Direct memory access

Direct memory access (DMA) is a feature of many computer systems that allows certain hardware subsystems to access main system memory independently of the central processing unit (CPU). Without DMA, programmed input–output must be used to transfer data which typically fully occupies the CPU for the entire duration of the transfer, and is thus unavailable to perform other work. With DMA, the CPU first initiates the transfer, then it does other operations while the transfer is in progress, and it finally receives an interrupt from the direct memory access controller (DMAC) when the operation is done. This feature is useful at any time that the CPU cannot keep up with the rate of data transfer, or when the CPU needs to perform work while waiting for a relatively slow data transfer. Many hardware systems use DMA, including disk drive controllers, graphics cards, network cards, sound cards and dedicated DMA controllers (acting on behalf of the aforementioned device types). DMA is also used for intra-chip data transfer in some multi-core processors. Computers that have DMA capability can transfer data to and from devices with much less CPU overhead than computers without DMA. DMA can also be used for memory-to-memory transfers. DMA can offload expensive memory operations, such as large copies or scatter-gather operations, from the CPU to a DMA controller. An implementation example is the I/O Acceleration Technology. DMA is of interest in network-on-chip and in-memory computing architectures. While hardware acceleration is usually vendor-specific, direct memory access can be used by the class driver.