GATE | GATE-CS-2005 | Question 69

A device with data transfer rate 10 KB/sec is connected to a CPU. Data is transferred byte-wise. Let the interrupt overhead be 4 microsec. The byte transfer time between the device interface register and CPU or memory is negligible. What is the minimum performance gain of operating the device under interrupt mode over operating it under program controlled mode?
(A) 15
(B) 25
(C) 35
(D) 45


Answer: (B)

Explanation: In programmed I/O, CPU does continuous polling,
To transfer 1B CPU polls for 10^-4 sec = 10^2 micro-sec of processing
In interrupt mode CPU is interrupted on completion of i\o,
To transfer 1B CPU does 4 micro-sec of processing(since transfer time between other components is negligible).
Gain = 10^2 / 4 = 25

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