2. HOME PREVIOUS TOPIC NEXT
PREVIOUS QUESTION PAPERS FOR OS
CPP TUTORIALS
http://raj-os.blogspot.in/ 2
3. Recap
In the last class, you have learnt:
• Various free space management techniques
http://raj-os.blogspot.in/ 3
4. Objectives
On completion of this period, you would be able to
know
• Why allocation methods are needed
• Various allocation methods
http://raj-os.blogspot.in/ 4
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
http://raj-os.blogspot.in/ 5
6. Allocation Methods
Three major methods of allocation of disk space
are
– Contiguous
– Linked
– Indexed
http://raj-os.blogspot.in/ 6
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
http://raj-os.blogspot.in/ 7
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
http://raj-os.blogspot.in/ 8
9. Contiguous Allocation
• The number of disk seeks are Minimal
• The seek time when a seek is needed Minimal
http://raj-os.blogspot.in/ 9
10. Contiguous Allocation
• Contiguous allocation file is defined by
– Disk address
– Length (in block units) of the block
http://raj-os.blogspot.in/ 10
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
http://raj-os.blogspot.in/ 11
12. Contiguous Allocation
• The directory entry for file has
– Name
– Starting address
– Length
http://raj-os.blogspot.in/ 12
15. Problems of Contiguous Allocation
• Finding space for a new file or a resized file.
• Determining size requirements.
• External fragmentation of the hard disk.
http://raj-os.blogspot.in/ 15
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
http://raj-os.blogspot.in/ 16
17. Summary
In this class, you have learnt
Various allocation methods
Their advantages and disadvantages
http://raj-os.blogspot.in/ 17
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
http://raj-os.blogspot.in/ 18
19. Quiz
• Allocation methods are for effective disk space utilization
[T/F]
True
• With contiguous allocation the disk movement is minimal
[T/F]
True
http://raj-os.blogspot.in/ 19
20. Quiz
Linked allocation support direct access [T/F]
– False
Space is wasted for pointers [T/F]
– True
http://raj-os.blogspot.in/ 20
21. Other subject materials
• Web designing
• Micro processors
• C++ tutorials
• java
home
http://raj-os.blogspot.in/ 21