Consider a disk drive wth 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

10

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40

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50

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25

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Solution

The correct option is D $25$
$R e v o l u t i o n P e r m i n u t e = 3000 R P M$

$O r 3000 / 60 = 50 R P S$

$I n o n e t r a c k r o t a t i o n i t c a n r e a d = 512 K B$

$I n 50 R P S i t c a n r e a d = 512 \times 50$

$F o r 1 b y t e r e a d = 1 / (512 \times 50) = 39.06 n s$

$\Rightarrow F o r 4 b y t e s i t t a k e s 156 n s$

$P e r c e n t a g e o f t i m e t h a t t h e C P U g e t s$
$b l o c k e d d u r i n g D M A o p e r a t i o n = \frac{40}{156} \times 100$

$≃ 25$

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