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Input and output (I/O) devices allow us to communicate with the computer system.
I/O is the transfer of data between primary memory and various I/O peripherals.
Input devices such as keyboards, mice, card readers, scanners, voice
recognition systems, and touch screens allow us to enter data into the computer.
Output devices such as monitors, printers, plotters, and speakers allow us to get
information from the computer.
These devices are not connected directly to the CPU. Instead, there is an
interface that handles the data transfers. This interface converts the system bus
signals to and from a format that is acceptable to the given device. The CPU communicates
to these external devices via input/output registers. This exchange of
data is performed in two ways. In memory-mapped I/O, the registers in the interface
appear in the computer’s memory map and there is no real difference
between accessing memory and accessing an I/O device. Clearly, this is advantageous
from the perspective of speed, but it uses up memory space in the system.
With instruction-based I/O, the CPU has specialized instructions that perform the
input and output. Although this does not use memory space, it requires specific
I/O instructions, which implies it can be used only by CPUs that can execute these
specific instructions. Interrupts play a very important part in I/O, because they are
an efficient way to notify the CPU that input or output is available for use.
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