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I have seen the word SIMM and DIMM a lot, and was ignoring what they mean, but it seems that SIMM and DIMM basically just means:

a circuit board that contains all the RAM components on it, to serve as 1 unit, for example, as a 2GB unit.

So a computer that has 4 DIMMs, each with 2GB, will be 8GB of RAM.

And SIMM just means the slot contacting points are redundant on both sides, while DIMM are independent contacting points. And SO-DIMM is half the size of a DIMM.

Is there any case where a SIMM or DIMM is actually more than 1 circuit board? Or do they ALWAYS refer to 1 circuit board?

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A long time ago, IIRC, you could only install SIMMS in twos. But I'm not that old. –  surfasb Oct 16 '11 at 20:38
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2 Answers

The last "M" in the acronym represents the word "module", which denotes a non-reduceable manufactured/assembled entity. So it typically refers to only one circuit board. The only instances where more than one PCB would be involved might be the situation where the boards were encased in a (sealed) enclosure or bonded together. For memory modules, the definitions of SIMM and DIMM do mean only one circuit board or PCB. The term "module" does not have any attribute such as a requirement that the modules be installed as a set (e.g. a matched pair).

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  • SIMM - Single In-line Memory module
  • DIMM - Dual In-line Memory Modules

SIMM modules are with twice less pins than DIMM modules, which allows DIMM to utilize the wider 64-bit bus, versus the 32-bit bus in case of SIMM.

It is actually possible to achieve 64-bit bus using the two SIMMs as well, by using these in a tandem configuration, but DIMMs eliminated the need for this.

Quite often there is misunderstanding of the memory module's rank term, by putting the equation sign between the rank and PCB or between the rank and side. In fact, two sided module could be single or dual ranked, depending of data width of the DRAM chips (x4 or x8)

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