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
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.
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.
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.
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.
What Will Happen?
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.