What is the need of DMA?

Answer 1

DMA, or Direct Memory Access, is a subcontroller that can access memory in sequential order without intervention from the processor. DMA is used to moving around large amounts of data in the computer without taking time away from the processor.

The first major uses of DMA included drive controllers and sound cards. Hard drives do not access single bytes of data, but instead an entire "sector" (512 bytes at once). They place this data in a buffer, which then is transferred to system memory. If the CPU had to read each byte and place it in memory, it would waste time that it could be performing other operations.

Similarly, sound had to be attended to constantly or it would "stutter" or have "clicks". To make sure that sound ran smoothly, developers would set up a memory to port DMA transfer, and the controller would smoothly feed the sound data into the sound card without requiring the programmer to constantly swap between drawing pixels and playing music or sound effects.

DMA is still in use today, and we use it for video, audio, and hard drive data transfers, just as we did back then. The processor is free to perform other tasks while waiting for data to load, for sound to play, or while performing blits to the screen.