Answer to Question #273609 in Electrical Engineering for Anil

Question

“There is a mechanism in which CPU relinquishes the control over the buses for carrying out various operations between memory unit and I/O unit.”Justify your answer and explain such process in detail with the help of suitable diagram.

 

Answer 1

DMA :

Direct Memory Access(DMA) : In the DMA the interface transfers the data into and out of the memory unit through memory bus.

The transfer of data between these devices is limited by the speed of the CPU because of the difference in the speed of Memory unit and I/O device.

Removing the CPU volunteering data transfer it increases the speed, without CPU data transfer us fast.

During the DMA technique, CPU is idle as it doesn't perform any action.

DMA controller takes the buses to manage the transfer directly between I/O and memory devices.

There are two control signals that initiates this functionality :

BR(Bus Request) : It is used by DMA Controller to request to CPU

BG(Bus Grant) : CPU activates this signal output to inform the external DMA that the Bus

Request (BR) can now take control of the buses to conduct memory transfer without

processor. 

DMA diagram :

Transfer can be made in several ways :

DMA Burst : Continuous Block sequence consisting of memory words is transferred in continuous burst while the DMA controller is master of the memory buses.

Cycle Stealing : It allows the DMA controller to transfer one data word at a time, after which it must returns control of the buses to the CPU.

DMA Controller has three register : 

Address Register

Word Count Register

Control Register

Below is the block diagram of DMA: