4.3. CALCULATING THE CAPACITY OF A MODULE

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4.3. CALCULATING THE CAPACITY OF A MODULE

Memory holds the information that the CPU needs to process. The capacity of memory chips and modules are described in megabits (millions of bits) andmegabytes (millions of bytes). When trying to figure out how much memory you have on a module, there are two important things to remember:

A module consists of a group of chips. If you add together the capacities of all the chips on the module, you get the total capacity of the module. Exceptions to this rule are:

• If some of the capacity is being used for another function, such as error checking.
  
  
• If some of the capacity is not being used, for example some chips may have extra rows to be used as back-ups. (This isn't common.)

While chip capacity is usually expressed in megabits, module capacity is expressed in megabytes. This can get confusing, especially since many people unknowingly use the word "bit" when they mean "byte" and vice versa. To help make it clear, we'll adopt the following standards in this book:

When we talk about the amount of memory on a module, we'll use the term "module capacity"; when we are referring to chips, we'll use the term "chip density". Module capacity will be measured in megabytes (MB) with both letters capital, and chip density will be measured in megabits (Mbit), and we'll spell out the word "bit" in small letters.

COMPONENT	CAPACITY EXPRESSION	CAPACITY UNITS	EXAMPLE
Chips	Chip Density	Mbit (megabits)	64Mbit
Memory Modules	Module Capacity	MB (megabytes)	64MB

CHIP DENSITY

Each memory chip is a matrix of tiny cells. Each cell holds one bit of information. Memory chips are often described by how much information they can hold. We call this chip density. You may have encountered examples of chip densities, such as "64Mbit SDRAM" or "8M by 8". A 64Mbit chip has 64 million cells and is capable of holding 64 million bits of data. The expression "8M by 8" describes one kind of 64Mbit chip in more detail.

In the memory industry, DRAM chip densities are often described by their cell organi-zation. The first number in the expression indicates the depth of the chip (in locations) and the second number indicates the width of the chip (in bits). If you multiply the depth by the width, you get the density of the chip. Here are some examples:

CURRENT AVAILABLE CHIP TECHNOLOGY

	CHIP DEPTH IN MILLIONS OF LOCATIONS	CHIP WIDTH IN BITS	CHIP DENSITY = DEPTH x WIDTH
16Mbit Chips 4Mx4 1Mx16 2Mx8 16MxI	4 1 2 16	4 16 8 1	16 16 16 16
64Mbit Chips 4Mx16 8Mx8 16Mx4	4 8 16	16 8 4	64 64 64
128Mbit Chips 8Mx16 16Mx8 32Mx4	8 16 32	16 8 4	128 128 128
256Mbit Chips 32Mx8	32	8	256

MODULE CAPACITY

It's easy to calculate the capacity of a memory module if you know the capacities of the chips on it. If there are eight 64Mbit chips, it's a 512Mbit module. However, because the capacity of a module is described in megabytes, not megabits, you have to convert bits to bytes. To do this, divide the number of bits by 8. In the case of the 512Mbit module:

 512Mbits

—————– = 64MB
8 bits per byte

You may hear standard memory modules in the industry being described as: “4Mx32” (that is, “4 Meg by 32”), or “16Mx64” (“16 Meg by 64”). In these cases, you can calculate the capacity of the module exactly as if it were a chip:

4Mx32 is 128Mbits.                     16Mx64 is 1024Mbits.

 128Mbits                              1024Mbits

—————– = 16MB module        —————– = 128MB module
8 bits per byte                        8 bits per byte

Here are some additional examples:

STANDARD MODULE TYPES

	STANDARD	MODULE DEPTH IN LOCATIONS	MODULE WIDTH IN DATA BITS	CAPACITY IN MBITS = DEPTH X WIDTH	CAPACITY IN MB = MBITS/8
72-Pin	1Mx32 2Mx32 4Mx32 8Mx32 16Mx32 32Mx32	1 2 4 8 16 32	32 32 32 32 32 32	32 64 128 256 512 1024	4 8 16 32 64 128
168-Pin	2Mx64 4Mx64 8Mx64 16Mx64 32Mx64	2 4 8 16 32	64 64 64 64 64	128 256 512 1024 2048	16 32 64 128 256

As we mentioned earlier, there’s only room for a certain number of chips on a PCB. Based on an industry standard 168-pin DIMM, the largest capacity module manufacturers can make using 64Mbit chips is 128MB; with 128Mbit chips, the largest module possible is 256MB; and with 256Mbit chips, the largest module possible is 512MB.

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