DMA is method of transfering data from the computer RAM to another part of the computer without processing while most data that is input or output from your computer process by the cpu, some data does not requered processing or can be process by anotherdevice.In this setuavation DMA can save processing time and is a more ifficient way to move data from the computer memory to other device.


                    Example: The sound card may need to access data storein the computer RAM but sence it can process the data it self ,it may use DMA to bypass the cpu. Many computer avoid bardaning to main cpu to do this work.CPU special prepose process called  DMAC (direct memory access cantroler) to insite a DMA  transfer. The host write DMA command block into memory.This block contains pointer to the shouce of transfer, a pointer to the transfer and a count of the number of bites to transfer.


                    The cpu write the address this command block to DMA cantroller than those on with other work. The DMA cantroller provide to operate the memory bus directive, spacing address on the bus to perform transfer without help of the main cpu.








1. Device driver is hold to a  transfer disk data to buffer address X.


2. Device driver tells dis cantroller to transfer C bites from disk to buffer at address X.


3. Disk  cantroller intialize the DMA transfer.


4. Disk cantroller send each by to DMA cantroller.


5. DMA cantroller transefer bites to buffer X ,increasse in memory address and decrease in C untial  C=0.


6. When C=0 DMA interrutent  the cpu to cegnal  transfer complition.




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by varsha   jain  in Operating System  on 3/31/2015 11:46:00 PM  

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