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Memory Size Specifications

A specific notation is used to indicate the size and bit width of memory modules. These can be quite confusing to many memory buyers, especially if they don't understand the concept of bit width. However, knowing what these notations mean can help you make sure that you get the right kind of memory for your PC.

Memory modules have a specific width. Each module of the same type has the same width, so all 72-pin parity SIMMs for example have the same width and all 72-pin non-parity SIMMs have the same width. Larger SIMMs of the same format (say a 16 MB 72-pin SIMM as opposed to an 8 MB 72-pin SIMM) have more storage because they have a greater depth of storage for each bit of width. A memory SIMM or DIMM is usually specified using a notation that looks something like this: "2x32-60". The "x" is read as "by", just as a "2x4" in the lumber yard is called a "two by four", so this SIMM would be a "two by thirty-two sixty". A more generic way to express the notation is "DxW-S". Here is what each of the numbers means:

• D: This is the depth of the module in millions. For each bit of width, there are this many megabits (not bytes) of storage. This number is usually 1, 2, 4 or 8. For smaller SIMMs, it can be 256 or 512; in this case it represent the number of kilobits of depth, instead of megabits.
• W: This is the width of the module in bits. Each SIMM or DIMM type has the same width. This number is usually 8, 32 or 64 for non-parity modules, or 9, 36 or 72 for parity or ECC modules.
• S: This is the speed of the module in nanoseconds. It is sometimes not specified directly as part of the specification but each module should be rated with its speed.

What is confusing is that the actual amount of storage in megabytes is usually not shown. You have to be able to figure that out from the numbers given, although it's not that hard once you get used to it. So how do you interpret a "2x32-60"? This is a 72-pin SIMM (because it has 32 bits of width). The size of the SIMM is 2 million (depth) x 32 bits (width). 32 bits is 4 bytes, so the size of the SIMM is 8 MB (2 million times 4). The speed of the SIMM is 60 ns. To take another example, let's look at a "4x9-70". This is a 30 pin SIMM (because it has only 9 bits of width). The depth is 4 million bits. The size of the SIMM is therefore 4 million x 9 bits. The "9" tells us that it is a parity SIMM; only 8 of the bits are used to store data, and the ninth is for error detection. So the total size is 4 million x 8 bits which is 4 MB. The speed is 70 nanoseconds.

This table shows the different size specifications for common modules. Note that not all of these modules are necessarily available; I don't think that 8 MB 30-pin SIMMs even exist, as 1 MB and 4 MB are the only ones I ever see used. 2 MB SIMMs existed at one point but now are almost never seen. Notice the difference between the specifications of a 4 MB 30-pin module (4x8) and a 4 MB 72-pin module (1x32). This shows how the 30-pin module is narrower than the 72-pin (only 8 bits) but deeper (4M instead of 1M):

 Size in MegaBYTES 30-Pin SIMM, Non-Parity 30-Pin SIMM, Parity 72-Pin SIMM, Non-Parity 72-Pin SIMM, Parity/ECC 168-Pin DIMM, Non-Parity 168-Pin DIMM, Parity/ECC 1 MB 1x8 1x9 256x32 256x36 -- -- 2 MB 2x8 2x9 512x32 512x36 -- -- 4 MB 4x8 4x9 1x32 1x36 -- -- 8 MB 8x8 8x9 2x32 2x36 1x64 1x72 16 MB 16x8 16x9 4x32 4x36 2x64 2x72 32 MB -- -- 8x32 8x36 4x64 4x72 64 MB -- -- 16x32 16x36 8x64 8x72

Tip: A simple rule of thumb to find the size in megabytes of any module from its "DxW" specification is as follows: take the D and W numbers and multiply them together (if D is 256 or 512, use 0.25 or 0.5 instead). Then, take the product and divide by 8 (for non-parity memory) or 9 (for parity). The result is the size in megabytes.

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