c-
Very high speed devices could place heavy demands on the CPU for I/O servicing if they relied on the CPU to copy data word by word. The DMA controller is a device which copies blocks of data at a time from one place to the other, without the intervention of the CPU. To use it, its registers must be loaded with the information about what it should copy and where it should copy to. Once this is done, it generates an interrupt to signal the completion of the task. The advantage of the DMA is that it transfers large amounts of data before generating an interrupt. Without it, the CPU would have to copy the data one register-full at a time, using up hundreds or even thousands of interrupts and causing the machine to grind to a halt
http://markburgess.org/os/os.html#SECTION00334000000000000000