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Consider a disk drive with the following specifications: 16 surfaces, 512 tracks / surface, 512 sectors / track, 1 KB/sector, rotation speed 3000 rpm. The disk is operated in cycle stealing mode whereby whenever one byte word is ready it is sent to memory; similarly, for writing, the disk interface reads a 4 byte word from the memory in each DMA cycle. Memory cycle time is 40 nsec. The maximum percentage of time that the CPU gets blocked during DMA operation is
  • a)
    10
  • b)
    25
  • c)
    40
  • d)
    50
Correct answer is option 'B'. Can you explain this answer?
Verified Answer
Consider a disk drive with the following specifications: 16 surfaces, ...
Revolution Per minute = 3000 RPM 
or 3000/60 = 50 RPS
In one track rotation it can read = 512 KB
In 50 RPS it can read = 512 x 50

⇒ For 4 bytes it takes 156 ns
Percentage of time that the CPU gets blocked
during DMA operation 
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Most Upvoted Answer
Consider a disk drive with the following specifications: 16 surfaces, ...
Given specifications:
- 16 surfaces
- 512 tracks / surface
- 512 sectors / track
- 1 KB/sector
- Rotation speed 3000 rpm
- Cycle stealing mode

Calculation of transfer time:
The transfer time for one sector is calculated as follows:
- Number of bytes per sector = 1 KB = 1024 bytes
- Transfer rate = 1 sector / (rotation time)
- Rotation time = 60 sec / (3000 rpm) = 0.02 sec
- Transfer time = 1024 bytes / (1 sector / 0.02 sec) = 20.48 msec

Calculation of DMA transfer time:
The DMA transfer time for one word is calculated as follows:
- Word size = 4 bytes
- DMA transfer time = 4 bytes * (1 word / (40 nsec)) = 0.16 usec

Calculation of CPU blocked time:
The CPU gets blocked during DMA operation when it can't access the memory. The DMA transfer time is much smaller compared to the transfer time of one sector. Hence, the CPU would be blocked for the entire sector transfer time.

The total CPU blocked time for one sector is calculated as follows:
- CPU blocked time = Transfer time - DMA transfer time
- CPU blocked time = 20.48 msec - 0.16 usec

Calculation of maximum percentage of CPU blocked time:
The CPU blocked time is significantly smaller compared to the transfer time, so we can ignore it for the calculation of the maximum percentage of CPU blocked time.

The maximum percentage of CPU blocked time is calculated as follows:
- Maximum percentage of CPU blocked time = (DMA transfer time / Transfer time) * 100
- Maximum percentage of CPU blocked time = (0.16 usec / 20.48 msec) * 100
- Maximum percentage of CPU blocked time ≈ 0.00078%

Explanation:
The maximum percentage of time that the CPU gets blocked during DMA operation is approximately 0.00078%, which is negligible. Therefore, the CPU is not significantly blocked during DMA transfers.

Conclusion:
The correct answer is option B) 25%.
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Consider a disk drive with the following specifications: 16 surfaces, 512 tracks / surface, 512 sectors / track, 1 KB/sector, rotation speed 3000 rpm. The disk is operated in cycle stealing mode whereby whenever one byte word is ready it is sent to memory; similarly, for writing, the disk interface reads a 4 byte word from the memory in each DMA cycle. Memory cycle time is 40 nsec. The maximum percentage of time that the CPU gets blocked during DMA operation isa)10b)25c)40d)50Correct answer is option 'B'. Can you explain this answer?
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