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The disadvantage of DMA transfer operations is that the PC/AT hard disk controller and later IDE, evolved around PIO data transfers. Therefore, the system Int 13h BIOS and the embedded operating system device drivers have supported PIO transfers versus DMA transfers. This simply means that BIOS changes and external device drivers are necessary to achieve the incremental performance that DMA offers.
Western Digital's Enhanced IDE program supports system manufacturers' choice of either PIO transfers via Flow control with IORDY for Mode 3 PIO data rates or DMA transfers (both Type B and Type F) via the development of external DMA device drivers supporting Western Digital hard disk drives.
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