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Determining a Memory Module's Size and Features

Most memory modules are labeled with a sticker indicating the module's type, speed rating, and manufacturer. If you are attempting to determine whether existing memory can be used in a new computer, or if you need to replace memory in an existing computer, this information can be essential. Figure 6.15 illustrates the markings on typical 512MB and 1GB DDR memory modules from Crucial Technologies.

However, if you have memory modules that are not labeled, you can still determine the module type, speed, and capacity if the memory chips on the module are clearly labeled. For example, assume you have a memory module with chips labeled as follows:

• MT46V64M8TG-75

By using an Internet search engine such as Google and entering the number from one of the memory chips, you can usually find the data sheet for the memory chips. Consider the following example: Say you have a registered memory module and want to look up the part number for the memory chips (usually eight or more chips) rather than the buffer chips on the module (usually from one to three, depending on the module design). In this example, the part number turns out to be a Micron memory chip that decodes like this:

• MT = Micron Technologies (the memory chip maker)
• 46 = DDR SDRAM
• V = 2.5V DC
• 64M8 = 8 million rows x 8 (equals 64) x 8 banks (often written as 64 Meg x 8)
• TG = 66-pin TSOP chip package
• –75 = 7.5ns @ CL2 latency (DDR 266)

From this information, you can determine that the module has the following characteristics:

• The module runs at DDR266 speeds using standard 2.5V DC voltage.
• The module has a latency of CL2, so it can be used on any system that requires CL2 or slower latency (such as CL2.5 or CL3).
• Each chip has a capacity of 512Mb (64 x 8 = 512).
• Each chip contains 8 bits. Because it takes 8 bits to make 1 byte, the capacity of the module can be calculated by grouping the memory chips on the module into groups of eight. If each chip contains 512Mb, a group of eight means that the module has a size of 512MB (512Mb x 8 = 512MB). A dual-bank module has two groups of eight chips for a capacity of 1GB (512Mb x8 = 1024MB, or 1GB).

If the module has nine instead of eight memory chips (or 18 instead of 16), the additional chips are used for parity checking and support ECC error correction on servers with this feature.

To determine the size of the module in MB or GB and to determine whether the module supports ECC, count the memory chips on the module and compare them to Table 6.17. Note that the size of each memory chip in Mb is the same as the size in MB if the memory chips use an 8-bit design.

Table 6.17. Module Capacity Using 512Mb (64Mbit x 8) Chips

 Number of Chips Number of Bits in Each Bank Module Size Supports ECC? Single or Dual Bank 8 64 512MB No Single 9 72 512MB Yes Single 16 64 1GB No Dual 18 72 1GB Yes Dual

The additional chip used by each group of eight chips provides parity checking, which is used by the ECC function on most server motherboards to correct single-bit errors.

A registered module contains 9 or 18 memory chips for ECC plus additional memory buffer chips. These chips are usually smaller in size and located near the center of the module, as shown previously in Figure 6.10.

You can also see this information if you look up the manufacturer and the memory type in a search engine. For example, a web search for Micron Unbuffered DIMM Design locates a table showing various DIMM organization, SDRAM density, and other information for listed modules.

As you can see, with a little detective work, you can determine the size, speed, and type of a memory module—even if the module isn't marked, as long as the markings on the memory chips themselves are legible.

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