A DMA controller can generate memory addresses and initiate memory read or write cycles. Standard DMA, also called third-party DMA, uses a DMA controller. The two large integrated circuits below the middle of the image are the DMA controller (l.) and - unusual - an extra dedicated DMA controller (r.) for the magneto-optical disc used instead of a hard disk drive in the first series of this computer model.
Motherboard of a NeXTcube computer (1990). DMA is of interest in network-on-chip and in-memory computing architectures. An implementation example is the I/O Acceleration Technology. DMA can offload expensive memory operations, such as large copies or scatter-gather operations, from the CPU to a dedicated DMA engine. Similarly, a 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.ĭMA can also be used for "memory to memory" copying or moving of data within memory. Computers that have DMA channels can transfer data to and from devices with much less CPU overhead than computers without DMA channels. DMA is also used for intra-chip data transfer in multi-core processors. Many hardware systems use DMA, including disk drive controllers, graphics cards, network cards and sound cards. 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. 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. 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. Direct memory access ( DMA) is a feature of computer systems that allows certain hardware subsystems to access main system memory independently of the central processing unit (CPU).
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