Test by Bikram | Mock GATE | Test 2 | Question: 41

Question

A $DMA$ module is transferring bytes to memory using cycle stealing mode from a device transmitting at $16 KB/s$. The processor is fetching instructions at the rate of $1 MB/s$. The percentage by which the processor will be slowed down due to the $DMA$ activity is ______.

Comments

https://gateoverflow.in/156067/gate-overflow

Answer 1

Device transfers 1 byte in 61 microsec. 16kB / sec
Memory operation transfers 1 Byte in 1microsec.  1MB / sec.
%CPU idle time = 1/(1+61) = 1.6%

Comments

It is given cycle stealing ,why pipelining is not considered ?

%cpu slowed down = 1/ 62.5 *100 = 1.6

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@Bikram sir,

@sushmita

@VS

ma'am

Is % CPU slow down and % CPU idle/blocked/consume will be same ???

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what should be the answer if it is operated in burst mode instead of cycle stealing mode?

Answer 2

Suppose each intruction size is 1  Byte, So, if DMA doesn't interrupt CPU then it will fetch $10^6$ instructions in one second.

To transfer 1 Byte $= \frac{1}{16K} = 62.5$ Microseconds

So DMA will interrupt CPU $\frac{1Sec}{62.5 Micro Sec}=16000 times$

When CPU can fetch $10^6$ instructions in 1 second, it will fetch now $10^6-16000 = 984000$ instructions only.

Hence CPU is slowed down by $=\frac{16000}{10^6} = 0.016 = 1.6\text{%}$