DMA controller in Computer Architecture

Hello students in this article we know about DMA controller in Computer Architecture, DMA controller registers, Working of DMA Controller and Advantages and Disadvantages of DMA controller

Now the execution of a pc program, it needs the synchronous operating of quite one part of a computer. For example, Processors – providing necessary management information, addresses…etc, buses – to transfer information and data to and from memory to I/O devices…etc. The attention-grabbing issue of the system would be the approach it handles the transfer of knowledge among processor, memory and I/O devices. Usually, processors control all the method of transferring data, right from initiating the transfer to the storage of data at the destination. This adds load on the processor and most of the time it stays within the ideal state, therefore decreasing the potency of the system. to hurry up the transfer of information between I/O devices and memory, DMA controller acts as station master. DMA controller transfers data with borderline intervention of the processor.


Direct Access Media (DMA) :

DMA Controller could be a hardware device that permits I/O devices to directly access memory with less participation of the processor. DMA managementler desires constant previous circuits of AN interface to speak with the central processing unit and Input/Output devices.

Fig-1 below shows the diagram of the DMA controller. The unit communicates with the CPU through knowledge bus and control lines. Through the utilization of the address bus and permitting the DMA and RS register to pick out inputs, the register among the DMA is chosen by the CPU. RD and WR are two-way inputs. once BG (bus grant) input is 0, the central processing unit will communicate with DMA registers. once BG (bus grant) input is 1, the CPU has relinquished the buses and DMA can communicate directly with the memory.

DMA Controller Diagram in Computer Architecture

DMA controller registers

Address register –It contains the address to specify the required location in memory.
Word count register –It contains the amount of words to be transferred.
Control register –It specifies the transfer mode.

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Note – All registers within the DMA seem to the CPU as I/O interface registers. Therefore, the C.P.U. will each scan and write into the DMA registers beneath program management via the information bus.

Block Diagram of DMA Controller

Working of DMA Controller

DMA Controller must share the bus with the processor to form the info transfer. The device that holds the bus at a given time is termed bus master. once a transfer from I/O device to the memory or vice verse has to be made, the processor stops the execution of this program, increments the program counter, moves data over stack then sends a DMA choose signal to DMA controller over the address bus.

If the DMA controller is free, it requests the control of bus from the processor by raising the bus request signal. Processor grants the bus to the controller by raising the bus grant signal, currently DMA controller is that the bus master. The processor initiates the DMA controller by causing the memory addresses, variety of blocks of knowledge to be transferred and direction of data transfer. when assignment the info transfer task to the DMA controller, rather than waiting ideally until completion of data transfer, the processor resumes the execution of the program after retrieving directions from the stack.

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DMA controller currently has the complete control of buses and might move directly with memory and I/O devices freelance of central processor. It makes the information transfer in step with the control directions received by the processor. once completion of information transfer, it disables the bus request signal and CPU disables the bus grant signal thereby moving control of buses to the CPU.

When an I/O device needs to initiate the transfer then it sends a DMA request signal to the DMA controller, that the controller acknowledges if it’s free. Then the controller requests the processor for the bus, raising the bus request signal. once receiving the bus grant signal it transfers the information from the device. For n channeled DMA managementler n variety of external devices is connected.

The DMA transfers the data in 3 modes that embrace the following.

a) Burst Mode: during this mode DMA relinquishing the buses to central processor solely after completion of whole data transfer. Meanwhile, if the CPU needs the bus it’s to remain ideal and look forward to data transfer.

b) Cycle Stealing Mode: during this mode, DMA offers control of buses to CPU after transfer of each byte. It incessantly problems asking for bus control, makes the transfer of 1 computer memory unit and returns the bus. By this central processor doesn’t have to be compelled to look forward to an extended time if it desires a bus for higher priority task.

c) Transparent Mode: Here, DMA transfers data only if CPU is death penalty the instruction that doesn’t need the utilization of buses.

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Advantages and Disadvantages of DMA controller

Advantages:
1)Transferring the info while not the involvement of the processor will speed up the scan-write task.
2)DMA reduces the clock cycle requires to read or write a block of data.
3)Implementing DMA also reduces the overhead of the processor.
Disadvantages :
1)As it’s a hardware unit, it would cost to implement a DMA controller within the system.
2)Cache coherence problem can occur whereas mistreatment DMA controller.

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