The document discusses storage management concepts including fragmentation and paging. It covers: - External and internal fragmentation that can occur with fixed memory partitions. - Paging concepts that divide memory into fixed-sized frames and logical memory into pages to avoid external fragmentation. - Address translation using page tables to map logical addresses to physical frames.

1. Storage Management
• Fragmentation
• Paging concepts
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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
 Multiple partition allocation
 Allocation of memory
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4. Objectives
On completion of this class, you will be able to
know
• Fragmentation
- External
- Internal
• Paging concepts
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5. Multiprogramming with Fixed Partitions
Memory
0k
• Divide memory into n
OS
4k
(possible unequal)
P1
partitions
16k
• Problem: P2
– Fragmentation 64k
Free Space
P3
128k
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6. External Fragmentation
Memory
• Enough total memory 0k
Operating
exists to satisfy a System
400k
request
P1
• But not contagious
1000k
• Results in external
P4
fragmentation 1700k
2000k Free Space
• Ex: Process size P3
2300k 560K
P1=600K, P4= 700K, 2560k
P3=300K, P5= 500K 6

7. Fixed Partitions
Memory
 Ex. A Hole Size is
18,464 Free Space
18,464 bytes OS
Bytes
 Process P3 request
is 18,462 bytes P1
 Allocate the request
Internal
 Hole of two bytes is P3 fragmentation
left (Internal
(cannot be
Fragmentation) P2
reallocated)
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8. Fragmentation
• Disadvantage
– Block of free memory between two process can not
be used for executing the process
– Which end of the free block is allocated (top or
bottom)
– If all free blocks of memory were together several
more process can be executed
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9. How Bad Is Fragmentation?
• Statistical arguments - Random sizes
• First-fit, Best-fit, Worst-fit
• Given N allocated blocks
• 0.5∗N blocks will be lost because of fragmentation
• Known as 50% RULE
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10. Solution to External Fragmentation
Compaction
 Shuffle memory contents to place all free memory
together in one large block as shown in slide no. 11
 Compaction is not always possible
 If relocation is static and is done at assembly or
load time compaction is not possible
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11. Solution to External Fragmentation
 Compaction is possible only if relocation is
dynamic and is done at execution time
 I/O problem
 Latch job in memory while it is involved in I/O
 Do I/O only into OS buffers
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12. Solution to External Fragmentation
• Compaction
5 Monitor Job 7 Job 5 Job 3 Job 8 Free
Job 6
6 Monitor Job 7 Job 5 Job 3 Job 8 Free
Job 6
7 Monitor Job 7 Job 5 Job 3 Job 8 Free
Job 6
8 Monitor Job 7 Job 5 Job 3 Job 8 Free
Job 6
9 Monitor Job 7 Job 5 Job 3 Job 8 Job 6 Free
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13. Internal fragmentation
Want to also avoid internal fragmentation?
 Memory is handed out in some fixed way (Power
of 2 for instance)
 and requesting program doesn't use it all
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14. Storage Management Problems
• Fixed partitions suffer from
– Internal fragmentation
• Variable partitions suffer from
– External fragmentation
• Compaction suffers from
– Overhead
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15. Paging
• Possible solution to the external fragmentation is
use of Paging scheme
• Paging avoids the problem of fitting varying-sized
memory chunks on to backing store
• We don’t need to assign contiguous memory
chunks
• Internal fragmentation can only occur on the last
page assigned to a process
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16. Paging
• Logical address space of a process can be
noncontiguous
• Process is allocated to physical memory whenever
the latter is available
• Divide physical memory into fixed-sized blocks called
frames
– Ex: size is power of 2, between 512 bytes and 8,192 bytes
• Divide logical memory into blocks of same size
called pages
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17. Paging
• Keep track of all free frames
• To run a program of size n pages, need to find n
free frames and load program
• Set up a page table to translate logical to
physical addresses
• Internal fragmentation
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18. Paging Hardware
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19. Address Translation Scheme
• Address generated by CPU is logical address
– is divided into two parts as shown in next slide
– Page number (p) – used as an index into a page table
which contains base address of each page in physical
memory
– Page offset (d) – combined with base address to
define the physical memory address that is sent to the
memory unit
– for given logical address space 2m and page size 2n
page number page offset
p d
m-n n 19

20. Paging Hardware
Physical
Logical address Physical
address
Memory
p d f d
CPU
p
Page table
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21. Paging Model of Logical and Physical
Memory
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22. Paging Model of Logical and Physical
Memory
• Example: mapping of user view of memory to
physical memory
– Page size of 4 bytes
– Physical memory of 32 bytes ( 8 pages )
– Logical address 0, page 0, offset 0
– Indexing into page table, we get page 0 is in frame 5
– Logical address 0 maps to physical address
20(=(5X4)+0)
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23. Paging Example
32-byte memory and 4-byte pages
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24. Summary
In this class, you have learnt
• Fragmentation
• Paging concepts
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25. Frequently Asked Questions
1. What is external fragmentation?
2. What is internal fragmentation?
3. Define a page and a frame
4. Explain the paging hardware with a neat
diagram
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26. Quiz
1. Enough total memory exists to satisfy a
request but not contagious results in
a) External fragmentation
b) Internal fragmentation
C) None
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27. Quiz
2.Shuffling memory contents to place all free
memory together in one large block is called
a) Fragmentation
b) Hole
c) Compaction
d) None
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28. Quiz
3. Fixed partitions suffer from
a) External fragmentation
b) Internal fragmentation
c) Compaction
d) None
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29. Quiz
4. Variable partitions suffer from
a) External fragmentation
b) Internal fragmentation
c) Compaction
d) None
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30. Quiz
5. Possible solution to the external fragmentation is
to use
a) Compaction
b) Paging scheme
C) None
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31. Quiz
6. ___________used as an index into a page
table
a) Page offset
b) Hole
c) Page number (p)
d) None
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32. Other subject materials
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33. Quiz
7. ______________combined with base address to
define the physical memory address
a) Page number
b) Page offset (d)
c) Compaction
d) None
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34. Quiz
8. Divide physical memory into fixed-sized blocks
called
a) External fragmentation
b) Frames
c) Compaction
d) None
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