D.M.A stands for direct memory access.D.M.A is also referred to as hardware controlled I/O, D.M.A is the only type of I/O transfer in which the Microprocessor is not involved in the transfer of data between the memory and the external device .
In this case there is direct data transfer between an I/O device and memory, data Is not transferred from an I/O device to one of the Microprocessor registers and then to memory a vice-versa.
In DMA, Microprocessor goes to the HOLD state. the address bus data and the data communication control signal like , .IO/ are all tri-stated and the external device which initiates the D.M.A through making HOLD control signal active, takes the command of the address bus BDB and BCB and The control signal for data transfer applications between the memory & device DMA is used primarily to transfer a number of words or black of data at high speed.
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