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Direct Memory Access:-

Direct Memory Access (DMA) is the capability to transfer data from a peripheral to main memory without the CPU's intervention. DMA boosts the performance of peripherals manyfold, because it doesn't burn CPU cycles to move data. PCI networking cards and IDE disk drives are common examples of peripherals relying on DMA for data transfer.

DMA is initiated by a DMA master. The PC motherboard has a DMA controller on the South Bridge that can master the I/O bus and initiate DMA to or from a peripheral. This is usually the case for legacy ISA cards. However, buses such as PCI can master the bus and initiate DMA transfers. CardBus cards are similar to PCI and also support DMA mastering. PCMCIA devices, on the other hand, do not support DMA mastering, but the PCMCIA controller, which is usually wired to a PCI bus, might have DMA mastering capabilities.

The issue of cache coherency is synonymous with DMA. For optimum performance, processors cache recently accessed bytes, so data passing between the CPU and main memory streams through the processor cache. During DMA, however, data travels directly between the DMA controller and main memory and, hence, bypasses the processor cache. This evasion has the potential to introduce inconsistencies because the processor might work on stale data living in its cache. Some architectures automatically synchronize the cache with main memory using a technique called bus snooping. Many others rely on software to achieve coherency, however.

DMA can occur synchronously or asynchronously. An example of the former is DMA from a system frame buffer to an LCD controller. A user application writes pixel data to a DMA-mapped frame buffer via /dev/fbX, while the LCD controller uses DMA to collect this data synchronously at timed intervals. An example of asynchronous DMA is, transmit and receive of data frames between the CPU and a network card.

System memory regions that are the source or destination of DMA transfers are called DMA buffers. If a bus interface has addressing limitations, that'll affect the memory range that can hold DMA buffers. So, DMA buffers suitable for a 24-bit bus such as ISA can live only in the bottom 16MB of system memory called ZONE_DMA PCI buses are 32-bits wide by default, so you won't usually face such limitations on 32-bit platforms.