Registered (also called buffered) memory modules have a register between the DRAM modules and the system’s memory controller. They place less electrical load on the memory controller and allow single systems to remain stable with more memory modules than they would have otherwise. When compared with registered memory, conventional memory is usually referred to as unbuffered memory or unregistered memory. When manufactured as a dual in-line memory module (DIMM), a registered memory module is called an RDIMM, while unregistered memory is called UDIMM or simply DIMM.
Registered memory is often more expensive because of the lower number of units sold and additional circuitry required, so it is usually found only in applications where the need for scalability and robustness outweighs the need for a low price – for example, registered memory is usually used in servers.
Although most registered memory modules also feature error-correcting code memory (ECC), it is also possible for registered memory modules to not be error-correcting or vice versa. Unregistered ECC memory is supported and used in workstation or entry-level server motherboards that do not support very large amounts of memory.