DIMM

DIMM is an abbreviation for Dual In-line Memory Module, it is an assembly of several RAM chips on a single PCB for usage in personal computers. DIMMs began to replace SIMMs as the predominant type of memory module when Intel's Pentium processors were in the mainstream market.

Related Topics:
RAM - PCB - Personal computer - SIMM - Intel - Pentium

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The most common types of DIMMs are:

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• 72-pin-DIMMs, used for SO DIMM
• 144-pin-DIMMs, used for SO DIMM
• 168-pin-DIMMs, used for SDRAM
• 184-pin-DIMMs, used for DDR SDRAM
• 240-pin-DIMMs, used for DDR2 SDRAM

There are 2 notch keys on the bottom edge of 168-pin-DIMMs, each has three different notch positions which represents three different types:

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The first notch is DRAM key position. It represents RFU, buffered and unbuffered.

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The second notch is voltage key position. It represents 5.0, 3.3 and Reserved.

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The upper one in the photo is an unbuffered 3.3V 168-pin-DIMM.

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Vendors have recently begun to distinguish between single and dual ranked DIMMs -- it is not yet clear that this is equivalent to dual banked DIMM, which just means that they have chips on both sides. See below for detail explanation of ranked DIMMs. Note that this terminology only applies to Registered DIMMs.

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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.

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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.

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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).

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