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Memory rank is a type of construction applied to dynamic random access memory (DRAM) chips. As a protocol, all ranks must consist of a 64-bit-wide bus and an 8-bit-wide chip, equaling a total width of 72 bits. While the memory rank must be 64-bits wide, the rank can consist of different size chips. Ranks can be single, dual, quad or octal, though most consumer computers only see single and dual. The higher ranks, which hold more memory, are typically seen on servers and higher-end computers.
A memory rank is the array of different DRAM chips that are connected. DRAM is different from regular random access memory (RAM) in that each piece of information is stored in a different capacitor in the chip. This allows the DRAM to recall information better than RAM. By creating a memory rank system, the DRAM is able to store more overall memory while still being compact and inexpensive.
The JEDEC Solid State Technology Association (JEDEC), formerly known as the Joint Electron Devices Engineering Council, is an independent council that decides on different protocols and standards for computer architecture and construction. The organization decided the most efficient way to build a memory rank would be by using a 64-bit bus, which refers to the size of the chip, along with an 8-bit error correction code (ECC) chip for a combined width of 72 bits. To be within these standards, rank cannot be larger or smaller than this width.
While the memory rank width is standard, the size of the chips does not have to be. For example, one company can make a rank with a single 64-bit chip, but another company can make the rank from eight 8-bit chips, and another can build a rank from 16 4-bit chips, and all three would be considered a standard memory rank. As long as the total is 64 bits, not including the ECC, then it fits the standard. There can also be different chips per layer; one DRAM piece may have a 16 4-bit layer, while another layer is made from eight 8-bit chips. Most companies prefer to use more chips, because this gives the DRAM more processing power and more areas in which to store data.
As of 2011, there are four types of rank: single or one layer, dual or two layers, quad or four layers and octal or eight layers. The more layers, the more memory the company can fit onto a chip. Commonly, consumers only find single- or dual-layer memory in their computers, while powerful server computers make use of the quad- and octal-layer ranked memory chips.