GATE Overflow Test Series | Computer Organization and Architecture | Test 1 | Question: 22

Question

A Disk drive with $8$ surfaces, $256$ tracks/surface, $256$ sectors/track, $512$ bytes/sector and rotation speed of $3600$ rpm is operated in cycle stealing mode whereby whenever one $2$ Byte word is ready, it is sent to memory. Similarly for writing, the disk interface reads a $2$ Byte word from memory in each DMA cycle. The memory cycle time is $20$ nanoseconds. Find the maximum percentage of time that a CPU gets blocked during DMA operations (rounded to $1$ decimal point).

Answer 1

Only one surface of a hard disk can be read at a time.
    
Ref: https://superuser.com/questions/25355/multiple-independent-read-write-heads-on-the-same-hard-drive-platter

Track capacity: $512\times 256 = 128$ KB

Data transfer rate:
    $\frac{60}{3600}s\rightarrow 128$ KB
    
    $2 \text{Byte} \rightarrow \frac{2}{60\times 128\times 1024}=254.3$ ns $= \left \lceil \dfrac{254.3}{20}\right \rceil = 13$ memory cycles 

Maximum % of time CPU gets blocked using cycle stealing:
    
$\qquad=\frac{1}{13}\times 100=7.69\%$

If we do not do ceil operation for getting memory cycles, we will get $7.8\%$ which is also fine as that can happen if DMA operation continues for a very long time. 

Comments

In same cycle we can transfer & prepare data

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@gatecse 

Only one surface of a hard disk can be read at a time.

This more concretely means that only one track per surface at a time right?

 

Im asking this as i want to clarify if access to disk is in the following manner:

Surface 1, track 0, sector 0# to #s

Surface 1, track 1, sector #0 to #s

.

.

Surface 1, track t, sector #0 to #s

Surface 2, track 0, ….. and so on (i have a hunch that this answer is implying this method)

OR

according to the following wikipedia page on <cylinder,head,sector> format, that is 1^st we finish a whole cylinder (but ofc one surface at a time) and then the next cylinder, starting from the top surface again.

https://en.wikipedia.org/wiki/Logical_block_addressing#CHS_conversion

Hoping you reply soon with a clarification, or maybe a link where i can clarify this. Thanks

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@rish-18

Only one surface of a hard disk can be read at a time.

one surface = one track in one rotation.