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I/O is one of the three main components in a computer system. The responsibility of I/O is to interface with external devices. Based on their applications, all I/O operations can be divided into three types, sensory input, control output, and data transfer. Once the nature of each type of I/O operation is understood, the hardware design and software for these operations can be understood easily.

Like memory components, I/O components have addresses and each I/O address usually consists of 8 or 16 bits of data. Since many sensory input and control output signals consist of just one bit of information, a single I/O address may be able to handle multiple input and output signals. For instance, dome bits of an 8-bit port can be used to receive input signals while other bits are used to send output signals. Therefore, sometimes one bit of an I/O address is called an I/O port and sometimes all bits of one I/O address are called an I/O port. When someone says how many ports are needed, you need to find out what his or her definition of port is.