DIMM
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Image:DIMMs.jpg A DIMM, or dual in-line memory module comprises a series of random access memory integrated circuits. These modules are mounted on a printed circuit board and designed for use in personal computers. DIMMs began to replace SIMMs (single in-line memory modules) as the predominant type of memory module as Intel's Pentium processors began to control the market.
The main difference between SIMMs and DIMMs is that SIMMs have a 32-bit data path, while DIMMs have a 64-bit data path. Since Intel's Pentium has a 64-bit bus width, it required SIMMs installed in matched pairs in order to use them. The processor would then access the two SIMMs simultaneously. DIMMs were introduced to eliminate this inefficiency. Another difference is that DIMMs have separate electrical contacts on each side of the module, while the contacts on SIMMs on both sides are redundant.
The most common types of DIMMs are:
- 72-pin-DIMMs, used for SO-DIMM
- 144-pin-DIMMs, used for SO-DIMM
- 168-pin-DIMMs, used for FPM, EDO and SDRAM
- 184-pin-DIMMs, used for DDR SDRAM
- 240-pin-DIMMs, used for DDR2 SDRAM
There are 2 notches on the bottom edge of 168-pin-DIMMs, and the location of each notch determines a particular feature of the module.
- The first notch is DRAM key position. It represents RFU, buffered, and unbuffered.
- The second notch is voltage key position. It represents 5.0V, 3.3V, and Reserved.
- The upper DIMM in the photo is an unbuffered 3.3V 168-pin DIMM.
A DIMM's capacity and timing parameters may be identified with SPD (Serial Presence Detect), an additional chip which contains information about the module type.
ECC DIMM RAM has additional Error Correcting Code housed in the DIMM package.
Registered DIMM ranking
Vendors have recently begun to distinguish between single and dual ranked DIMMs -- it is not yet clear whether this is equivalent to dual banked DIMM, which just means that a module has two memory banks, often with one on each side. See below for detail explanation of ranked DIMMs. Note that this terminology only applies to registered DIMMs.
JEDEC decided that the terms "dual-sided," "double-sided," or "dual-banked" were not correct when applied to registered DIMMs. Registered DIMMs can be built using "x4" (by 4) memory chips or "x8" (by 8) memory chips. "x4" or "x8" refer to the width of the memory chips in bits.
In the case of the "x4" Registered DIMMs, the data width per side is 36 bits; therefore, the memory controller (which requires 72 bits) needs to address both sides at the same time to read or write the data it needs. In this case, the two-sided module is single-ranked.
For "x8" Registered DIMMs, each side is 72 bits wide, so the memory controller only addresses one side at a time (the two-sided module is dual-ranked).
External links
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