The document discusses different allocation methods for managing disk space for files in an operating system. It describes contiguous, linked, and indexed allocation. Contiguous allocation allocates files in contiguous blocks, simplifying access but wasting space. Linked allocation uses pointers to non-contiguous blocks, avoiding fragmentation. Indexed allocation uses an index block to store block addresses, requiring multiple accesses to retrieve a block.

1. Allocation Methods
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2. HOME PREVIOUS TOPIC NEXT
PREVIOUS QUESTION PAPERS FOR OS
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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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