MB vs Mbits

Megabyte (MB) vs. Megabit (Mb)

Big B vs. little b. What’s the difference? Ever get the feeling that the more you learn, the less you know?

Drill into the technical details of a DRAM data sheet and you just might see what I’m talking about. One topic that frequently stumps people is deciphering megabits and megabytes.

Let’s say you buy a 128 megabyte module from Samsung and decide you want to know what all those little numbers on the memory chips mean. So you log on to the Samsung Web site and find the appropriate data sheet. After reading a few lines, you realize that according to the data sheet, you have a module with 64 megabit parts. Wait a minute? You paid for a 128MB upgrade!

Don’t worry, you have one.

Bits and Bytes
A bit is a single character of data (a 0 or a 1). A byte is eight characters of data. Therefore, eight bits make a byte. Your computer processes information in a series of eight bits, or, one byte.

Here’s where the numbers can get confusing. To make a 128 megabyte module, you need eight 128 megabit parts. (Remember, it takes eight bits to make a single byte. So multiply 128 megabits by eight parts and you get a total of 128 megabytes.)

Reading data
When your computer reads data from memory, it reads one bit from each of the eight memory chips to make a byte. That means all of the chips on your module are always working together, rather than a single chip working at one time.

If the module had ECC, it would have nine chips instead of eight. The ninth chip would also work in conjunction with the other chips, except it would do the error checking and correction.

Big “B” versus little “b”
If the similar names aren’t confusing enough, the abbreviations are worse. Megabit is abbreviated with a lower case b (Mb) and megabyte is abbreviated with a capital B (MB). Memory modules are almost always referred to in terms of megabytes (MB).

Different densities
Another thing to consider is that the number of megabits in each component doesn’t always match the total number of megabytes in your module.

For instance, in our last example, the 128 megabyte module was made with eight 128 megabit parts.

There are other ways to make 128 megabyte modules. You can use sixteen 64 megabit parts or four 256 megabit parts. Either way, the total number of bits adds up to 128 megabytes. This gives the module assembler flexibility to use larger RAM densities as they become available and to take advantage of DRAM pricing changes to build modules using the most cost effective method.


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