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Question about DMA

Posted on 1998-02-15
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Last Modified: 2013-12-27
Hi, I am doing a Operation System Homework problem. However, I just have no idea about this problem (I can't find the answer in the book.)  I wonder if someone here can explain it for me?

The question is:
In virtually all systems that include DMA modules, DMA access to main memory is given higher priority than processor access to main memory. Why?


Thanks,
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Question by:kelly_host
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Stonewal earned 50 total points
ID: 1806318
Because it is called "Direct Memory Access"? or because it has no use for the CPU and only needs room to work?

That was off the wall   hehehehe
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Expert Comment

by:Otta
ID: 1806319
It's a matter of "priorities".
The hard-drive is physically "spinning",
and is either sending data from the
hard-drive into RAM, or needs to copy
data from RAM onto the hard-drive,
and it needs access to RAM *RIGHT* *NOW*,
i.e., at the highest priority.
The CPU must "wait" for the accesses to be
done; then, the CPU can fetch the next instruction to be executed.
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