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RAM Sizes and Speeds

Now that we have discussed the various types of RAM and their function, it is appropriate to discuss the various sizes and speeds that RAM can be found in. One of the more difficult things to do in the world of PC hardware is to Identify how much memory is on a particular module without some kind of label, decal, or screen-printed text on the module itself. Most modern memory modules however will have a series of numbers printed on the chips themselves. It commonly follows a DxW-S format, which is explained below.

• D: The first letter specifies the bit depth, measured in millions of bits. Each number represents how many megabits of memory cells. Be very clear that we are dealing with megabits and not megabytes.

• W: The next letter specifies the bit width of each cell in the module. A 30-pin SIMM has an 8-bit width, 72-pin SIMMs have a 32-bit width, and DIMMs have a 64-bit width. ECC RAM variations have 9, 36, or 72 bit width respectively. To determine the size of the module that you're looking at, you would take DxW / 8.

This will yield the size of the module in megabytes. For example, if we had 168-pin DIMM and it was labeled as being an 8x64 module, this would make it a 64MB (8 x 64 / 8) DIMM module. It is important to note that when calculating parity or ECC modules, the extra bits used for parity information are not counted. For example in a 9-bit module only 8-bits are used in the calculation.

• S: The S specifies the speed of the module nanoseconds. This be defines how fast a module can read or write to the memory bus. Refer to the table below to see the different depths, widths, and speeds of various SIMM and DIMM memory modules.
 SPEED-O-RAMThe speed of a RAM module’s access time is measured in nanoseconds.

 Size in MB 30-pin SIMM Non-Parity 30-Pin SIMM Parity 72-Pin SIMM Non-Parity 72-Pin SIMM Parity 168-Pin DIMM No-Parity 168-Pin DIMM Parity 1 MB 1x8 1x9 256kx32 256kx36 -- -- 2 MB 2x8 2x9 512kx32 512kx36 -- -- 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 128 MB -- -- 32x32 32x36 16x64 16x72 256 MB -- -- 64x32 64x36 32x64 32x72 512 MB -- -- 128x32 128x32 64x64 64x72

 How Memory Accesses OccurAccessing memory in a PC is a very detailed and complex operation. It is certainly not required knowledge for the A+ Core Exam. However, certain issues surrounding the process can affect system performance.

Let’s begin by explaining exactly how memory is addressed by the system.

As you have learned, memory is organized into banks. A bank of memory contains modules of memory. The modules are made up of memory chips, which are in turn divided into cells. Each cell contains a certain number of bits, which is expressed by the bit width.

For example, a 16x32 module contains 16M (16,777,216) cells of 32 bits each. This yields a memory module with a total capacity of 64 MB (67,108,864 bits). Referring back to our packaging chart, this would be a 64MB 72-pin SIMM. If we used this in a Pentium class system, we would need a matching SIMM to complete a 128MB bank of memory (Still with me?).

Continuing with this example, using binary math we would determine that the system would require a 24-bit address bus to address the 16M cells (224 = 16,777,216). However, memory cells are logically organized into a “square” containing rows and columns. You can think of this like a spreadsheet. If I wanted to access cell M12, I would find row 12, then move to column M. The memory controller breaks accesses up the same way.

For example, our processor needs to access cell 15,342,195 in our SIMM module. This would have a binary equivalent of:

111010100001 101001110011

The memory controller takes the first 12-bits as the row address, then it uses the last 12-bits as the column address. Using this method, the address bus need only be 12 bits wide instead of 24 bits. This equals lower costs to the manufacturer.

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