GATE | GATE-CS-2005 | Question 70

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

Answer: (B)
Explanation: Time takes for 1 rotation = 60/3000
It reads 512*1024 Bytes in one rotation.
Time taken to read 4 bytes = 153 ns
153 is approximately 4 cycles (160ns)
Percentage of time CPU gets blocked = 40*100/160 = 25
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