More about DMA

Although a DMA controller can transfer data without intervention by a processor its operation must be under the control of a  program executed by the processor. To initiate the transfer of a block of words, the processor sends the starting address, the number of words in the block, and the direction of the transfer. On recieving this information, the DMA controller proceeds to perform the required information. When the entire block has been transferred the controler informs the processor by by raising an interrupt signal.

When a DMA transfer is taking place, the program that requested the transfer cannot continue,and the processor can be used to execute another program. After the DMA transfer is completed the processor can return to the program that requested the transfer. I/O operations are always performed by the operating system of the computer in response to a request from an application program.