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Direct memory access in operating system-computer world

                             Direct memory access 

Direct memory access : DMA stands for  direct memory access and it is another mechanism where input output device is given full transfer block of data to from memory without going through the CPU. In other word we can say that it is a method of transferring data from the computers RAM to another part of computer. The DMA controller operate the memory bus directly without the help of main CPU so DMA controller improves the system performance.

Whenever a device uses DMA , it has to specify a DMA channel . Ultra DMA hard drive uses DMA to transfer at high speed. Whereas earlier hard drives were the first to transfer data from CPU which took more time. 

In older computers , the four DMA channel were 0, 1, 2 and 3 were used , when the 16 bit ISA bus was introduced, the channels 5, 6 and 7 were also added to it.

DMA transfer modes

1. Burst mode : Burst mode is the fastest DMA mode in which two or more bytes can be continuously transferred.

2. Cycle steal mode : Cycle steal mode is slower than burst mode. In this mode only one byte is transferred at a time.

3. Transparent mode : This is the slowest data transfer mode in which the processor executes some status that makes it different from the system bus.

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