An I/O device transfers data at a rate of 10MB/s over a 100MB/sbus. The data is
ID: 3607936 • Letter: A
Question
An I/O device transfers data at a rate of 10MB/s over a 100MB/sbus. The data is transferred in 4KB blocks. If the processoroperates at 500MHz, and it takes a total of 5000 cycles to handleeach DMA request, find the fraction of CPU time handling the datatransfer with and without DMA.
Question # 2
A magnetic disk has an average seek time of 6 ms. The transferrate
is 50 MB/sec. The disk rotates at 10,000 rpm and the controlleroverhead is 0.2 msec. Find the average time to read or write 1024bytes.
Explanation / Answer
Dear, Question 1
Without DMA Theprocessor here copies the data into memory as it is sent over thebus. Since
the I/O device sends data at a rate of 10MB/s over the 100MB/s bus,10 % of each second
is spent transferring data. Thus 10% of the CPU time is spentcopying data to memory.
With DMA the Time required in handling each DMA request is 5000cycles. Since 2500 DMA requests are issued (10MB/4KB) the totaltime taken is 12,500,000 cycles. As the CPU clock is 500MHZ, thefraction of CPU time spent is 12,500,000/(500x106) or 2.5%.
Question 2 :
Given data :
Average seek time=6ms
Average rotate time=0.6*60/10,000=3.6 ms
Transfer rate=1KB/50MB=0.02ms
controller time=0.2ms
The total time taken = Seek time +Rotate time+Transfer rate +Controller time
= 6+3.6+0.02+0.2
= 9.82 ms
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