Direct memory access (DMA) is a feature of modern computers that allows certain hardware subsystems within the computer to access system memory independently of the central processing unit (CPU).
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 initiates the transfer, does other operations while the transfer is in progress, and receives an interrupt from the DMA controller when the operation is done. This feature is useful any time the CPU cannot keep up with the rate of data transfer, or where the CPU needs to perform useful work while waiting for a relatively slow I/O data transfer. Many hardware systems use DMA, including disk drive controllers, graphics cards, network cards and sound cards. DMA is also used for intra-chip data transfer in multi-core processors. Computers that have DMA channels can transfer data to and from devices with much less CPU overhead than computers without a DMA channel. Similarly, a processing element 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.
Read more about Direct Memory Access: Principle, Cache Coherency
Famous quotes containing the words direct, memory and/or access:
“However strongly they resist it, our kids have to learn that as adults we need the companionship and love of other adults. The more direct we are about our needs, the easier it may be for our children to accept those needs. Their jealousy may come from a fear that if we adults love each other we might not have any left for them. We have to let them know that it’s a different kind of love.”
—Ruth Davidson Bell. Ourselves and Our Children, by Boston Women’s Health Book Collective, ch. 3 (1978)
“The pure serene of memory in one man,—
A ripple widening from a single stone
Winding around the waters of the world.”
—Theodore Roethke (1908–1963)
“The last publicized center of American writing was Manhattan. Its writers became known as the New York Intellectuals. With important connections to publishing, and universities, with access to the major book reviews, they were able to pose as the vanguard of American culture when they were so obsessed with the two Joes—McCarthy and Stalin—that they were to produce only two artists, Saul Bellow and Philip Roth, who left town.”
—Ishmael Reed (b. 1938)