1. Allocation Methods
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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:
• Various free space management techniques
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4. Objectives
On completion of this period, you would be able to
know
• Why allocation methods are needed
• Various allocation methods
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5. Allocation Methods
• An allocation method refers to how disk blocks
are allocated for files
• Allocation methods are for
- effective disk space utilization
– allow fast file access
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6. Allocation Methods
Three major methods of allocation of disk space
are
– Contiguous
– Linked
– Indexed
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7. Contiguous Allocation
• Each file occupies a set of contiguous blocks on
the disk
• Simple – only starting location (block #) and length
(number of blocks) are required
• Random access
• Wasteful of space (dynamic storage-allocation
problem)
• Files cannot grow
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8. Contiguous Allocation
• Accessing block b+1 after block b
(assuming that only one job is accessing the
disk )
– Requires no head movement
– When needed only one track to the next
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9. Contiguous Allocation
• The number of disk seeks are Minimal
• The seek time when a seek is needed Minimal
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10. Contiguous Allocation
• Contiguous allocation file is defined by
– Disk address
– Length (in block units) of the block
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11. Contiguous Allocation
Example:
• If file is n block long and starts at location b, then
– The blocks occupied are b, b+1, b+2…..b+n-1
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12. Contiguous Allocation
• The directory entry for file has
– Name
– Starting address
– Length
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13. Example:
Fig 1
Contiguous allocation
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14. Contiguous Allocation
Fig 2
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15. Problems of Contiguous Allocation
• Finding space for a new file or a resized file.
• Determining size requirements.
• External fragmentation of the hard disk.
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16. Indexed Allocation Disadvantages
Wasteful if only a few pointers are stored in the
index block
Number of disk access necessary to retrieve the
address of target block address
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17. Summary
In this class, you have learnt
Various allocation methods
Their advantages and disadvantages
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18. Frequently Asked Questions
1.Why allocation methods are needed
2. Explain various allocation methods
3.List the advantages and disadvantages of
various allocation methods
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19. Quiz
• Allocation methods are for effective disk space utilization
[T/F]
True
• With contiguous allocation the disk movement is minimal
[T/F]
True
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20. Quiz
Linked allocation support direct access [T/F]
– False
Space is wasted for pointers [T/F]
– True
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21. Other subject materials
• Web designing
• Micro processors
• C++ tutorials
• java
home
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