Megabyte (MB) versus Megabit (Mb)
Bits and Bytes
A bit is a single character of data (0 zero or 1 one). 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 is where the numbers could get a bit (no pun intended) confusing. To make a
128 megabyte memory 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 one 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 has ECC, (Error Correcting Code) it will 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"
Now, 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), and memory modules are almost always
referred to in terms of megabytes (MB). |
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Some things don't always match - different
densities
Another thing to consider is that the number of
megabits in each component don't always match the total number of megabytes in
your module. As noted above, the 128 megabyte module was made with eight 128
megabit parts.
There are several ways of making 128 megabyte
modules. You can use sixteen 64 megabit parts or four 256 megabit parts. 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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