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One-bank and two-bank SDRAM modules

What is actually a “bank”. In the DRAM world the notion “bank” has three different meanings:

  • there are internal banks on the chip as discussed in the article ….
  • the term "bank" is also sometimes used for the individual memory slots in a computer
  • and finally there are also the “banks” of a DRAM module, this is what we are going to discuss in the following.

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It is often assumed that if chips a build on one side of a DRAM DIMM, then the module is a 1-bank module and if the chips a build on both sides of the DIMM then we are dealing with a 2-bank module. Unfortunately the reality is not so simple. The DRAM components, i.e. chips, can be divided in three different organizations: x4, x8, and x16. This actually means that the chips were fused in a such a way that 4, 8 or 16 chip-DQs are used to read/write the data. Now, in order to “saturate” the 64DQs of a DIMM the following number of chips can be used: 16 chips with a x4 organization, or 8 chips with a x8 organization, or finally 4 chips with a x16 organization. The number of chips on your module is actually determining the capacity/volume of memory you have on this module. A 16 x4 module has a higher memory capacity than 8 x8 or 4 x16 modules (assuming that all chips have the same memory capacity).

For unbuffered modules typically x8 chips are used. This means that 8 chips are needed per module saturate the 64DQs. These 8 chips fit on one side of the DIMM board. With 256Mb (mega-bit) chips that would be a 256Mb x8 module and overall results a 256 MB (mega-byte) module.

However, since one side of the DIMM board is still free, in order to increase/double the total capacity of the module additional 8 chips can be build on this side. The 64DQs of the chips from the “front side” of the DIMM will be addressed by acting the chip select signals CS0 and CS2, while the 64DQs of the chips from the “back side” of the DIMM will be addressed using CS1 and CS3. In this way a 2-bank module has been build where the bank 1 actually in on the on side of the DIMM, whereas bank 2 in on the other side of the DIMM. If registered modules with a lower capacity are needed then x16 chips are used and in this case a 1-bank module with 4 chips build on the DIMM will result.

For Registered modules typically x4 devices are used. In this case 18 chips per module are needed to saturate the 72DQs of the module. Note that registered modules are manufactured only with parity. These 18 chips will no longer fit on one side of the DIMM board and therefore must be shared between the two sides! With 256Mb-chips a module with 256Mb x (16+2-parity) =512MB will result. In this case we have a 1-bank module, however the back of the DIMM board is now no longer free and the chips of this bank are on both sides of the DIMM. In order to make such a module a 2-bank module, and thus to double the capacity, some radical measures should be taken, such as stacking of chips or doubling the DIMM dimensions. However, if registered modules with a lower capacity are needed then x8 chips are used and in this case a 1-bank module with 8 chips build on the DIMM will result.

For SODIMMs (used in notebooks) is something else again. Because the module is much smaller than conventional DIMMs only 4 chips can fit on each side of the SODIMM. Therefore, there are three constructions for this type of modules: the 1-bank module with 4 chips in x16 organization, a 1-bank module with 8 chips in x8 configuration or a 2-bank module with 8 chips in x16 organization.