Clock speed and MIPS both reflect the potential processing power of a machine but the processor is not the only component in a real machine. A typical system is comprised of a processor, memory, a video display and disk drives.
In running a program data has to be moved between these system components - from memory to processor, from memory to disk, from processor to video etc.. Connections are made between the different components by sets of wires called buses. In practice a bus is more than just a set of wires, it also includes sets of rules or protocols that govern how data should be transferred, what its destination is etc.. Another name for a data bus is a data highway which is more descriptive of its role in transferring data from one system component to another.
The speed of these transfers between the components of a system affects how long a program takes to run. Different programs will make demands on different types of data transfer. For example, a database program will spend much of its time transferring data from disk to memory and vice verse whereas a DTP program will spend more of its time transferring data to and from the video display. The importance of high data transfer rates between the different system components depends very much on the type of work that a program is engaged in. In an ideal world a system would optimize the transfer rates between all system components but in the real world this results in a very expensive machine! A more realistic approach is to optimize those transfer rates that are critical to the application.
Data transfer rates are usually measured in either MBytes per second or MBits per second and it is important to be aware of which measure is in use. 1MByte per second is eight times faster than MBit per second.
There are two factors that affect data transfer rates - clock speed and band width. Clock speed is simply the rate at which a unit of data can be transferred and band width is the size of the unit of transfer. In other words, at each clock pulse an item of data is transferred but the total amount of data transferred can be increased by sending more per clock pulse. In physical terms band width corresponds to the number of wires in a bus available for data transfer. In simple terms it needs one wire connection to transfer a single bit of data between two units.
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