Peripheral devices (also called I/O devices) are pieces of equipment that exchange data with a CPU

Examples: switches, LED, CRT, printers, keyboard, keypad

Speed and characteristics of these devices are very different from that of CPU so they cannot be connected directly

Interface chips are needed to resolve this problem

Main function of an interface chip is to synchronize data transfer between CPU and I/O device

Data pins of interface chip are connected to CPU data bus and I/O port pins are connected to I/O device

Since a CPU may have multiple I/O devices, CPU data bus may be connected to data buses of multiple interface

An address decoder is used to select one device to respond to the CPU I/O request

Different CPUs deal with I/O devices differently
Some CPUs have dedicated instructions for performing input and output operations (isolated I/O)

Other CPUs use the same instruction for reading from memory and reading from input devices, as well as writing data into memory and writing data into output devices (memory-mapped I/O)
8051 is memory mapped

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