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'''Programmed input/output''' ('''PIO''') is a method of transferring data between the [[Central processing unit|CPU]] and a peripheral, such as a network adapter or an [[AT Attachment|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 not involved in the data transfer.
The term ''Programmed I/O'' can refer to either [[Memory-mapped I/O|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<ref>Computer Organization and Architecture 9th Edition. Stallings, William. Pearson, 2012</ref> 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 [[AT Attachment|ATA]] interface; however, this interface can also be operated in any of several [[Direct Memory Access|DMA]] modes. Many older devices in a PC also use PIO, including legacy serial ports, legacy parallel ports when not in ECP mode, the PS/2 keyboard and mouse ports, legacy MIDI and joystick ports, the interval timer, and older network interfaces.
== PIO mode in the ATA interface ==
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