Programmed input/output
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The programmed input/output (PIO) interface was the original method used to transfer data between the CPU (through the IDE controller) and an IDE/ATA device. The PIO interface is grouped into different modes that correspond to different transfer rates. The electrical signalling among the different modes is similar - only the cycle time between transactions is reduced in order to achieve a higher transfer rate. All IDE/ATA devices support the slowest mode - Mode 0. By accessing the information registers (using Mode 0) on an IDE/ATA drive, the CPU is able to determine the maximum transfer rate for the device and configure the IDE controller for optimal performance.
The PIO modes require a great deal of CPU overhead to configure a data transaction and transfer the data. Because of this inefficiency, the DMA (and eventually UDMA) interface was created to increase performance. The simple digital logic required to implement a PIO transfer still makes this transfer method useful today (especially if high transfer rates are not required).
The modes are given below:
| Mode | Maximum Transfer Rate (MB/s) | Standard Where Spec Was Defined |
|---|---|---|
| Mode 0 | 3.3 | ATA |
| Mode 1 | 5.2 | ATA |
| Mode 2 | 8.3 | ATA |
| Mode 3 | 11.1 | ATA-2 |
| Mode 4 | 16.7 | ATA-2 |