1. Chapter3 Memory Management Techniques

2. What is meant by memory management and how to manage memory?
Ready Queue
Process need to be executed
Memory management is how to organize active processes (processes currently in the ready queue) so that:
Processes can be easily reached.
Maximize memory space utilization.

3. Show how a loader stores an executable file into the memory assuming:
File of Size =20 memory words (instructions and data).
Using Contiguous allocation method.
Repeat the problem three different times using:
First fit.
Best fit.
Worst Fit.
Executable file
(Size =20 memory words)
Memory
Loader ?

4. Using First Fit Memory OS Process Process Process Process ProcessOS
1040 20
Base
Limit
Start address of process
Legal range
999
1000
1010
Process
1040
Process
1060
Executable file
(Size =20 memory words)
1070
Process
1100
1125
Process
1150
Loader
1200
Process
1250
1255

5. Using Best Fit Memory OS Process Process Process Process Process
1100 20
Base
Limit
Start address of process
Legal range
1000
1010
Process
1040
Executable file
(Size =20 memory words)
1070
Process
1100
Process
1120
1125
Process
Loader
1150
1200
Process
1250
1255

6. Using Worst Fit Memory OS Process Process Process Process Process
1150 20
Base
Limit
Start address of process
Legal range
1000
1010
Process
1040
Executable file
(Size =20 memory words)
1070
Process
1100
1125
Process
Loader
1150
Process
1170
1200
Process
1250
1255

7. How to manage memory in multi-programming environment?
Memory Management In multi-programming environment
Paging
Swapping
Contiguous allocation

8. Swapping:
A process can be swapped out of memory to disk, and then brought back into memory for continued execution.

9. Multi-programming environment with “priority scheduling”
Ex1:
Multi-programming environment with “priority scheduling”
Assume a system with 32 kb memory size.
5kb are used for OS, 10 kb for the low priority process.
Hence, the available space is 17 kb.
A higher priority process arrives and needs 20 kb.
OS (5 kb)
Low Priority process (10 kb)
Available
17 Kb
27 Kb
High Priority process (20 kb)
7 Kb
Swap in the high priority process
high priority process finishes execution
Swap out the low priority process to disk
Disk
Swap in the low priority process again to resume execution
High
Priority
20 Kb

10. Contiguous Allocation
What is meant by contiguous allocation, what are its different types?
In contiguous allocation, each process is contained in a single contiguous section of memory.

11. Methods for Contiguous Allocation
Simple Method
General Method

12. Simple Method Divide memory into several fixed-sized partitions.
Each partition contains one process.
Degree of multiprogramming is bound by the number of partitions.
When a partition is free, a process is selected from the input queue and is loaded into the free partition.
When the process terminates, the partition becomes available for another process.
its no longer used because it has various drawbacks like:
Degree of multiprogramming is bounded by the number of partitions.
Internal fragmentations.

13. As shown This method suffers from internal fragmentations.
Assuming a memory of 30 KB divided into three partitions as following:
Process 1
(7 KB)
Input Queue
(in the disk)
10 KB
Internal Fragmentation
Process 2
(9 KB)
4 KB
8 KB
9 KB
7 KB
10 KB
Process 3
(8 KB)
10 KB
As shown
This method suffers from internal fragmentations.
The degree of multiprogramming is bounded to 3 although it can be 4.

14. General Method
Initially, all memory is available for user processes, and is considered as one large block of available memory.
When a process arrives and needs memory, we search for a hole large enough for this process using.
If we find one, we allocate only as much memory as is needed, keeping the rest available to satisfy future requests.
First Fit
Best Fit
Worst Fit

15. X
There are three different methods to find the suitable hole for a process?:
First fit: allocate the first hole that is big enough (fastest method).
Best fit: allocate the smallest hole that is big enough (produces the smallest leftover hole).
Worst fit: allocate the largest hole (produces the largest leftover hole which may be more useful than the smaller leftover hole from a best-fit approach.

16. OS
Process 4
(4 KB)
Process 1
(7 KB)
Input Queue
(in the disk)
Process 5
(9 KB)
Process 2
(9 KB)
9 KB
4 KB
8 KB
9 KB
7 KB
Process 3
(8 KB)
As shown:
The degree of multiprogramming changing according to the number of processes in the memory (in ready queue).

17. After a period of time External Fragmentations appearOS
Process 4
Input Queue
(in the disk)
Process 2
External Fragmentations
Process 3
4 KB
8 KB
9 KB
7 KB
Process 9
Process 20
As shown:
This method suffers from external Fragmentations.

18. Compaction OS OS Compaction:
Process 2
Process 3 OS
Process 4
Process 9
Process 20 OS
Process 4
Input Queue
(in the disk)
Process 2
Process 3
4 KB
8 KB
9 KB
7 KB
Process 9
A new hole to store a new process
Process 20
Compaction:
Is a movement of the memory contents to place all free memory in a one large block sufficient to store new process.
It is a solution for the external fragmentation but it is expensive and is not always possible.

19. Paging
Paging is a memory-management scheme that permits the physical-address space of a process to be noncontiguous.
it is commonly used in most operating systems.
Divide physical memory into fixed-sized blocks called frames.
Divide Process into blocks of same size called pages.
Use a page table which contains base address of each page in physical memory. X

20. Example A process P is divided into 4 pages.1 2 3
Page number
Page number
Frame number
Process P
The process will be loaded into 4 frames.
The page number is used as index to the page table.
The page table contains the frame number for each page.

21. Paging Example Sol: Memory size=32 Byte =32 words.
32-byte memory, each memory word of size =1 byte (can store only one character), the size of page (and frame also)= 4-byte.
Show how to store 4 pages process into memory using a page table.
according to your page table, what are the physical addresses corresponding to the logical addresses 4 and 13.
Sol:
Memory size=32 Byte =32 words.
Page size = Frame size = 4 bytes =4 words.
Number of Frames = 32/4 = 8 Frames (addressed from 0  7)

22. Logical address
Page 0 Contents
Physical address
Page 0
Page 1
Page 2
Page 3
4 Page process
Frame 0
Frame 1
Frame 2
Frame 3
Frame 4
Frame 5
Frame 6
Frame 7

23. Advantages of paging: Problems in paging: No external fragmentation.
Allow the process components to be noncontiguous.
Problems in paging:
A possibility of internal fragmentations that can not be used.
Q: Explain when you have internal fragmentations when using the paging technique.
Sol:
when the contents of the last page of the process is less than the frame size, then the remaining part of the frame will be an internal fragmentations that can not be used.