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Registered memory


Not to be confused with ECC memory, although memory modules often use both technologies.

Registered (also called buffered) memory modules have a register between the DRAM modules and the system's memory controller. They place less electrical load on the memory controller and allow single systems to remain stable with more memory modules than they would have otherwise. Registered memory is more expensive because of the lower volume and the additional components, so it is usually found only in applications where the need for scalability and stability outweighs the need for a low price (servers, for example). Although most server-grade memory modules are both ECC and registered, there are both registered non-ECC modules and non-registered ECC modules.

There is a performance penalty for using registered memory. Each read or write is buffered for one cycle between the memory bus and the DRAM, so the registered RAM can be thought of as running one clock cycle behind the equivalent unregistered DRAM. With SDRAM, this only applies to the first cycle of a burst.

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The memory bus is the computer bus which connects the main memory to the memory controller in computer systems. Originally, general-purpose buses like VMEbus and the S-100 bus were used, but to reduce latency, modern memory busses are designed to connect directly to DRAM chips, and thus are designed by chip standards bodies such as JEDEC. Examples are the various generations of SDRAM, and serial point-to-point buses like SLDRAM and RDRAM. An exception is the Fully Buffered DIMM which, despite being carefully designed to minimize the effect, has been criticized for its higher latency.


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DIMM
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Two types of DIMMs: a 168-pin SDRAM module (top) and a 184-pin DDR SDRAM module (bottom). Note that the SDRAM module has two notches on the bottom edge, while the DDR1 SDRAM module has only one. Also note that both modules have 8 RAM chips, but the lower one has an unoccupied space for a 9th.
3 SDRAM DIMM slots

A DIMM, or dual in-line memory module, comprises a series of dynamic random access memory integrated circuits. These modules are mounted on a printed circuit board and designed for use in personal computers, workstations and servers. DIMMs began to replace SIMMs (single in-line memory modules) as the predominant type of memory module as Intel's Pentium processors began to gain market share.

The main difference between SIMMs and DIMMs is that DIMMs have separate electrical contacts on each side of the module, while the contacts on SIMMs on both sides are redundant. Another difference is that standard SIMMs have a 32-bit data path, while standard DIMMs have a 64-bit data path. Since Intel's Pentium has (as do several other processors) a 64-bit bus width, it requires SIMMs installed in matched pairs in order to complete the data bus. The processor would then access the two SIMMs simultaneously. DIMMs were introduced to eliminate this practice.

The most common types of DIMMs are:

* 72-pin SO-DIMM (not the same as a 72-pin SIMM), used for FPM DRAM and EDO DRAM
* 100-pin DIMM, used for printer SDRAM
* 144-pin SO-DIMM, used for SDR SDRAM
* 168-pin DIMM, used for SDR SDRAM (less frequently for FPM/EDO DRAM in workstations/servers)
* 172-pin MicroDIMM, used for DDR SDRAM
* 184-pin DIMM, used for DDR SDRAM
* 200-pin SO-DIMM, used for DDR SDRAM and DDR2 SDRAM
* 204-pin SO-DIMM, used for DDR3 SDRAM
* 214-pin MicroDIMM, used for DDR2 SDRAM
* 240-pin DIMM, used for DDR2 SDRAM, DDR3 SDRAM and FB-DIMM DRAM


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