Direct Memory Access

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Direct Memory Access, usually abbreviated as DMA, refers to a now-very-common technique used to transfer data between devices and main memory. In DMA transfers, data is not moved by the CPU (e.g. by reading it from the device, and the writing it to the memory); rather, the device transfers the data itself.

There are two main approaches for performing this. In the first, the CPU and devices share a bus which can perform read or write cycles to memory; the device must gain control of the bus, and then performs a memory cycle. In the second, the memory has multiple ports, and the device has direct access to the memory via one of the ports.