Direct  Memory Access

We have discussed the data transfer between the processor and I/O devices. We have discussed two different approaches namely programmed I/O and Interrupt-driven I/O. Both the methods require the active intervention of the processor to transfer data between memory and the I/O module, and any data transfer must transverse a path through the processor. Thus both these forms of I/O suffer from two inherent drawbacks.

To transfer large block of data at high speed, a special control unit may be provided to allow transfer of a block of data directly between an external device and the main memory, without continuous intervention by the processor. This approach is called direct memory access or DMA.

DMA transfers are performed by a control circuit associated with the I/O device and this circuit is referred as DMA controller. The DMA controller allows direct data transfer between the device and the main memory without involving the processor.