Direct memory access or DMA is the generic term used to refer to a transfer protocol where a peripheral device transfers information directly to or from memory, without the system processor being required to perform the transaction. DMA has been used on the PC for years, traditionally over the ISA bus for devices like sound cards, using DMA channels which are a standard system resource. The floppy disk interface also uses conventional DMA.
Most newer hard disks support the use of DMA modes over the IDE/ATA interface, which means that the built-in controller on the hard disk can do transfers to memory without the system processor's involvement. This is in direct contrast to the more conventional use of PIO modes, which require the CPU to do the work. However, this is a different type of DMA than that used on the ISA bus.
The standard ISA bus DMA channels are an old design dating back to the early days of the PC. They use the DMA controller built into the system chipset, to perform third-party DMA transfers, where the "third party" is the controller, working with the memory and the device. This sort of DMA is not used by modern hard disks for performance reasons.
Modern IDE/ATA hard disks use "first party" DMA transfers, where the peripheral device itself does the work of transferring data to and from memory. This is also called bus mastering. Bus mastering allows the hard disk and memory to work without relying on the old DMA controller built into the system, or needing any support from the CPU. It requires the use of the PCI bus (older buses like MCA also supported bus mastering but are no longer in common use).
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