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Direct memory access
Direct memory access (DMA) is a feature of modern computers that allows certain hardware subsystems within the computer to access system memory for reading and/or writing independently of the central processing unit. Computers that have DMA channels can transfer data to and from devices much smaller CPU load than computers without a DMA channel.

The Principle Of The DMA

DMA is an essential feature of all modern computers, as it allows devices to communicate without subjecting the CPU with a heavy head. Otherwise, the CPU would have to copy each piece of data from source to destination. It is usually slower than copying a normal memory blocks, since access to i/o devices on baseboard peripheral bus generally slower than normal system RAM. During this time, the processor will not be available for any other tasks related to access by bus the CPU, though it can continue to do any work that does not require access to the bus.

DMA transfer essentially copies a block of memory from one device to another. While the CPU initiates the transfer, it does not execute it. For so-called "third party" DMA, usually used with the ISA bus transfer is done using the DMA controller, which is typically part of the motherboard chipset. More advanced designs, such as the PCI bus, as a rule, use bus mastering DMA, where it takes the control of the bus and performs the translation.

A typical use of the DMA copies a block of memory from RAM or from a buffer to the device. Such an operation does not stall the processor, which as a result can be scheduled to perform other tasks. DMA is essential to high performance embedded systems. It is also important in providing so-called zero-copy implementations of peripheral device drivers as well as functionalities such as network packet routing, audio playback and streaming video.

The DMA controller

Processing unit, which oversees the DMA is known as DMA controller. Typically, the DMA controller is working to establish the page link between memory and i/o devices, with permission from the microprocessor, so that data can be transmitted at a much lower cost processor.