Consider a disk drive with the following specifications: 16 surfaces, 512…
2005
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
Attempted by 54 students.
Show answer & explanation
Correct answer: B
Solution summary: compute data rate and fraction of time the DMA steals memory cycles.
Bytes per revolution = 512 sectors/track × 1024 bytes/sector = 524,288 bytes.
Revolutions per second = 3000 rpm ÷ 60 = 50 rps. Data rate = 524,288 × 50 = 26,214,400 bytes/s.
For writing, each DMA memory access reads 4 bytes. Time between DMA memory accesses = 4 bytes ÷ 26,214,400 bytes/s ≈ 152.59 ns.
Each DMA memory access blocks the CPU for one memory cycle = 40 ns. Blocked fraction = 40 ns ÷ 152.59 ns ≈ 0.262 ≈ 26.2%.
Therefore the closest choice is 25%.
Note: If the interface transferred only 1 byte per DMA memory access (reading case), the time between accesses would be about 38.15 ns, which is shorter than the 40 ns memory cycle and would saturate memory (effectively blocking the CPU nearly 100%). The problem's available choices and the statement about 4-byte DMA cycles indicate the intended calculation is the 4-byte case above.