The data bus carries the data that is to be written or has been read from memory.
It is a bidirectional bus as it can carry data to or from memory.
The width of the data bus is directly related to the largest number that the bus can carry. For example an 8 bit bus can represent 2 to the power of 8 unique values. This equates to the numbers 0 to 255. A 16 bit bus can carry the values 0 to 65535 and so on.
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