The acronym GPIOⁱ stands for general purpose input and output. A GPIOⁱ is a simple peripheral which can read input from and produce output to physical wires. This allows a microprocessor to read and write physical wires in a computer system without being directly connected to those physical wires.

This is accomplished using two registers per physical wire. First, the tristate register is used to select between input mode and output mode for the wire. Second, the data register is used to store the state of the wire when in input mode or drive the state of the wire when in output mode.