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Although control of external devices is essential, for most devices, control functions
are secondary. The primary function of external devices is data transfer. Indeed,
most of the architectural choices surrounding external devices focus on mechanisms that
permit the device and processor to exchange data.
We will consider several questions regarding data transfer. First, exactly how is
data communicated? Second, how is the transfer controlled (i.e., does the processor or
the device initiate the operation)? Third, what techniques and mechanisms are needed
for the highest-speed transfers?
What voltages are used to communicate with an external device, and how is data
encoded? The answers depend on the type of device, the speed with which data must
be transferred, the type of cabling used, and the distance between the processor and the
device. In most cases, the digital signals used internally by a processor are not sufficient
for communication with an external device.
Because the voltages and encodings used for external connections differ from those
used internally, special hardware is needed to translate between the two representations.
We use the term interface controller to refer to the hardware that provides the interface
to an external device. Figure 13.2 illustrates that controllers are needed at both ends of
a physical connection.
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