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The bus (connections between and within the CPU, memory, and peripherals) used to carry data. Other connections are the address bus and control signals.
The width and clock rate of the data bus determine its data rate (the number of bytes per second it can carry), which is one of the main factors determining the processing power of a computer. Most current processor designs use a 32-bit bus, meaning that 32 bits of data can be transferred at once. Some processors have an internal data bus which is wider than their external bus in order to make external connections cheaper while retaining some of the benefits in processing power of a wider bus.
See also data path.