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Limitations of Standard DMA
While the use of DMA provided a significant improvement over processor-controlled data transfers, it too eventually reached a point where its performance became a limiting factor. DMA on the ISA bus has been stuck at the same performance level for over 10 years. For old 10 MB XT hard disks, DMA was a top performer. For a modern 8 GB hard disk, transferring multiple megabytes per second, DMA is insufficient.
On newer machines, disks are controlled using either programmed I/O (PIO) or first-party DMA (bus mastering) on the PCI bus, and not using the standard ISA DMA that is used for devices like sound cards. Hard disk transfer modes are discussed in detail here. This type of DMA does not rely on the slow ISA DMA controllers, and allows these high-performance devices the bandwidth they need. In fact, many of the devices that used to use DMA on the ISA bus use bus mastering over the PCI bus for faster performance. This includes newer high-end SCSI cards, and even network and video cards.
Next: DMA Controllers