel increases, the electrical load on the CPU's memory controller grows — more chips mean more capacitance and signal degradation. RDIMMs solve this by placing a register (also called a buffer) on the memory module that re-drives the command and address signals, reducing the load on the controller.
The result: servers with RDIMMs can support more DIMMs per channel and higher total memory capacity. A server platform supporting 8 DIMM slots per channel with RDIMMs can populate all 8 slots; the same platform with UDIMMs might be limited to 2 per channel at full speed.
RDIMMs are the standard for enterprise servers — Intel Xeon and AMD EPYC platforms are designed around them, and they always include ECC. The buffering adds a negligible amount of latency in exchange for far higher memory capacity. A further variant exists for the very highest-density configurations.