What Is RAM Density?

Step 1: RAM Density Definition

RAM Density is the capacity or size of a DIMM module or the capacity or size of each DRAM unit on a DIMM or RIMM. Density is normally referred to in megabits (Mb) when referring to DRAM units and in megabytes (MB) when referring to an entire DIMM module.

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Step 2: Density vs. Speed

When trying to determine what kind of RAM you need, the density of each DRAM unit on a DIMM is probably more important than the speed of the DIMM. For example, PC133 DIMMs are backward compatible to PC100 and PC66. On the other hand, 256 and 512 megabit technology DIMMs will not work in many motherboards.

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Step 3: How To Determine RAM Density

It's hard to figure out the density of a DIMM because there are multiple industry "standards" on how describe a DIMM, many of which are contradictory. Some retailers list the capacity of each DRAM unit in MB and the number of DRAM units on the DIMM. For example, a 256 MB 16X16 PC133 Non-ECC Unbuffered DIMM has 16 DRAM chips, and each chip is 16 MB in size. The density of each DRAM unit is found by multiplying 16 MB by 8 to get the value in bits rather than bytes (one byte is 8 bits). In this case, the density is 128 Mb.

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Step 4: What Your Motherboard Will Take

You may know that the maximum size RAM module your computer will accept is 256 MB and that it needs to meet a specific specification, such as PC133. Assuming the naming scheme of the last step, if you were to purchase a 256 MB 32X8 Non-ECC PC133 Unbuffered DIMM, the density is 256 Mb. This was determined by multiplying the number 32 by 8 bits. But because compatibility is based on the density of EACH DRAM unit on a memory module instead of the capacity of the DIMM as a whole, this DIMM probably won't work in your machine.

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Step 5: Explanation

A motherboard that only accepts 256 MB and smaller sized DIMMS will work only with memory that is based on 128 Mb technology or lower. The DIMM in this example has 8 DRAM units, each with a capacity of 32 MB. If 16 DRAM units were used, the capacity of this DIMM would be 512 MB, and it would only work in motherboards that accept 512 MB DIMMs or higher, even though it is just 256 MB in size. This is due to the density of each DRAM unit on the DIMM.

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Step 6: What Will Happen?

Remember that maximum capacity is based on the density of each DRAM unit on the DIMM and assumes the maximum number of DRAM units are on the DIMM. If you try to install a higher density DIMM than your computer can handle, one of two things will happen: 1) Your computer will not boot, and you will receive memory POST beeps, letting you know there is a problem. 2) The motherboard will only recognize half of the memory.