In this script, we will discuss the most important topic of Microprocessors and Interfacing. Now we will discuss

  • Direct Memory Access 

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Direct Memory Access 

The data transfer between a fast storage media such as Magnetic Disk and Memory unit is limited by

the speed of the CPU. 

Thus we can allow the peripherals directly communicate with each other using the memory busses,

removing the intervention of the CPU.. This type of data transfer technique is known as DMA. 

During DMA the CPU is idle and it has no control over the memory buses.

The DMA controller takes over the buses to manage the transfer directly between the I/O devices

and the memory unit. 




Bus Request: It is used by the DMA controller to request the CPU to relinquish control of the bus.


Bus  Grant: It is activated by the CPU to inform the external DMA controller that the buses are in a high impedance state and the requesting DMA can take control of the buses. Once the DMA has taken control of the buses it transfers the data. This transfer can take place in many ways.


Bus Transfer: In which block sequence consisting of memory works is transferred in a continuous burst where the DMA controller is the master of the memory buses. This mode is needed for fast devices like magnetic disks.


Cyclic Stealing: In this DMA Controller transfers one word at a time after which it must return the control of the buses to the CPU. The CPU merely delays its operation for one memory cycle to allow the direct memory I/O transfer to steal one memory cycle.