The document discusses disk structure and secondary storage. It describes how disks are organized with circular platters divided into tracks and sectors. Disks use magnetic heads to read and write data by moving across tracks. There are two main types of heads: fixed heads which each cover a single track, and moving heads which can access different tracks by positioning over the disk surface. The document outlines how disk capacity is increased by improving recording density, using both sides of platters, and stacking multiple disks.

1. Disk Structure
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2. HOME PREVIOUS TOPIC NEXT
PREVIOUS QUESTION PAPERS FOR OS
CPP TUTORIALS
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3. Recap
In the last class, you have learnt:
• Working set model
• Page fault frequency
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4. Objective
On Completion of this period, you would be able
to know :
• Requirements of secondary memory
• Types of secondary memory
• Disk structure
• Head types
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5. Secondary Storage Management
Ideal requirement – Programs and data must reside in
main memory permanently for process execution but,
Constraints:
• Main memory is a volatile storage
• Too small
• Expensive
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6. Requirements of Secondary Memory
• Should hold large amounts of data
• Should be nonvolatile
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7. Types of Secondary Memory
• Magnetic tapes
• Magnetic disks
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8. Types of secondary memory
• Magnetic Tapes
– Early secondary storage device
– Slow, cannot provide random access
– Currently used for backup of infrequently used
information
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9. Types of Secondary Memory
• Magnetic Disks
– Fast and provides random access
– Provides bulk of secondary storage
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10. Disk Structure
Fig 1 moving head disk mechanism
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11. Physical Structure
• Flat circular Shape
• Two surfaces are covered with magnetic material
• Information is recorded on both surfaces
• The disk surface is logically divided into tracks
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12. Operating System View of Disk
• One dimensional array of disk blocks
• Each block is a sector
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13. Operating System view of Disk
Mapping of Disk address
Disk address
Cylinder i
Surface j
Sector k
Block number = k + s ( j + i * t )
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14. Working Principle
• The drive motor spins, disk at high speed
• The read write head is positioned just above the
surface of the disk
• Information is stored magnetically on the track
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15. Head Types
• Fixed-head disk
• Moving-head disk
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16. Fixed-head Disk
– Separate head for each track
– High speed access
– Large number of Heads
– Very expensive
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17. Moving-head disk
– Only one head
– Moves in and out to access different tracks
– Less expensive
– Additional hardware required to move the head
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18. Increasing Storage Capacity
• Improving recording density
– Increasing number of tracks per inch
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19. Increasing Storage Capacity
• Using two sides of the platter for storing information
– Additional read write head required
– Storage capacity is doubled
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20. Increasing storage capacity
• Stacking several disks
– Additional read/write heads are required for each
disk
– Only one motor drive is sufficient
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21. Disk Characteristics
• Read, modify and write it back in the same place
• Access any block directly
• Sequential or random access is allowed
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22. Summary
In this class, you have learnt
•Types of secondary memory
•Disk structure
•Head types
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23. Frequently Asked Questions
1. Explain Disk structure
2. Write a note on the Secondary storage devices
3. Explain different types of R/W heads
4. Explain the head disk characteristics
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24. Quiz
• Moving Head is more expensive than fixed head
mechanism [T/F]
•False
• Storage capacity can be increased by
−Increasing density
− Using two surfaces of the platter
−Stacking several disks
−All the above
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25. Quiz
• Magnetic tapes provide direct access [T/F]
False
• The disk has a Flat circular shape [T/F]
True
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26. Other subject materials
• Web designing
• Micro processors
• C++ tutorials
• java
home
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