Direct Memory Access

Direct Memory Access (DMA) allows certain hardware subsystems within a computer to access system memory for reading and/or writing independently of the main CPU. Examples of systems that use DMA: Hard Disk Controller, Disk Drive Controller, Graphics Card, Soundcard.

DMA is an essential feature of all modern computers, as it allows devices of different speeds to communicate without subjecting the CPU to a massive interrupt load.

"Scatter-gather" DMA allows the transfer of data to multiple memory areas in a single DMA transaction. It is equivalent to the chaining together of multiple simple DMA requests. Again, the motivation is to off-load multiple I/O interrupt and data copy tasks from the CPU.

DRQ stands for DMA request; DACK for DMA acknowledge.