What is the difference between PIO and UDMA transfers?

There are two modes in which data can be transferred between an ATA hard disk drive and the computer system, PIO and DMA. Programmed Input/Output (PIO) mode is the slower of the two modes having the capability of transferring data at a maximum burst rate of 16.7 MBytes per second. PIO mode is also very CPU intensive has no built in error correction.

Ultra DMA (UDMA), which is also referred to as Ultra ATA, incorporates a Cyclic Redundancy Check (CRC) for error detection and correction. Single and Multiword DMA transfers do not support CRC. Single Word DMA is now considered obsolete and multiword DMA, the predecessor to Ultra DMA, was never widely implemented.

See the following tables for associated transfer rates (burst rates).
Mode Burst Speeds
PIO Modes Single Word DMA Multiword Word DMA Ultra DMA
Mode 0 3.33MB/s 2.08MB/s 4.17MB/s 16.7MB/s
Mode 1 5.22MB 4.17MB/s 13.3MB/s 25MB/s
Mode 2 8.33MB 8.33MB/s 16.7MB/s 33MB/s
Mode 3 11.1MB/s     44MB/s
Mode 4 16.7MB/s     66MB/s
Mode 5       100MB/s

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