PIO is Programmed I/O.

PIO is the oldest method is transferring memory over a IDE/ATA interface. The technique of a PIO involves using the CPU and support hardware to control the transfer of memory between the system itself and the hard drive. The speed of the PIO is called a PIO Mode. It starts at Mode 0 and increases with each mode having a faster cycle rate in nanoseconds. So, quite obviously, higher modes are better due to the fact that memory can be transferred so much faster due to increased Cycle Rates and Maximum Transfer Rates.

Support for PIO modes is built directly into system BIOS, meaning drivers are not necessary for them. The simplicity of design of PIO’s made them the most commonly used method of transferring memory for many years. It’s wide support and stability were satisfactory until Ultra DMA appeared, basically wiping out the usage of PIO.