DIRECT MEMORY ACCESS (DMA).

DIRECT MEMORY ACCESS (DMA).
Is there another way of performing the I/O activity without wasting the processor’s time? Computer systems also employ a similar technique. It is called direct memory access (DMA). In DMA, the processor gives the command such as “transfer 10 KB to I/O port 125” and the DMA performs the transfer without bothering the processor. Once the operation is complete, the processor is notified. This notification is done by using an interrupt mechanism. We use DMA to transfer bulk data, not for single word transfers. A special DMA controller is used to direct the DMA transfer operations.
DMA is implemented by using a DMA controller. The DMA controller acts as a slave to the processor and receives data transfer instructions from the processor. Figure 31 shows the difference between programmed I/O and DMA transfer. In programmed I/O, the system bus is used twice as shown in Figure 31a. The DMA transfer not only relieves the processor from the data transfer chore but also makes the transfer process more efficient by transferring data directly from the I/O device to memory.