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According to the ATA specs, PIO mode must always be supported by all ATA-compliant drives as the default data transfer mechanism.
 
PIO mode uses a tremendous amount of CPU resources, because every byte of data transferred between the disk and the CPU must be sent through the CPU's [[Inline Assembly/Examples#I\O access|IO port bus]] (not the memory). On some CPUs, PIO mode can still achieve actual transfer speeds of 16MB per sec, but no other processes on the machine will get any CPU time.
 
However, when a computer is just beginning to boot, there are no other processes. So PIO mode is an excellent and simple interface to utilize during bootup, until the system goes into multitasking mode.
 
Please note that this article deals with what are now styled PATA hard disks, as opposed to SATA hard disks.
 
 
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