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Modules

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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.

What is SDRAM parity?

Modern desktop processors operate with a 64-bits bus. However, some SDRAM DIMMs are 72-bit wide, why? Additional 8-bit are called “parity bits”, or "parity byte". The parity bits offer the computer the possibility for error detection and correction. In order to realize this useful feature during the write procedure, for each byte of the bus (64 bits = 8 x 8 bits, i.e. the bus is 8 bytes wide) the so called checksum is build and stored in a single parity bit. Thus, for a 64-bit wide bus 8-parity-bits are needed.

What are the Elements of a SDRAM Module?

"PCBs" is the abbreviation for "Printed Circuit Board" and is actually the board on which the whole module is built. For standard DIMMs the PCB is an epoxy resin that is dyed green and consists of several (e.g., four) insulating layers which are separating the conductive layers (consisting of copper) from each other. The individual copper conductive layers are connected with each other through contact holes, also called "VIA's. From several conductive layers only some of them are made of structured conducting paths, e.g. the front and posterior outer layer (these conducting paths are then visible) and the middle layer. Between the layers with structured conducting paths, two continuous/bulk (without structured conducting paths) conducting layers are present, one of them being connected to the power supply VCC and the other one is connected to ground. By means of these two continuous copper layers a good distribution of the supply voltage over the entire module is provided and in addition they are playing the role of capacities (charge reservoirs) able to “catch” undesired big current peaks. The DIMM pins are usually made of gold, deposited on the boards by means of electroplating.

Registered and Stacked SDRAM Modules

For synchronous DRAM (SDRAM) unbuffered, registered and stacked modules are usual. In the following I will explain what registered and stacked modules are:

Buffered and Unbuffered DRAM Modules

For non-synchronous DRAM, like EDO/FPM, buffered and unbuffered modules were usual. What is the difference actually between unbuffered and buffered DRAM modules?

SDRAM Module Types

The following types of DRAM modules can be found in computers and notebooks: SIMMs, RIMMs, DIMMs and SODIMMs.

SDRAM Module Photos

The memory modules evolved parallel to the development of new processor generations. This is due to the fact that the processors have become faster and faster and the data quantity processed by them has expanded. Please see bellow examples of DRAM modules.

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