Test by Bikram | Computer Organization and Architecture | Test 2 | Question: 1

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

1.57% should be the correct answer.. I don't know why here concepts get modified as per one's comfort.

Sir why haven't you use (y/x+y)*100 . Values will change in accordance to the modes we use.

Answer 1

Here DMA is using cycle stealing mode to transfer data to memory. In cycle stealing mode 1 Byte is transferred in each cycle.

Device transfers 16 kb in 1 sec 

=> 1 kb in (1/16 ) sec 

now 1 kb = 1000 Bytes 

so 1000 B in 0.0625 sec 

=> 1 B in ( 0.0625 / 1000 ) sec = 0.0000625 sec , 1 sec = 10⁶  =1000000  μs

That means Device transfers 1 Byte in 62.5 μs.

Memory operation transfers  1 MB in 1 sec

=> 10⁶ B in 1 sec

=> 1 B in (1/10⁶) secs = (1/10⁶ )* 10⁶ μs = 1 μs

so memory transfer rate is 1 Byte in 1 μ second. ( same as above calculation as processor fetch at 1MB / s ).

To fetch 1 Byte data processor needs 1 Microseconds 

% time cpu is idle is for 1 cycle  , while DMA is transferring data 

so cpu time is 1 μs and total transfer time is 62.5 μs 

%CPU idle time = 1/(62.5) * 100 = 0.016 * 100 = 1.60 %

Comments

@bikram sir

1 kb = 1000 Bytes  //it should not b 1024 bytes?

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@joshi

No it should not be 1024 , as it is bandwidth , 1 KB = 1000 Bytes used always means 10³ Bytes.

You can read above comments by me for more clarification .

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got it sir,thnks.

Answer 2

DMA will interrupt CPU to transfer 16KB in 1 second, as Cycle Stealing method is used by DMA, it will interrupt CPU 16 K times in 1 second.
If DMA doesn't interrupt CPU, then CPU will fetch 1 million (10^6) instruction 1 second.

because of DMA steal 16K cycles from CPU, CPU will be idle for 16K cycles and now it will fetch 1,000,000 - 16,000 = 984,000 instructions hence it is slow down by 1 - (984000/1000,000)=0.016 or 1.6%

Comments

I got 1.56 with some calculation above.Can you once check why is that incorrect.Its in comment section under bikram sir answer

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Why have you considered KB and MB as 10^3 and 10^6 respectively? Why not 2^10 and 2^20?

 I know we do this in Cn ,but i have not used it in CO?As it can b e numerical type snwers also,so answer can change if we do approximation of 1024 to 1000.

Please correct me if i am wrong?

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@Manu Thakur as you said

$\frac{2^{20}-2^{{14}}}{2^{^{20}}}$ *100

comes out to be 1.5625

Is it like for Bandwidth or data transfer M is taken as 10^6.