Consider a disk drive with the following specifications: 16 surfaces, 512…

2005

Consider a disk drive with the following specifications: 16 surfaces, 512 tracks per surface, 512 sectors per track, 1 KB per sector, and a rotation speed of 3000 rpm. The disk uses cycle-stealing DMA. For reading, whenever one 4-byte word is ready, it is sent to memory; similarly, for writing, the disk interface reads one 4-byte word from memory in each DMA cycle. The memory cycle time is 40 ns. Which offered value is closest to the maximum percentage of time for which the CPU is blocked during DMA operation?

Answer: B. 25 — ConceptIn cycle-stealing DMA, each DMA transfer temporarily occupies the memory bus, so the CPU-blocking fraction equals the DMA memory-cycle time divided by…

  1. A.

    10

  2. B.

    25

  3. C.

    40

  4. D.

    50

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Show answer & explanation

Correct answer: B

Concept

In cycle-stealing DMA, each DMA transfer temporarily occupies the memory bus, so the CPU-blocking fraction equals the DMA memory-cycle time divided by the interval between successive DMA transfers.

The disk transfer rate depends on bytes per revolution and revolutions per second; the total number of surfaces and tracks determines capacity, not the instantaneous transfer rate of the active track.

Application

  1. One revolution transfers 512 sectors per track × 1024 bytes per sector = 524,288 bytes.

  2. The rotation rate is 3000 ÷ 60 = 50 revolutions per second.

  3. Therefore, the disk data rate is 524,288 × 50 = 26,214,400 bytes per second.

  4. Each DMA cycle transfers 4 bytes, so DMA performs 26,214,400 ÷ 4 = 6,553,600 memory cycles per second.

  5. Each DMA cycle blocks the CPU for 40 ns. Thus, the blocked fraction is 6,553,600 × 40 × 10−9 = 0.262144.

  6. As a percentage, 0.262144 × 100 = 26.2144%, whose closest offered value is 25%.

Cross-check

The interval between 4-byte DMA transfers is 4 ÷ 26,214,400 seconds ≈ 152.59 ns. The ratio 40 ÷ 152.59 ≈ 0.262 confirms the same blocked fraction.

Result

The CPU is blocked for approximately 26.2% of the time, so the closest offered value is 25%.

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