What is Direct memory access?

Direct memory access (DMA) is a process wherein an external device takes over the control of system bus by the CPU.DMA is for high-speed data transfer by/to mass storage peripherals, for example: hard disk drive, CD-ROM, magnetic tape and sometimes video controllers. For illustration, a hard disk may boast a transfer rate of 5 M bytes per second, that is 1 byte transmission every 200 ns. To generate such data transfer through the CPU is both unnecessary and undesirable.