Editing Direct memory access (section)

{{Short description|Feature of computer systems}}
{{more footnotes needed|date=May 2023}}

'''Direct memory access''' ('''DMA''') is a feature of computer systems that allows certain hardware subsystems to access main system [[computer memory|memory]] independently of the [[central processing unit]] (CPU).<ref name="DMAfundamentals" />

Without DMA, when the CPU is using [[programmed input/output]], it is typically fully occupied for the entire duration of the read or write operation, 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 DMA 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 I/O data transfer.

Many hardware systems use DMA, including [[disk drive]] controllers, [[graphics card]]s, [[network card]]s and [[sound card]]s. DMA is also used for intra-chip data transfer in some [[multi-core processor]]s. Computers that have DMA channels can transfer data to and from devices with much less CPU overhead than computers without DMA channels. Similarly, a [[processing element|processing circuitry]] inside a multi-core processor can transfer data to and from its local memory without occupying its processor time, allowing computation and data transfer to proceed in parallel.

DMA can also be used for "memory to memory" copying or moving of data within memory. DMA can offload expensive memory operations, such as large copies or [[scatter-gather]] operations, from the CPU to a dedicated DMA engine. An implementation example is the [[I/O Acceleration Technology]]. DMA is of interest in [[network-on-chip]] and [[in-memory computing]] architectures.