Memory rank

A memory rank is a set of DRAMs connected to the same chip select, and which are therefore accessed simultaneously. In practice they also share all of the other command and control signals, and only the data pins for each DRAM are separate (but the data pins are shared across ranks).

The term “rank” was created and defined by JEDEC, the memory industry standards group. On a DDR, DDR2, or DDR3 memory module, each rank has a 64-bit wide data bus (with an optional extra 8-bit ECC on some DIMMs). The number of physical DRAMs depends on the their individual widths. For example, a rank of x8 (8-bit) DRAMs would consist of 8 physical chips (plus one for ECC), but a rank of x4 (4-bit) DRAMs would consist of 16 physical chips (plus two for ECC). Multiple ranks can coexist on a single DIMM, and modern DIMMs can consist of one rank (single rank), two ranks (dual rank), four ranks (quad rank), or eight ranks (octal rank).

There is little difference between a dual rank UDIMM and two single rank UDIMMs in the same memory channel, other than that the DRAMs reside on different PCBs. The electrical connections between the memory controller and the DRAMs are almost identical (with the possible exception of which chip selects go to which ranks). Increasing the number of ranks per DIMM increases memory density in the channel, but does not enhance performance; too many ranks in the channel can cause excess loading and decrease the speed of the channel. DRAM load can be lessened by using registered memory.

Memory operations to a single rank are generally faster than memory operations that span ranks, as individual ranks have different flight times and may require the memory controller to insert small delays when switching between ranks. However, modern memory controllers attempt to spread memory operations across ranks for power and thermal reasons.

See also