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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 4 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____________
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rpm 3000 means

60/3000 = 1/50 sec roatate 1 track

track capacity = 512 * 1 KB

1/50 sec access 512 KB

1 byte access take 1/(50 * 512 * 2^10)  sec

4 byte access take 4/(512 * 2^10 ) sec

 

in stealing mode 4 byte take 40 n sec

 

so % cpu gets blocked = ( cycle stealing mode / cpu mode )

                                   =   (40 * 10 ^ -9 *512 * 2^10 /4 ) *100 == 26 %

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