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Synchronous (Clocked) Data Transfer
The system uses various signals to control the flow of information (data) around the PC. The system clock (or clocks) are key control signals that are used to synchronize most of the operations that occur within a PC. One of the most important functions of the clock signal is to control the transfer of data over an interface or bus. This is called synchronous or clocked data transfer.
Most interfaces involve one or more data signals that run between devices, as well as various control signals. The control lines tell various devices on the interface when to begin sending data, and when to look for data being sent by other devices. They also facilitate negotiation, which is the process or determining whose turn it is to use a system bus.
Once a data transfer is ready to occur, the clock related to that interface or bus controls the transfer of each piece of data. In conventional operation, one bit of data is transferred across each data line, for each cycle of the clock. The "ticking of the clock" is recognized by triggering on either the rising edge or falling edge of the clock signal. Each subsequent rising or falling edge of the clock triggers the next chunk of data to move across the data line(s) from the sending to the receiving device.
Since the pace of the clock controls the transfer of data, this means that the speed of the clock is also the speed of the bus or interface. Speeding up the clock means that data is transferred more quickly. The total throughput of any bus or interface is equal to the speed of the interface multiplied by the width of the data bus (how many data signals transfer data at once.) See this discussion of system bus operation for more details on this.