Programmed input–output
term
Programmed input–output (also programmed input/output, programmed I/O, PIO) is a method of data transmission, via input/output (I/O), between a central processing unit (CPU) and a peripheral device, such as a Parallel ATA storage device. Each data item transfer is initiated by an instruction in the program, involving the CPU for every transaction. In contrast, in direct memory access (DMA) operations, the CPU is uninvolved in the data transfer.
The term can refer to either memory-mapped I/O (MMIO) or port-mapped I/O (PMIO). PMIO refers to transfers using a special address space outside of normal memory, usually accessed with dedicated instructions, such as IN and OUT in x86 architectures. MMIO refers to transfers to I/O devices that are mapped into the normal address space available to the program. PMIO was very useful for early microprocessors with small address spaces, since the valuable resource was not consumed by the I/O devices.
The best known example of a PC device that uses programmed I/O is the Parallel AT Attachment (PATA) interface; however, the AT Attachment interface can also be operated in any of several DMA modes. Many older devices in a PC also use PIO, including legacy serial ports, legacy parallel ports when not in ECP mode, keyboard and mouse PS/2 ports, legacy MIDI and joystick ports, the interval timer, and older network interfaces.
PIO mode in the ATA interface
The PIO interface is grouped into different modes that correspond to different transfer rates.
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