To transfer data between memory and I/O devices, DMA controller takes over the control of the system from the processor and transfer of data take place over the system bus. For this purpose, the DMA controller must use the bus only when the processor does not need it, or it must force the processor to suspend operation temporarily. The later technique is more common and is referred to as cycle stealing, because the DMA module in effect steals a bus cycle.

The typical block diagram of a DMA controller is shown in the Figure 6.9.