Memory management allows multiple processes to reside in memory simultaneously through memory allocation algorithms. There are two main approaches: fixed and variable partitioning. Fixed partitioning divides memory into equal or variable sized partitions, which can lead to internal and external fragmentation. Variable partitioning dynamically allocates memory blocks of varying sizes to processes using first fit, best fit, or worst fit allocation strategies. Compaction is needed with worst fit to consolidate free memory and reduce external fragmentation.

1. MEMORY
MANAGEMENT

2. memory management
 In a multiprogramming system, in order to
share the processor, a number of processes
must be kept in memory.
 Memory management is achieved through
memory management algorithms.
 In order to be able to load programs at
anywhere in memory, the compiler must
generate relocatable object code.
 Also we must make it sure that a program in
memory, addresses only its own area, and no
other program’s area.
1

3.  Memory Allocation
Single Partition Allocation
In this scheme Operating system is
residing in low memory and user processes
are executing in higher memory.
Advantages
It is simple.
It is easy to understand and use.
Disadvantages
It leads to poor utilization of processor and
memory.
Users job is limited to the size of available
memory.
2

4. Multiple-partition Allocation
 One of the simplest methods for allocating
memory is to divide memory into several
fixed sized partitions.
 There are two variations of this.
 Fixed Equal-size Partitions
 Fixed Variable Size Partitions
3

5. Fixed Equal-size Partitions
 It divides the main memory into equal number of fixed
sized partitions, operating system occupies some fixed
portion and remaining portion of main memory is
available for user processes. Advantages
 Any process whose size is less than or equal to the
partition size can be loaded into any available partition.
 It supports multiprogramming.
 Disadvantages
 If a program is too big to fit into a partition use overlay
technique.
 Memory use is inefficient, i.e., block of data loaded into
memory may be smaller than the partition. It is known as
internal fragmentation.
4

6. Fixed Variable Size Partitions
 By using fixed variable size partitions we can
overcome the disadvantages present in fixed
equal size partitioning
5

7. 3.2 Variable Partitioning
 With fixed partitions we have to deal with the problem of
determining the number and sizes of partitions to
minimize internal and external fragmentation.
 If we use variable partitioning instead, then partition
sizes may vary dynamically.
 In the variable partitioning method, we keep a table
(linked list) indicating used/free areas in memory.
6

8. fragmentation
memory
OS
2K
6K Empty (6K)
12K empty
Empty (3K)
P2 (9K)
P1 (2K)
If a whole partition is
currently not being used,
then it is called an
external fragmentation.
If a partition is being
used by a process
requiring some
memory smaller than
the partition size, then
it is called an internal
fragmentation.
7

9. 3.2 Variable Partitioning
 Initially, the whole memory is free and it is considered as
one large block.
 When a new process arrives, the OS searches for a
block of free memory large enough for that process.
 We keep the rest available (free) for the future
processes.
 If a block becomes free, then the OS tries to merge it
with its neighbors if they are also free.
8

10. 3.2 Variable Partitioning
 There are three algorithms for searching the list of free
blocks for a specific amount of memory.
 First Fit
 Best Fit
 Worst Fit
9

11. first fit
 First Fit : Allocate the first free block that is
large enough for the new process.
 This is a fast algorithm.
10

12. first fit
OS
P1 12 KB
<FREE> 10 KB
P2 20 KB
<FREE> 16 KB
P3 6 KB
<FREE> 4 KB
Initial memory
mapping
11

13. first fit
OS
P1 12 KB
<FREE> 10 KB
P2 20 KB
<FREE> 16 KB
P3 6 KB
<FREE> 4 KB
P4 of 3KB
arrives
12

14. first fit
OS
P1 12 KB
P4 3 KB
<FREE> 7 KB
P2 20 KB
<FREE> 16 KB
P3 6 KB
<FREE> 4 KB
P4 of 3KB
loaded here by
FIRST FIT
13

15. first fit
OS
P1 12 KB
P4 3 KB
<FREE> 7 KB
P2 20 KB
<FREE> 16 KB
P3 6 KB
<FREE> 4 KB
P5 of 15KB
arrives
14

16. first fit
OS
P1 12 KB
P4 3 KB
<FREE> 7 KB
P2 20 KB
P5 15 KB
<FREE> 1 KB
P3 6 KB
<FREE> 4 KB
P5 of 15 KB
loaded here by
FIRST FIT
15

17. Best fit
 Best Fit : Allocate the smallest block among those that
are large enough for the new process.
 In this method, the OS has to search the entire list, or it
can keep it sorted and stop when it hits an entry which
has a size larger than the size of new process.
 This algorithm produces the smallest left over block.
 However, it requires more time for searching all the list
or sorting it
 If sorting is used, merging the area released when a
process terminates to neighboring free blocks, becomes
complicated.
16

18. best fit
OS
P1 12 KB
<FREE> 10 KB
P2 20 KB
<FREE> 16 KB
P3 6 KB
<FREE> 4 KB
Initial memory
mapping
17

19. best fit
OS
P1 12 KB
<FREE> 10 KB
P2 20 KB
<FREE> 16 KB
P3 6 KB
<FREE> 4 KB
P4 of 3KB
arrives
18

20. best fit
OS
P1 12 KB
<FREE> 10 KB
P2 20 KB
<FREE> 16 KB
P3 6 KB
P4 3 KB
<FREE> 1 KB
P4 of 3KB
loaded here by
BEST FIT
19

21. best fit
OS
P1 12 KB
<FREE> 10 KB
P2 20 KB
<FREE> 16 KB
P3 6 KB
P4 3 KB
<FREE> 1 KB
P5 of 15KB
arrives
20

22. best fit
OS
P1 12 KB
<FREE> 10 KB
P2 20 KB
P5 15 KB
<FREE> 1 KB
P3 6 KB
P4 3 KB
<FREE> 1 KB
P5 of 15 KB
loaded here by
BEST FIT
21

23. worst fit
 Worst Fit : Allocate the largest block among those that
are large enough for the new process.
 Again a search of the entire list or sorting it is needed.
 This algorithm produces the largest over block.
22

24. worst fit
OS
P1 12 KB
<FREE> 10 KB
P2 20 KB
<FREE> 16 KB
P3 6 KB
<FREE> 4 KB
Initial memory
mapping
23

25. worst fit
OS
P1 12 KB
<FREE> 10 KB
P2 20 KB
<FREE> 16 KB
P3 6 KB
<FREE> 4 KB
P4 of 3KB
arrives
24

26. worst fit
OS
P1 12 KB
<FREE> 10 KB
P2 20 KB
P4 3 KB
<FREE> 13 KB
P3 6 KB
<FREE> 4 KB
P4 of 3KB
Loaded here by
WORST FIT
25

27. worst fit
OS
P1 12 KB
<FREE> 10 KB
P2 20 KB
P4 3 KB
<FREE> 13 KB
P3 6 KB
<FREE> 4 KB
No place to load
P5 of 15K
26

28. worst fit
OS
P1 12 KB
<FREE> 10 KB
P2 20 KB
P4 3 KB
<FREE> 13 KB
P3 6 KB
<FREE> 4 KB
No place to load
P5 of 15K
Compaction is
needed !!
27

29. compaction
 Compaction is a method to overcome the external
fragmentation problem.
 All free blocks are brought together as one large block of
free space.
 Compaction requires dynamic relocation.
 Certainly, compaction has a cost and selection of an
optimal compaction strategy is difficult.
 One method for compaction is swapping out those
processes that are to be moved within the memory, and
swapping them into different memory locations
28

30. compaction
OS
P1 12 KB
<FREE> 10 KB
P2 20 KB
P4 3 KB
<FREE> 13 KB
P3 6 KB
<FREE> 4 KB
29
Memory
mapping before
compaction

31. compaction
OS
P1 12 KB
P2 20 KB
P4 3 KB
P3 6 KB
30
Swap out
P2

32. compaction
OS
P1 12 KB
P2 20 KB
P4 3 KB
P3 6 KB
Swap in
P2
Secondary
storage
31

33. compaction
OS
P1 12 KB
P2 20 KB
P4 3 KB
P3 6 KB
Swap out
P4
Secondary
storage
32

34. compaction
OS
P1 12 KB
P2 20 KB
P4 3 KB
P3 6 KB
Swap in
P4 with a
different
starting
address
Secondary
storage
33

35. compaction
OS
P1 12 KB
P2 20 KB
P4 3 KB
P3 6 KB
Swap out
P3
Secondary
storage
34

36. compaction
OS
P1 12 KB
P2 20 KB
P4 3 KB
P3 6 KB
Swap in
P3
Secondary
storage
35

37. compaction
OS
P1 12 KB
P2 20 KB
P4 3 KB
P3 6 KB
<FREE> 27 KB
Memory
mapping after
compaction
Now P5 of 15KB
can be loaded
here
36

38. compaction
OS
P1 12 KB
P2 20 KB
P4 3 KB
P3 6 KB
P5 12 KB
<FREE> 12 KB P5 of 15KB is
loaded
37

39. Fragmentation
 There are two types of fragmentation in OS which
are given as: Internal fragmentation, and External
fragmentation.
 Internal Fragmentation:
Internal fragmentation happens when the
memory is split into mounted sized blocks.
Whenever a method request for the memory, the
mounted sized block is allotted to the method.
just in case the memory allotted to the method is
somewhat larger than the memory requested,
then the distinction between allotted and
38

40.  External Fragmentation:
External fragmentation happens when there’s
a sufficient quantity of area within the memory
to satisfy the memory request of a method.
however the process’s memory request cannot
be fulfilled because the memory offered is
during a non-contiguous manner.
39

41. 40
External fragmentation

42. Comparison
41
•Internal fragmentation happens when
the method or process is larger than
the memory.
•The solution of internal fragmentation
is best-fit.
•Internal fragmentation occurs when
memory is divided into fixed sized .
•The difference between memory
allocated and required space or
memory is called Internal
fragmentation.
•External fragmentation happens when
the method or process is removed.
block.
•Solution of external fragmentation is
compaction, paging and segmentation
partitions.
•External fragmentation occurs when
memory is divided into variable size
partitions based on the size of
processes.
•The unused spaces formed between
non-contiguous memory fragments are
too small to serve a new process, is
called External fragmentation .