Special input/output (Special I/O or SIO) are inputs and/or outputs of a microcontroller designated to perform specialized functions or have specialized features.
Specialized functions can include: Hardware interrupts, analog input or output PWM output Serial communication, such as UART, USART, SPI bus, or SerDes. External reset Switch debounce Input pull-up (or -down) resistors open collector output Pulse counting Timing pulses
Some kinds of special I/O functions can sometimes be emulated with general-purpose input/output and bit banging software.
See also
Atari SIO
References