What is a memory rank?

The term “rank” was created by JEDEC, the memory industry’s standards group. The concept of memory rank applies to all memory module form-factors, such as desktop DIMMs, notebook SODIMMs, workstation and server Registered DIMMs; in general, though, it is only critical on server platforms due to their higher memory capacity. A memory rank is, simply put, a block or area of data that is created using some or all the memory chips on a memory module.

A rank must be 64 bits of data wide; on memory modules which support Error Correction Code (ECC), the 64-bit wide data area requires an 8-bit wide ECC area for a total width of 72 bits. Depending on how memory modules are engineered, they can contain one, two, or four areas of 64-bit wide data areas (or 72-bit wide areas, where 72 bits = 64 data bits and 8 ECC bits).

Why is the total number of memory ranks important?

Servers based upon Intel’s E7320/E7520 chipset (code-named “Lindenhurst”) limit the number of memory ranks of PC2700 or PC2-3200 memory. That means that the combined number of memory ranks for all the memory modules must not exceed the specified maximum number of ranks. If the number of memory ranks exceeds the specified maximum ranks, the server may not boot or may not operate reliably.

How many ranks do typical module types have?

Single-sided modules are always single-rank. Doubled-sided unbuffered DIMMs and SODIMMs are always dual-rank.

Registered DIMMs, used for servers and workstations, vary from a singlerank to up to four ranks, as illustrated in the chart below:

In the first example, a single-sided module is always single-rank. A doubled-sided Registered DIMM can be:

• Single-sided, with all the memory chips on both sides representing a single 64-bit wide + 8-bit ECC area, or
• Dual-rank (with one rank per side), or
• Quad-rank (with 2 ranks per side for a total of 4 ranks)

How do Intel Lindenhurst servers limit ranks?

The number of ranks is important based upon the server’s memory speed:

• If PC2100 memory is used, the number of ranks is not critical and either single or dual rank memory module pairs can be used.
• If PC2700 or PC2-3200 memory is used, there is a maximum of 8 ranks per server. For example, if two pairs of dual-rank memory modules are inserted into the memory sockets, that would add up to 8 ranks (4 modules x 2-ranks each = 8 ranks); in this case, empty sockets cannot be used for upgrading memory.

Here’s a generalized chart showing the memory limitations:

Why do the single- and dual-rank memory modules vary in price?

In general, single-rank memory modules are built using x4 (“By 4”) DRAM chips and are more expensive than dual-rank memory modules (which are built using x8 DRAM chips); both module types have the same number of chips but the x4 DRAMs are more expensive than x8 DRAMs. Dual-rank memory modules may limit future upgradeability and capacity of servers when using PC2700 or PC2-3200 memory. This tradeoff between memory cost and capacity is important to consider when purchasing memory modules for Intel Lindenhurst-based servers.

How can I find out whether a ValueRAM Registered DIMM is single- or dual-rank?

The ValueRAM part numbers will designate whether memory modules are single-, dual-, or quad-rank:

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