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I/O is one of the three main components of a computer system. The responsibility of I/O is to interface with external devices. Depending on their applications, I/O operations can be divided into three groups: 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, some bits of an 8-bit port can be used to receive input signals while other bits are used to send output signals. Depending on how they are used, one bit of an I/O address may be referred to as an I/O port or the set of all bits at a single I/O address may be referred to as 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.