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PIO stands for Programmed Input-Output. It is nowdays a slow and inefficient data transfer mode. The data is transferred by the processor, byte by byte. This requires processor time and therefore causes a high and unwanted processor load. These issues are somewhat solved with DMA

In Linux, hard drive PIO modes may be tweaked with hdparm. For instance, to set hda to PIO mode 4 (if supported) type

# hdparm -X12 /dev/hda 
PIO Modes
Mode Mbyte/sec hdparm option
PIO Mode 0 3.3 -X08
PIO Mode 1 5.2 -X09
PIO Mode 2 8.3 -X10
PIO Mode 3 11.1 -X11
PIO Mode 4 16.6 -X12