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Definition: A set of  parallel conductors, which allow devices attached to it to communicate with the CPU.

The bus consists of  three main parts:

Control lines

These allow the CPU to control which operations the devices attached should perform, I.E. read or write.

Address lines

Allows the CPU to reference certain (Memory) locations within the device.

Data lines

The meaningful data which is to be sent or retrieved from a device is placed on to these lines.

Diagram of a bus

The Bus is set to run at a specified speed which is measured in MHz
Q: Why do you think the Bus speed is pre-set?